Aphids are true insects within the order Hemiptera. Aphids are a relatively small group of insects, yet they are of serious economic concern in a variety of crops. About 25% of known crop species are affected by aphids. About 100 species are of serious economic importance . They have a worldwide distribution, but are most commonly found in temperate regions. They can travel large distances passively on strong winds.
Most adult aphids do not develop wings, but a some do . They feed on plants by utilizing a needle-like organ called a stylet. This allows them to bypass the plant cuticle and cells that contain defensive compounds.
Aphids are phloem-feeding, piercing, sucking pests that present a threat to crops through at least 4 mechanisms:
- Feeding damage- diverts nutrients intended for plant growth and reproduction to the aphids, causes physical plant damage which can lead to lower photosynthetic rates, increased transpiration, and lesions that can lead to pathogen infections.
- The injection of phytotoxins in to the plant during aphid feeding which can damage plant health and growth.
- Aphids are the most common vector of plant viruses. Nearly 50% of plant viruses can be transmitted by aphids , including viruses confirmed in Cannabis such as Cucumber Mosaic Virus (CMV) and Alfalfa Mosaic Virus (AMV) .
- Honeydew left on foliage by aphids can support the growth of sooty mold on leaf surfaces, leading to negatively impacted photosynthetic rates.
Cannabis produces the terpene β– farnesene, which acts as an alarm pheromone in the aphids .
What Aphid Species Affect Cannabis?
At least six species have been reported to infest Cannabis plants . However, more recent information has challenged previous claims about which species colonize Cannabis. In Colorado, M. persicae and P. humuli were unable to survive on hemp plants after manual inoculation. It may be that previous reports of M. persicae and P. humuli as pests of Cannabis were actually misidentification of the Cannabis aphid, P. cannabis. Aphis fabae has only been reported once in the literature, and it may be possible that these other species may be misidentified or uncommon pests . Given this information, IPM choices should generally focus on controlling P. cannabis, not other aphid species.
Myzus persicae (Green peach aphid)
A green (sometimes yellowish or even pink) aphid that is fairly large (about 2.0-3.4 mm long). The nymphs of the winged adults (alatae forms) are usually pink or red, while the adults have black-brown heads and a black spot on their abdomen.
In general, aphids are heteroecious, meaning that they migrate between two different hosts. M. persicae overwinters on Prunus species such as cherry or peach trees as eggs laid on limbs or tree bud axils. When eggs hatch, they give birth to fundatrices (stem mothers). These fundatrices can reproduce parthenogenically (asexually); they give live birth to 60-100 wingless clonal larvae called fundatrigeniae. The fundatrigaeniae give birth to more wingless females asexually (apterae or apterous viviparae). This continues until spring, when the first winged (alatae) aphids develop. These alatae females fly to alternate hosts such as Cannabis and produce more apterae offspring. Each aptera gives birth to 30-70 offspring until food becomes restricted, at which point new alatae (summer migrants) are produced to find new Cannabis plants. To reach maturity, each aptera offpring goes through about four molts on Cannabis. At the end of the summer, the aphids return to the primary host by special alatae called sexuparae. Upon return to Prunus hosts, the sexuparae give birth to ten sexuales.
In controlled indoor or greenhouse environments, M. persicae can replicate all year long parthenogenically on Cannabis.
can be male/female or alatous/apterous. When sexuales mate, females lay 5-10 eggs which overwinter on Prunus species. Using this life cycle, they are able to asexually amplify their populations on primary and secondary hosts.
Black Bean Aphid– Aphis fabae
Overwintering host- Euonymus or Virburnum species.They represent at least 4 subspecies, and which subspecies which attacks Cannabis is unknown. Like M. persicae, eggs hatch in to fundatrices, followed by fundatrigeniae, then apterae. The rest of the life cycle is similar to M. persicae, and in the spring, winged females may infest Cannabis plants, which then produce more aphids asexually. They are found in all temperate regions except Australia, they infect many crops, and may vector over 30 viruses.
Bhang Aphid/Hemp Louse- Phorodon cannabis
These aphids are about 25% smaller than M. persicae (1.9-2.7 mm long). Their color can vary from colorless to bright green with dark green stripes. Alatae (winged) larvae are smaller than apterae larvae and have dark patches on the head and abdomen. Unlike M. persicae, P. cannabis does not alternate hosts (autoecious), and so sexuparae lay eggs directly on Cannabis, particularly on flowering tops. It has evolved to complete its entire life cycle on Cannabis plants.
As previously mentioned, P. cannabis is most damaging to Cannabis flowers; it prefers to feed on flower sepals and in seeded Cannabis, and shelter in between seeds and sepals. The bhang aphid has been reported as a common vector of Cannabis viruses including the hemp streak virus, hemp mosaic virus, cucumber mosaic virus, alfalfa mosaic virus, hemp mottle virus, and hemp leaf chlorosis virus. P. cannabis may have evolved from P. humuli, because it is autoecious but is indistinguishable from P. humuli aside from the smaller head of P. humuli and fewer bristles.
Much like M. persicae, P. humuli is heteroecious and overwinters on Prunus species. In the CA bay and southern England, spring migration of P. humuli alatae females to Cannabis happens in early and late June, respectively. Females can travel up to 150 km by wind. P. humuli normally infests hops, but can also infest Cannabis .
Melon aphid (A. gossypii)
First noted on marijuana in India, they are fairly small at 1-2 mm long. Color may vary from light yellow to dark green. The species has a broad host range but prefers warmer temperatures around 27°C and is a common vector of plant viruses.
An aphid with a fairly large host range that has been reported in California in 2018 . There is little information on this aphid, but it appears to be primarily asexual, with females producing parthenogenetically . I am unsure how aggressive of pests they are, but they have been reported from both indoor and outdoor grows.
Light green aphids- Consider M. persicae, P. humuli, and P. cannabis, though M. persicae is the largest of these species.
Dark green to black aphids- Points towards A. fabae or A. gossypii.
Distinguishing features between aphid species are usually based on the insect size, color/length of tubercles and cornicles or the size/presence of a midline knob. For instance, P. humuli and P. cannabis are smaller than M. persicae.
Signs and Symptoms of Aphid Infestation
Like mites, aphids tend to concenctrate on the undersides of leaves. Some species prefer lower leaves, such as Myzus persicae, and some species tend to feed on upper leaves Aphis fabae . Some species of aphids even feed on the buds themselves, such as Phorodon humuli and P. cannabis, though they also feed on leaves.
Early symptoms are hard to detect, because they tend to be light colored spots on the bottoms of leaves near veins. It is important to check all parts of the foliage when you scout. As infestations progess, you will begin to notice distortions and strange growth patterns on developing foliage or flowers. Very severe infestations can result in plant wilt and even death.
Honeydew is another symptom of aphid infestation. It can sometimes be seen as shiny spots on the leaves, but it tends to not be very apparent on Cannabis leaves.
Symptoms such as wilting or yellowing can sometimes be confused with symptoms caused by mites or whitelfies. Aphids have long wings at least twice the span of their body length, winged insects such as fungus gnats have shorter wings.
There is not a lot of information on control of the Cannabis aphid, as it is a fairly new pest in North America. Most recommendations will be based on control methods demonstrated to be effective on other aphid systems.
For indoor grows, sanitation and pest exclusion are very important. All air intake should be filtered or screened to exclude alatus (winged) females. Those entering the grow space should wash and not wear any clothing worn outside.
Genetic resistance introduced through selective breeding is a likely source of future control techniques, but right now there is not much knowledge as to which cultivars may be more resistant than others to aphid infestation.
Bacillus thuringiensis-based products may have minor efficacy against some aphid species, but generally speaking is not a good choice for controlling aphids or piercing-sucking pests in general.
Grandevo CG- Chromobacterium subtsugae
This product contains heat-killed bacteria as well as secreted metabolites. It works through various mechanisms of action
C. subtsugae is part of a recommended control program of the hop aphid (Phorodon humuli) along with B. bassiana and azadirachtin. Based on this, it has also been suggested as a control method for the very closely related Cannabis aphid (P. cannabis) .
Venerate CG- Burkholderia spp. strain A396
Like Grandevo, Venerate consists of heat-killed bacteria and secreted metabolites. Venerate is quite effective against M. persicae and is comparable with spirotetramat. It is more effective than C. subtsugae (i.e. Grandevo) for the green peach aphid . For more broad control of various aphid species and other insect pests, it is recommended to rotate Venerate and Grandevo on a 7 day cycle.
B. bassiana metabolites may have insecticidal activity towards aphids even without viable cells. B. bassiana culture filtrate (no living cells) is effective at controlling M. persicae , and even endophytic B. bassiana may negatively impact aphid populations on the colonized host plant . B. bassiana has been shown to be effective against other aphids such as Macrosiphum rosae (the rose aphid), and Aphis gossypii (melon aphid) [8, 9]. It may also be effective against the cannabis aphid, as it has been demonstrated to be effective against the closely related hop aphid (Phorodon humuli) . It is important that relative humidity is fairly high for B. bassiana to be effective. Ambient humidity levels should be at least 50% and the humidity at leaf surface would preferably be close to 80%.
Like B. bassiana, I. fumosorosea consists of live fungal spores and can spread throughout an insect population as the fungus sporulates on deceased insects. It has been demonstrated to be effective against some aphid species such as the brown citrus aphid , M. persicae . It has limited effects on beneficial insects or predators and can be used along with other microbial insecticides. I. fumosorosea effectiveness may be decreased by the use of horticultural oils .
Purchase product PFR-97 from Certis USA, not available on Amazon.
Aphid Predators and Parasitoids
Green Lacewing (Chrysoperla carnea)
This larvae of this species are effective at keeping aphid populations low and preventing outbreaks without chemical control. Only the larvae are predatory. Adults lay eggs proximal to aphid colonies. They are also effective control for other common pests such as whiteflies. They tend to be most effective in enclosed locations with environmental control (indoor and greenhouses). They can tolerate warm temperatures, but do not operate well in cold environments. The ideal temperature and RH level is 20 ° C to 31 ° C and 70% respectively. Maintenance release rates are usually around 1,000-2,000 eggs/acre and infestation response can be up to 10,000 eggs/acre.
I do not generally recommend ladybug releases for aphid control in Cannabis as a control method, unless in an enclosed environment with very high release rates. There needs to be a high population of aphids to keep them alive, and may require over 1,000 lady bugs per plant to actually get the issue under control. However, they can potentially work when released in high numbers in proper environments. For populations to persist, they may need to be misted. Most commercially available lady beetles are convergent lady beetles, as they are easily collected in large numbers from overwintering sites. It may take a couple weeks after release in optimal conditions for them to become fully active predators.
The parasitoids in the subfamily Aphidiinae are small wasps (0.08 – 0.12 inches). Some species are commercially available for aphid control. Adult wasps lay eggs inside of aphid hosts. The eggs hatch inside aphids and the larvae begin to feed on aphid tissue. In later development, the larvae kill the aphids and pupate inside the mummies. They exit the aphids as mature wasps.
Commercially available parasitoids are within the genera Aphidius (Hymenoptera: Braconidae) and Aphelinus (Hymenoptera: Braconidae). Like predators, parasitoids are most effective when aphid populations are low. However, in certain systems, it has been demonstrated that using both predators and parasitoids together is more effective than using one or the other [14. 15].
These parasitoids are effectively used in glasshouses and outdoors in temperate regions. Ideal conditions are 70-77°F and relatively high humidty (~80%). Temperatures above 85°F halt parasitoid development, but in 50°F temperatures, parasitoids may still develop. Humidity is relatively less important, and development may still occur even in low humidity. If the humidity is sufficient for They are sold as aphid mummies from which wasps emerge or as newly emerged adults. They are useful in the winter because they are not affected by short days, but aphids are generally not a problem in cold weather anyways. They are commonly used for control of M. persicae, but have also been observed to parasitize the more feared and recently introduced cannabis aphid (Phorodon cannabis).
This is another parasitoid that has been found to complete its life cycle using P. cannabis as the host. Life cycle and environmental conditions are similar to A. colemani
Aphid Midge (Aphidoletes aphidimyza)
These midges are cecidomyiid flies that are effective aphid predators. Like green lacewings, it is the larvae that feed on the aphids. It is common to release aphid midges along with green lacewings to increase predator diversity, as each predator may fill different niches better. Adults are small (2-3mm) and resemble mosquitos, while larvae look like small orange maggots that can also reach 2-3mm in size. Eggs are laid in foliage, and the larvae begin feeding on aphids on the plants after hatching, then they drop to the soil within a week to pupate. Larvae can eat up to 80 aphids but require at least 7 to complete the life cycle . These midges are less affected by azadirachtin than green lacewings. While green lacewings larvae are not killed by azadirachtin, it may interfere to some degree with the egg-laying of female lacewings. For this reason, it may be worth it to use midges either in place of or supplementary to release of lacewings if you are also using neem or azadirachtin . They will go in to diapause if the day length drops below 12 hours or temperatures are below 50 farenheit. They require soil and will not work in hydroponic systems .
Minute Pirate Bugs (Orius insidiosus)
Beware! These insects have a painful bite. They are good generalist predators that do well with controlling thrips and aphids. They reproduce fairly quickly, completing their life cycle in under a month. They do well at temperatures between 64-82°F, with 60% RH. It is recommended to release 100-2000 per acre depending on pest population.
Most beneficial insects are not available on Amazon. Check out Arbico Organics at arbico-organics.com
Azadirachtin and Neem Oil
Both neem oil and azadirachtin are effective in controlling various aphid species, but the efficacy of the compounds varies depending on the aphid species . It appears that insecticidal activity is not due to antifeedant activity, but instead inhibits reproduction and molting. Neem appears relatively harmless to many beneficial insects and can usually be used along with other strategies. When utilizing fungal biocontrol such as B. bassiana or I. fumosorosea, azadirachtin may be more compatible than complex mixtures such as neem oil.
The active ingredients of these soaps are potassium salts of fatty acids. The soap is able to disrupt the cuticle of the insect, essentially leading to death through desiccation. They are contact pesticides that have no residual pesticidal activity. They are also very broad spectrum pesticides and will kill beneficial insects as well as pests. Because of this, it is not recommended to use insecticidal soaps concurrently with beneficial insect release. However, they can be used as a useful knock down spray to reduce initial populations. Water sprays can be effective at reducing the populations just by knocking aphids of the plants, but using insecticidal soaps ensures that aphids knocked off do not find their ways back to the plants. Soaps can also be mixed with other insecticides including pyrethrins, horticultural oil, essential oils, triglycerides, or azadirachtin and neem products. While good as a part of an IPM program, neem oil and azadirachtin are not as effective against P. humuli as pyrethins, insecticidal soaps, or beneficial fungi . Based on anecdotal evidence, I believe neem is less effective on P. cannabis than on some other aphid species, but should still be used preventatively or tank mixed with other insecticides as a knock down treatment.
Pyrethrins are insecticides found in some chrysanthemum. Due to the source, they can be used in organic production. Usually, formulations have six chemicals found in the flowers. They are non-persistent and typically degrade within a few days when exposed to the environment and sun. Synthetic pesticides called pyrethroids have been made that mimic the activity of pyrethrins, do not degrade readily, but are not approved for use in Cannabis. Pyrethrins are much lower risk than pyrethroids for both consumer health and meeting pesticide testing limits. However, there have been reports of flower failing residue limit tests of pyrethrins and it is not fully understood why. Piperonyl butoxide (PBO) can be mixed with pyrethrins as a synergist. PBO works by inhibiting insect enzymes that may help break down pyrethrins. Pyrethrins are quite effective as an initial knockdown.
Pyrethrins can be mixed with insecticidal soaps or neem products such as azadirachtin
Horticultural Oils (Triglycerides, Mineral Oil, Essential Oils)
Most horticultural oils, particularly mineral oils and plant fat oils, work by suffocating insects and blocking respiration. Mineral oils are composed of higher alkanes derived from petroleum, and plant oils are mainly triglycerides. Triglycerides are composed of a glycerol molecule bound to three fatty acids which may be saturated or unsaturated. Most horticultural oils are most effective when mixed with a surfactant that helps spread the oils and may help deliver the oils more effectively to insect spiracles. Horticultural oils are also quite effective against fungi, particularly epidermal-limited pathogens such as powdery mildew.
Plant oils tend to be harsher on the plant epidermis than refined mineral oils and can more easily result in leaf burn. However, many plant oils contain other fat-soluble secondary metabolites that may have insecticidal activity. For instance, neem oil contains azadirachtin which inhibits insect reproduction and molting, but it also has triglycerides that work through suffocation. Other common plant oils used as insecticides include corn oil, soybean oil, canola oil, and other vegetable oils.
Various plant-derived essential oils can also have insecticidal activity. Essential oils can be derived in a variety of ways, most frequently by steam distillation followed by separation of the non aqueous layer or by solvent extraction. Steam distillation results in a high concentration of low molecular weight, nonpolar, volatile (easily becomes gaseous) chemicals including small terpenes, aldehydes, alcohols, and esters. Fatty acids are not easily recovered through distillation. Solvent extraction with nonpolar solvents such as hexane can also recover these chemicals, but during the solvent removal, many of the smaller volatiles may be lost and larger nonpolar molecules may remain. Essential oils also can be present in cold press extracts along with fatty acids.
One such essential oil product is Trifecta Crop Control. Along with essential oils, the product contains corn oil, citric acid, and an emulsifier. According to the Trifecta website, Trifecta works through more complex mechanisms than simply suffocation: clove oil contains eugenol: a terpene that causes cell lysis in fungal species, peppermint oil for reproductive inhibition, garlic oil for repellency and to treat systemic fungal infections, thyme oil for endocrine disruption in soft body pests, corn oil as a suffocant, and citric acid for chelating calcium in insect’s exoskeletons .
If you wish to avoid chemical control altogether, it is possible to achieve good levels of prevention using predator insects and parasitoids. It is generally recommended to use both parasitoids and insects within the generalist predator guild. For instance, weekly to biweekly release of green lacewings (5,000-10,000 eggs/acre) and parasitoid Aphidius species (500 aphid mummies/acre) can provide good control and release numbers can be increased up to tenfold at the first sign of infestation. Other insects that can be purchased and released are convergent lady beetles (Hippodamia convergens) and minute pirate bugs (Orius insidiosus).
Compared to many synthetic insecticides, neem and azadirachtin products tend to be relatively benign to beneficial insect populations . Undoubtedly, neem has negative effects on the total predator population, but the number of surviving predators relative to the number of surviving aphids is not significantly different, and the aphid parasitism rate by Aphidius can actually increase . However, it is important to mitigate the negative effects of neem on predators by using proper application timing. For instance, spraying neem 24 hours before predator release will allow for the knockdown effect of neem while preventing the predators from coming in to direct contact with the neem. If releasing predators on a biweekly basis, neem can also be sprayed at the same frequency but one day prior to release. If you alternate between release and neem on a weekly basis, or if you spray neem directly after predator release, you can negatively effect the predator populations .
In regards to entomopathogenic fungi, B. bassiana is fairly similar to neem in that it has little effect on beneficial insects if it is sprayed before release of beneficials. B. bassiana can colonize some host plant endophytically, and the predatory effects of green lacewings does not appear to be negatively affected by feeding on aphids that have been sprayed with B. bassiana spores or aphids that are feeding on plants that have been endophytically colonized by B. bassiana . However, spraying green lacewings directly with B. bassiana spore suspensions can negatively affect lacewing populations . B. bassiana may be incompatible with lady beetles as well. Therefore, it is a good idea to time applications to minimize off target effects. One study that looked at the timing of B. bassiana in relation to release of parasitoid A. colemani demonstrated that ideal B. bassiana spray timing is approximately 3 days after M. persicae parasitism by A. colemani. There is speculation that immature parasitoids may produce fungistatic or fungicidal metabolites to prevent the entomopathogenic fungi from killing the hosts. Furthermore, aphids colonized by B. bassiana prior to oviposition by parasitoids appear to reduce the success of parasitoids .
B. bassiana can infect minute soldier bugs quite well, as well as convergent lady beetles, but it can be feasibly used with lacewings and parasitoids by using a timing such as the following:
- At first sign of aphid infestation, do a knockdown spray with a product such as pyrethrins (recommended for Cannabis aphid), or pyrethrins mixed with insecticidal soap.
- When dry, release parasitoid Aphidius wasps (up to 4,000/acre depending on population). These are high release rates and in most cases, fewer should be sufficient.
- 4-5 days after wasp release (if using adults), spray with B. bassiana product such as Botanigard
- After dry, release green lacewings up to 10,000 per acre, aphid midges, and up to 5,000 minute pirate bugs/acre, depending on populations. This is only in severe infestations, even half of these numbers should be sufficient for more infestations.
- Do weekly release of predators and parasitoids at lower levels of 400 wasps, 200 minute pirate bugs, 1,000 lacewings, and 1,000 aphid midges per acre to suppress levels. This is a mixture of 4 good beneficials, but you can cut back to parasitoid wasps and one or two of the predators rather than using all of them.
- Use a rotation of Grandevo, Venerate, and Neem Oil. A rotation of any 2 products in this list could also be used. Both Grandevo and Venerate do not affect beneficial insects, and neem oil has low toxicity that can be mitigated with proper application timing.
If you do not want to use pyrethrins as a knockdown, I would recommend using a product such as Trifecta that contians an insecticidal soap, citric acid, essential oils, and corn oil. Please do not use Spinosad or synthetic pesticides in commercial operations as they are not labeled for use in Cannabis and are tested for.
Isaria fumosorosea may be even more effective than B. bassiana, at least in the case of M. persicae , and is of similar effectiveness as B. bassiana on the hop aphid (Phorodon humuli), closely related to P. cannabis . For all applications of B. bassiana, it may be worth either trying I. fumosorosea or rotating between application of B. bassiana and I. fumosorosea. For both of these fungi, high humidity is ideal and ambiet humidity should not be below 50%, with higher humidities being more effective.
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- González-Mas, N., Cuenca-Medina, M., Gutiérrez-Sánchez, F., & Quesada-Moraga, E. (2019). Bottom-up effects of endophytic Beauveria bassiana on multitrophic interactions between the cotton aphid, Aphis gossypii, and its natural enemies in melon. Journal of Pest Science, 92(3), 1271–1281. https://doi.org/10.1007/s10340-019-01098-5
- Portilla, M., Snodgrass, G., & Luttrell, R. (2017). Lethal and sub-lethal effects of Beauveria bassiana (Cordycipitaceae) strain NI8 on Chrysoperla rufilabris (Neuroptera: Chrysopidae). Florida Entomologist, 100(3), 627–633.
- Emami, F., Alichi, M., & Minaei, K. (2013). Interaction between the entomopathogenic fungus, Beauveria bassiana (Ascomycota: Hypocreales) and the parasitoid wasp, Aphidius colemani Viereck (Hymenoptera: Braconidae). Journal of Entomological and Acarological Research, 45(1), e4. https://doi.org/10.4081/jear.2013.e4
- Meng, H., Tian, J., Fu, S., Diao, H., & Ma, R. (2014). Pathogenicity of Isaria fumosorosea and Beauveria bassiana against the green peach aphid, Myzus persicae. Acta Phytophylacica Sinica, 41(6), 717–722.
- Dorschner, K. W., Feng, M.-G., & Baird, C. R. (1991). Virulence of an Aphid-Derived Isolate of Beauveria bassiana (Fungi: Hyphomycetes) to the Hop Aphid, Phorodon humuli (Homoptera: Aphididae) . Environmental Entomology, 20(2), 690–693. https://doi.org/10.1093/ee/20.2.690
- Mohammed, A. A., Kadhim, J. H., & Kamaluddin, Z. N. A. (2018). Selection of highly virulent entomopathogenic fungal isolates to control the greenhouse aphid species in Iraq. Egyptian Journal of Biological Pest Control, 28(1), 71. https://doi.org/10.1186/s41938-018-0079-3
- Aphidoletes aphidimyza. (n.d.). Retrieved May 3, 2020, from https://biocontrol.entomology.cornell.edu/predators/Aphidoletes.php
- Azadirachtin & Neem Oil. (n.d.). Retrieved May 3, 2020, from https://www.growertalks.com/Article/?articleid=23465
- Aphidoletes for Aphid Control. (n.d.). Retrieved May 3, 2020, from https://greenmethods.com/aphidoletes/
- Insects/Mites that Feed on Hemp – Fluid Feeders: Cannabis Aphid. (n.d.). Retrieved May 5, 2020, from https://webdoc.agsci.colostate.edu/hempinsects/PDFs/Cannabis aphid August 2018 revision.pdf
- SYSTEMATIC TREATMENT OF APHID GENERA. (n.d.). Retrieved May 5, 2020, from http://www.aphidsonworldsplants.info/d_APHIDS_A.htm#Abstrusomyzus
Mites are probably the most feared arthropod pests of Cannabis. There are three types of mites that are of particular concern for Cannabis growers: broad mites, russet mites, and spider mites. Spider mites are the most widespread and pervasive issue, but broad mites and russet mites are harder to deal with and can result in devastating losses.
What is a mite?
A mite is an arachnid in the subclass Acari (Phylum Euarthropoda, Class Arachnida). Acari also contains ticks, but ticks are not included in the mite classification. Mites are quite small and the majority are under 1mm in length.
Not all mites are pests
Mites span a large range of lifestyles. Many mites are integral parts of the soil food web and act as decomposers of organic matter. In forest soils, up to 400,000 mites can be found in a square meter of soil . Some mites act as predators or parasites and may feed on other arthropods or fungi. Some of these mites are used in biological methods for controlling pest populations. A small number of mites actually pose a substantial risk to plants.
Spider mites are within the family Tetranychidae which contains about 1,200 known species . Spider mites feed on plant cells, and they spin webs over plants for protection and transportation.
What Species of Spider Mites Infest Cannabis?
Reportedly, two species of spider mites cause the most damage on Cannabis plants, the two-spotted spider mite Tetranychus urticae and the carmine spider mite T. cinnabarinus . Both species are very similar in their life cycles and appearance, but T. cinnabarinus thrives in higher temperatures than T. urticae and vice versa for cooler temperatures . Generally speaking, T. urticae only causes significant damage in semi-tropical regions with temperatures above 34°C .
T. urticae Identification, Symptoms, and Signs
Adult spider mite identification
- Adults are very small (about 0.4 mm in length), and males are slightly smaller than females. They are straw-colored to green and have large dark spots on either side of the abdomen
Other signs of spider mite infestation (not including visualization of adults)
- Females lay eggs on the undersides of leaves. Eggs are large in relation to the female (around 0.14 mm) and can be used as indicators of spider mite infestation. Frass (exrement) can also accumulate on leaf tissue.
- Adults weave extensive webs on the plants that are commonly the first indicators that growers notice on their plants. This webbing can help protect them from predators and can facilitate movement.
Plant tissue damage and symptoms from feeding
Spider mites feed on plant cells with stylet-like mouthparts, ‘sucking’ out cell contents and causing cell death.
- Stippling is a leaf pattern of numerous tiny necrotic specks (aka ‘flecking’). Stippling is usually concentrated on the undersides of leaves, but should also be apparent on the tops of leaves. Feeding on one side of the leaf can cause stippling on the opposite side as well.
Stippling begins with a small amount of greyish specks
The greyish stippling continues to spread across the leaf, and the leaves start to become yellowish, copper-brown, and in heavy infestations, can even die.
Spider Mite Life Cycle
The life cycle of spider mites is fairly straighforward. After hatching from an egg, the spider mite goes through three instars and molting cycles before reaching adulthood. In optimal conditions (high heat (81°F-86°F) and low humidity), the entire life cycle can be completed in just over a week , whereas lower temperatures (60°F-64°F) can slow the life cycle turnover by up to three times (20+ days) .
Effects on yield and quality
You may wonder why it is a big deal for plants to be infested.
- Appearance- Stippling and bronzing can occur on both sugar leaves and flower calyxes, leading to lower bag appeal. Webbing is also hard to remove and would likely be present in your final product.
- High Stress and Lower Productivity- As spider mites kill leaf cells and disrupt the plant cuticle, plants begin to lose control over local transpiration rates. Not only will this lead to lower growth rates and yield, it can lead to symptoms that may commonly appear to be severe drought such as leaves becoming dry and brittle. The numerous necrotric areas result in a lower total photosynthetic rate due to loss of photosynthetically active cells. Yield losses can reach 50% in heavily infested fields .
Favorable Environmental Conditions
Optimal environmental conditions for spider mites are high temperatures (30°C ideal) and low humidity . In Farenheit, high 80s is ideal, but temperatures above 90°F may prove inhibitory. I can not find sources on exact RH levels that benefit or inhibit the spider mites, but all publications that I have come across say that low humidity favors spider mites and high humidity inhibits spider mites. However, the humidity levels in most grow rooms that follow vapor pressure deficit guidelines are not excessively high (40%-65% depending on leave temperatures and life cycle stage) and I would certainly recommend following VPD guidelines over running excessively high RH due to prevent issues with bud rot.
Hemp Russet Mite (Aculops cannabicola)
Russet mites are more inconspicuous that spider mites. In very low populations, they don’t cause visible symptoms . In higher populations, symptoms begin to show on plants. In 1965, they were reported in central Europe, and in Kansas in 1971. They are now fairly ubiquitous in the United States and has been a big problem on the west coast for a few years now.
- *russet mites suppress JA-related plant defenses with compounds introduced to the plant through feeding *
- A common symptom on Cannabis is a pervasive and conspicuous upward curling of leaf edges. However, this does not always happen depending on cultivar, environment, and infestation severity .
- Leaves can become glossy and blistered, almost appearing wet. Interveinal regions can turn a crimson-like color (russeting). Some of these symptoms may be misdiagnosed as Hemp Streak Virus (HSV).
- As symptoms progress, leaves become brittle, dry, and more russeted, eventually leading to leaf death. Leaves may break off at the petiole.
- Russet mites can also infest plant stems and cause visible masses, especially near cola tops. This can result in wilting and even death of plant tops. They can even feed on flower petioles, leaving female flowers sterile .
Identifying Adult Russet Mites
- Russet mites are less than half the size of spider mites (0.2 mm long) and may need magnification to visualize . 60x magnification is ideal.
- Unlike many arachnids, Russet mites have 2 pairs of legs instead of 4.
- They are soft-bodied, quite pale, and have 2 segments, the gnathosoma (mouth parts) and the idiosoma (the rest of the body).
- One publication says that the life cycle of the hemp russet mite at 27°C and 70% RH takes 30 days . However, there is no study cited for this claim, and most other resources have vastly shorter estimations. This estimation is likely quite off.
- Most eriophyid mite species share a similar life cycle that includes the egg, two nymphal instars, and the adult . Almost all eriophyid mites complete their life cycle in approximately 7 days .
- Females can overwinter in plant debris, especially in plant stems and petiole bases.
Favorable Environmental Conditions
Generally speaking, russet mites can proliferate at lower temperatures than spider mites. In a study on the tomato russet mite, the ideal environmental conditions for proliferation were found to be around 80°F and 30% RH . In regards to higher humidity and lower temperatures, another study found that they were able to reproduce in conditions of 15-24 degrees C (59-75 degrees F) and 70-80% relative humidity .
Did Cal Trans Cause a Russet Mite Outbreak on the West Coast?!
I want to address this topic because I see a lot of people saying that the Russet Mite outbreak in CA was due to Cal Trans releasing mites for control of thistles. This idea is based on the .pdf file Enhanced Biological Control of Yellow Starthistle and Tumbleweed (Russian Thistle) by the Department of Transportation , which discussed the use of a blister mite, Aceria salsolae as a control agent for invasive Russian thistle. First of all, this mite is not the same as the Cannabis russet mite, it is specific to the Russian thistle. Second of all, the release of these mites was never approved . Hemp russet mites are spreading more rapidly due to higher temperatures, and can be quickly spread in an industry such as the Cannabis industry that does not have clean stock programs or good practices related to seed and clone disinfestation.
Broad Mites (Polyphagotarsonemus latus)
Like russet mites, broad mites adults are around 0.2 mm in length, and the eggs are under 0.1mm. You will not be able to see them with the naked eye. The best magnification to see them with is about 60x. Unlike russet mites, they tend to reside only in new leaves and don’t feed on fully opened leaves. However, they tend to be quite pale, almost transparent, but slightly greenish-goldish. They have two pairs of front legs, a pair in the middle of the abdomen, and a whispy pair of leg-like appendages in the back. Broad mites can walk short distances, but can also be wind-borne or attract and hitch rides on other insects including aphids and whiteflies . You will often only find broad mites in new apical leaves because they can hide easier and feed on young tissue that has not developed as many defenses .
The life cycle is basically the same as the russet mite of which there is a diagram in that section. There is an egg, a larva, a nymph, and an adult.
Each female can lay 40-50 eggs. It takes about 2 days for eggs to hatch, and
the larval and pupal development takes 2-3 days. Males hatch first and carry female pupae to young tissues. Females mate right away, but if they do not mate, they will still lay eggs in an asexual manner to all male mites. The male mites can then may mate with the female for sexual production to occur (same genes but recombination occurs). In optimal conditions, the life cycle takes about a week. They generally reproduce and reside in new developing leaves and are rarely found on fully opened leaves .
The symptoms can be indistinguishable from russet mite damage. In my opinion, broad mites tend to cause the leaves to curl down more than russets but the very edges of the leaves tend to ‘taco’ in both cases. Leaves tend to be a bit more deformed because broad mites secrete a toxin in their saliva that affects newly developing leaves .
Broad mites thrive in warm temperatures, but indoors they can be active year round . Unlike russet mites and spider mites, broad mites thrive in high humidity (80% is ideal). In regards to temperature, the optimum temperature for broad mite reproduction is 30°C (86°F) with close to 100% survival from egg and about a 3.5 day development period to adulthood. Over 90°F begins to be inhibitory (20-30% survival rate from egg to adulthood), although development time is still fast at this temperature (4-5 days). Lower temperatures (around 60°F have survival rates around 30% but have long development times of almost 2 weeks) . Humidity under 60% RH will help control broad mites.n Unlike russet mites, broad mites don’t overwinter in plant stems or the soil and may not survive in cold regions over the winter.
How To Prevent and Treat Mite Infestations
The first concept I want to get across is that an ounce of prevention is worth a pound of cure. If you are in an area that has problems with any of these mites, you should have an IPM program in place to prevent them from infesting your plant. There are a broad range of ways to do this, including pesticide sprays, or beneficial fungi and predator mites.
This method has a lot of benefits, especially preventatively or in flower when you are avoiding sprays.
This mite thrives best in conditions similar to the broad mite. It can stand a wide range of temepratures from 20°C to 30°C. It prefers humid conditions of 70%-80% RH, but can still complete its life cycle well at 40% RH if it is kept cooler . However, keeping RH above 50% is highly recommended. It is a good choice for IPM because it is very aggressive on thrips, but can also feed on stages of russet mites, broad mites, and spider mites. They need pests to feed on, so if they control your problem, they will begin to decline in population. There are slow release packets available that will release mites from a packet with a bran substrate for the bran mites that act as a temporary food source. A packet works well for about a month.
Satchets:1,000 Live Neoseiulus Amblyseius Cucumeris – Guaranteed Live Delivery! Nature’s Good Guys 10 X 1,000 Live Neoseiulus Amblyseius Cucumeris – Guaranteed Live Delivery! Nature’s Good Guys 25 X 1,000 Live Neoseiulus Amblyseius Cucumeris – Guaranteed Live Delivery!
Mites on carrier for sprinkling on plants:50,000 Live Neoseiulus Amblyseius Cucumeris – Guaranteed Live Delivery!
This mite primarily feeds on spider mite populations and can also feed on microscopic mites such as russets, broad mites, and cyclamen mites. They thrive well in any Cannabis growing environment. They can tolerate wide ranges of heat (10°C-33°C) and humidity, and can even survive short periods of under freezing temperatures. They do well in humidity levels of 40%-80%, which falls within the common ranges used to follow VPD recommendations in indoor grows. They do prefer humidity on the higher ends . They do especially well in environments that get hot and low humidity. Satchets can be ordered for this mite as a preventative for various mites or can be applied at first sign of spider mite infestation.
Mites on Carrier:Nature’s Good Guys 2,000 Live Adult Predatory Mites Packed in a 16 oz Container – P. persimilis a Predatory Mite Species for Spider Mite Control – Ships Next Business Day! 5,000 Live Adult Predatory Mites – Neoseiulus (Amblyseius) Californicus a Predatory Mite Specie for Spider Mite Control – Ships Next Business Day!l
This is probably one of the best choices for controlling broad mites, russet mites, and cyclamen mites. It is a fairly broad predator and also feeds on spider mites, thrips, and whiteflies. At 60% RH, mites develop between 18°C and 36°C, though for good activity the daytime temperature should be above 22°C . Optimum temperatures are around 25-28 °C. These mites can be used at higher temperatures than A. cucumeris provided RH remains high on leaf surface. Unlike A. californicus, they have poor tolerance to freezing temperatures. A. swirskii prefers high humidity and does not do as well as A. californicus with low humidity.
Satchets:Nature’s Good Guys Amblyseius swirskii 25 Sachets with 250 Mites Amblyseius Swirskii (25,000 in Hanging Sachets)
This is one of the best choices for targeting mites specifically. They prefer feeding on mites over thrip larvae and feed on spider mites, cyclamen mites, broad mites, and russet mites. It has an extremely large active humidity range of 6-40ºC. These mites are also very good preventatives for pest mites because they can survive on fungi and honedew as well. A slow release satchet will provide about a month of protection and maybe more depending on if the population can persist on other food sources in your environment. They can survive in some lower humidities (at 20ºC, when the mites are actively feeding, there is about a 50% survival rate to adulthood at 50% RH). These mites should not be used in grow areas with humidity below 50% RH.
Order from mite supplier- not available on Amazon
This is another mite-targeting species that feeds on all the types of mites discussed in this article. These mites love hot temperatures (27-43ºC) and may go in to hibernation in daytime temperatures under 27ºC. It can tolerate high humidity, but is most effective in RH between 30 and 60% RH . However, it can tolerate even lower humidities. it is a great choice for particularly dry glasshouses that may have below 40% RH. However, it does not do well in cold temperatures.
Order from mite supplier- not available on Amazon
This predatory mite is specifically useful for the two spotted spider mite and specifically feeds on web-weaving mites, but not so much against the microscopic foliage mites. The are about a half millimeter long and are striking orange. It controls the two-spotted spider mite at temperatures from 15°C to 27°C with humidity from 60% to 90%. Ideal conditions for control are obtained at 27°C (around 80°F) and 60%–85% R.H. At lower temperatures of 21°C (70°F), control can be obtained at humidity down to 40%, but at higher temperatures of 27°C, 40% RH is too low for effective control . Time to adulthood can range from 25 days at 15°C to 5 days at 30°C. At ideal temperatures and RH levels, generation times are typically around a week .
Mites on CarrierNature’s Good Guys 2,000 Live Adult Predatory Mites Packed in a 16 oz Container – P. persimilis a Predatory Mite Species for Spider Mite Control – Ships Next Business Day! Nature’s Good Guys 20,000 Live Adult Predatory Mites – P. persimilis a Predatory Mite Species for Spider Mite Control – Ships Next Business Day! Nature’s Good Guys 10,000 Live Adult Predatory Mites – P. persimilis a Predatory Mite Species for Spider Mite Control – Ships Next Business Day!
Entomopathogenic Fungi and Bacteria
Some fungi have adapted to infect, colonize, and consume living insects. Entomopathogenic means disease-causing on insects. Mites are not insects, but some of these fungi also actively kill and colonize mites. I believe these products are quite useful in the battle against aggressive microscopic foliar mites.
Beauveria bassiana strain GHA
B. bassiana spores germinate and infect mites through their cuticle. Infection is usually lethal within a few days.
- One study found up to 49.5% mortality of two-spotted spider mites within 4 days and did not affect any predatory mites tested . It may negatively affect some predatory mite species such as A. swirskii depending on spray timing and development stages but not affect other species such as A. cucumeris [46, 47]. In tomato russet mites, B. bassiana was particularly effective at reducing Russet mites as compared to other fungi, azadirachtin, and pyrethrins . In broad mites, B. bassiana can cause over 80% mortality, and was found to be more effective than other tested fungi .
- The ideal environmental condition for fungal pathogenicity is 100% RH and 25-30ºC, but some isolates grow best as low as 20ºC . Spores can become inactive above 30ºC and have very inhibited growth at 15ºC. However, no grower keeps their grow at this humidity level because of other pests and diseases that can be caused at such high temperatures. The fungus is still active at humidity levels down to 30% RH, albeit at a lower rate . Leaf humidity should be at least 60% RH to have a significant effect and the higher the humidity, the more active the fungus.
- Products include Mycotrol, Botanigard, and Velifer for B. bassiana only sprays. Botanigard Maxx also has pyrethrins in it. According to the manufacturers of Botanigard Maxx, it can be used up to 3 weeks before harvest (in Cannabis).
- B. bassiana is also useful for controlling vectors of mites such as aphids and whiteflies.
- B. bassiana may affect beneficial mites some, but in general has been found to be compatible with Amblyseius species.
In a well-controlled environment, this entomopathogenic fungus can effectively kill over 90% of two-spotted spider mite, but may lose effectiveness at high temperatures . It is generally used more for aphids and whiteflies, but may also help with control of various mites. I would not use this as the only biological control in an IPM program, but it may be useful in a rotation with other fungal sprays such as B. bassiana. I have not found studies on the efficacy on broad mites or russet mites, but it is at least demonstrated to be useful on spider mites in particular environmental conditions. Many anecdotes from growers say this is effective on russet mites and broad mites as well and use this in rotation with other biologicals including B. bassiana and C. subtsugae .
- Recommended product is PFR-97 20% WDG
- Should not be applied within 5 days of fungicide
- Ideal environment: 80% RH or higher for 8-10 hours when air movement is low and temperatures are 70-90°F . The humdity of the leaf surface is usually much higher than that of ambient air, and so an ambient humidity from 40%-50% may be sufficient if air movement is low. It is best applied in the early evening or morning where light is low, temperatures are lower, and humidity begins to rise outdoors.
Buy from your pesticide provider or from Certis USA. Not available on Amazon
There are some growers I have talked to who have not had success with PFR-97 in field conditions. In fact, the earlier study cited in regards to the effectiveness of PFR-97 does state that in greenhouse, the product did not work to control the spider mites.
Burkholderia spp. strain A396
Burkholderia is a genus of gram negative, obligate aerobic proteobacteria. The genus contains various pathogenic species on a wide range of organisms including plants an animals. This particular strain of a Bulkholderia species is pathogenic on arthropods and in Cannabis is particularly useful against aphids, broad mites, russet mites (can also be used for spider mites), and thrips. It was described as a novel Bulkholderia species isolated from the soil known as Burkholderia rinojensis . It contains various bacterial metabolites, enzymes, and heatkilled bacteria that kill insects using multiple mechanisms of action. In spider mites, over a 90% mortality rate was observed within 3 days of application .
- Venerate CG by Marrone Bioinnovations is a Cannabis-driven product that has a MRL tolerance exemption and a 0 day preharvest interval. There is no limit on application numbers. This is a highly recommended product for both knockdown and prevention of spider mites.
- Avoid using at the same time as beneficial mites. Give at least 48 hrs between spraying Venerate and releasing mites or beneficial insects.
- Use with a spreader-sticker such as Oroboost to maximize effectiveness.
Just a heads up: you may be able to buy smaller quantities directly from Marrone Bioinnovations. I will link what is available on Amazon, which is a gallon size.
Similar to Bulkholderia, C. subtsugae products are heat-killed bacteria and the fermentation media they grew in . The product contains various metabolites and enzymes produced by the bacteria resulting in a broad range of activity mechanisms. It only affects leaf-feeding insects and so is good to use in conjunction with predator mites.
- Recommended product: Grandevo from Marrone Bioinnovations
- It does not act as quickly as some other products, and may require 2-3 weekly application before a decline in population is noticed.
- One study found C. subtsugae to be ineffective on broad mites . It has been demonstrated to be lethal to the two-spotted spider mite but takes multiple days to begin to kill the pests .
- It does not appear to be very effective as a standalone product against russet and broad mites, but can help slow them down and can contibute to a good IPM program for russet mite control because it can be used at lower humidities and in combination with other sprays.
- A spreader-sticker such as Oroboost should be used with contact insecticides such as Grandevo, Venerate, or pyrethrins.
Sulfur is a broad spectrum fungicide, insecticide, and miticide. Because of this, it will kill pests on plant surfaces, but it will also kill beneficial insects and mites. Sulfur can be rough on the epidermis of the plants, and application might chew up the leaves a little bit, but Cannabis can usually handle it well.
- If you are growing indoors, do not burn elemental sulfur as a control because as it is burned, elemental sulfur becomes sulfur dioxide which can form sulfuric acid with moist leaves, resulting in more severe leaf damage.
- Sulfur should not be applied in flower, it will persist on the buds and undoubtedly affect the flavor of the buds. However, it can be applied up until flower.
- Sulfur dust is best applied through a sprayer as a wettable powder. Sulfur should be applied as a wettable powder at about 3 tbsp sulfur/gallon of water. You should test it on a plant before committing to spraying your entire field to make sure that they can handle it.
- You can also use products such as Safer Brand Garden 3-in-1 spray that contains both sulfur and potassium salts of fatty acids, which also have miticidal activity.
- I recommend using this as a knock-down if any symptoms are observed and should be followed up with applications of predatory mites and/or fungi. However, I personally would recommend other products as a population knock-down such as pyrethrins where they are allowed.
Neem and Azadirachtin
Azadirachtin is a compound found in neem oil. It is a very good general insecticide and miticide that is found in many IPM programs. It becomes systemic in plants, may act as an antifeedant and may disrupt the maturation process of arthropods that ingest it. In spider mites, it showed a 50% reduction in mites that made it to adulthood . I never recommend using this product in flower because there is some evidence of toxicity and allergy in humans, especially when consumed in large doses . However, this is a very good product to use as a preventative and works well in most IPM programs.
- Azadirachtin such as AzaMax can be used by itself, as neem oil, or in mixtures. Azera combines azadirachtin with pyrethrins which would be a good knock-down spray to use at first signs of infestation.
- Ortho tree & shrub fruit tree spray combines neem oil with pyrethrins and piperonyl butoxide. I like to spray neem products at least biweekly during veg to help prevent a broad range of pests including insects, mites, and fungi.
Phosphorous acid, or phosphite, is a very useful tool in the garden. First of all, it is also registered as a fertilizer under different brands and can be used as a foliar spray for feeding phosphorous. It can be in salt form as potassium phosphite which will also provide potassium and may be used as a ‘bloom booster’ in early flower. Recent studies have shown phosphite is not a good fertilizer choice for when phosphate is deficient, however.
- Phosphorous acid, aside from being a fertilizer, is also a systemic plant resistance activator that helps protect against various fungi including powdery mildew and fusarium.
- There is a product called MITE-PHITE ZM registered for use in Oregon, but I have not seen wide adoption of this for mite control.
- At least in the case of spider mites, phosphite may greatly reduce the reproductive rate , but it may also be helpful with russet, broad, and cyclamen mites.
- This is a great general addition to a Cannabis IPM program where approved. I like to spray this at least one time around the time of flipping to flower for both the nutrient content (I use potassium phosphite) and the pest resistance.
Mite Phite is not available on Amazon, but the following products have the same active ingredients (these are registered as fungicides):
Pyrethrins are insecticidal compounds found in the chrysanthemum flower. Please note that pyrethrins are different from pyrethroids. Pyrethrins are extracted directly from the chrysanthemum flower and degrade quickly (within a couple of days) whereas pyrethroids are synthetically produced and are designed to persist for longer periods of time. Pyrethrins are generally a mix of 6 insecticidal compounds. They are neurotoxic to insects but have low toxicity to mammals. Pyrethrins break down fairly quickly on the plant surface. after 5 days, only 3% remains.
- I recommend buying a pyrethrin product with other additives as well for diversification. However, pyrethrin only products are also effective and may be useful as tank mixes with other insecticides.
- PyGanic is a great pyrethrin-only product. Finding a product that also contains piperonyl butoxide will improve the effectiveness of the pyrethrins. One such product is Garden Safe Houseplant & Garden Insect Killer
- For instance, Azera insecticide combines pyrethrins with azadirachtin (another insecticide derived from neem oil)
- Mighty combines pyrethrins with canola oil. Canola oil is composed mostly of triglycerides which have miticidal activity.
- Ortho tree & shrub fruit tree spray is a great product that combines pyrethrins, neem oil extract, and piperonyl butoxide. Piperonly butoxide acts to inhibit enzymes that may help insects and mites detoxify the pyrethrins.
- Safer Brand Pyrethrin & Insecticidal Soap concentrate contains pyrethrins as well as potassium salts of fatty acids.
- Botanigard Maxx combines pyrethrins with a beneficial fungus, Beauveria bassiana, and this is a great product to use as an initial knockdown that will have some long-lasting effects if the fungi become established in the mite population. Coupled with predatory mite release, this product seems like a good choice.
Mighty is not found on Amazon. Purchase from NPK Industries
Horticultural oils: Mineral Oil, Cottonseed Oil, Soybean Oil, Neem Oil
These oils are hydrophobic liquids that act by suffocating arthropod pests by blocking spiracles. I do not use oils during flower at all because they may affect the flavor or microbial counts on your buds. As I mentioned earlier, I use neem oil during vegetative growth in my IPM program for insects, mites, and powdery mildew issues. Furthermore, if I spray potassium carbonate for powdery mildew control, I also add cottonseed oil if the plant is still in vegetative growth just to add some insecticidal activity to the spray as well. Personally, I recommend sticking to neem oil and cottonseed oil for horticultural oils. Neem oil sprays are also not toxic to P. persimilis, but may be mildly toxic to some Amblyseius species [38, 39]. It is likely the oil itself, not the azadirachtin that is causing issues in Amblyseius, one study found little effect of Azadirachtin on an Amblyseius mite . However, neem oil and mineral oil are both considered compatible with Amblyseius species tested due to a positive population increase , but heavy oil applications may slow down population growth.
Essential Oils are made by distilling plant materials and collecting volatile compounds that come out of these plants. These are rich in terpenes and other volatile compounds.
- Garlic oil
- Rosemary oil
- Rosemary oil
- Thyme oil
One product, Biomite, was found to significantly reduce spider mite adults and eggs and also did not negatively affect predator mites . Biomite contains Citronella oil, Farnesol, Geraniol, and Nerolidol. This is one of the most cost effective essential oil sprays.
Biomite is not available on Amazon, order from Brandt or Arbico Organics
Some of the best broad spectrum mixtures that contain all approved materials in California include Ed Rosenthal’s Zero Tolerance Herbal Pesticide and Dr. Earth Final Stop Yard & Garden Insect Killer (which also contains a horticultural sesame oil). Bonide mite contains a more insecticidal horticultural oil, cottonseed.
For spider mites, I have heard reports that rosemary oil is the most effective of plant essential oils. For microscopic mites such as broad mites and russet mites, clove oil has been reported as one of the most effective contact killers [personal correspondences].
One of my favorite sprays, Lost Coast Plant Therapy, uses a combination of essential oils, horticultural oil, citric acid, and isopropyl alcohol to kill fungi and suffocate/desiccate insects and mites.
Trifecta crop control is a good product that has a large array of essential oils, corn oil, and citric acid. It is good for contact killing and leaves essential oils that may act as repellants and antifeedants.
Not currently available on Amazon
Citric acid is a great product to include in any grow. It can act as a fungicide by adjusting leaf pH. It is also a contact miticide and may have over 90% mortality of spider mites . When combined with isopropyl alcohol, it is particularly effective at dessicating mites. However, it is not specific and should not be used with beneficial mites and it does not have activity after the spray.
- For just citric acid, I recommend using Flying Skull’s product Nuke Em.
- For mixes of citric acid, isopropyl alcohol, and oils, I recommend either Green Cleaner (Central Coast Garden Products) or Plant Therapy (Lost Coast).
- Trifecta crop control uses citric acid in conjunction with essential oils, corn oil, and soap.
Insecticidal Soaps (Potassium Salts of Fatty Acids)
Insecticidal soaps work by disrupting cell membranes and disrupting cuticles. They kill on contact and degrade rapidly. They do not leave an insecticidally active residue after drying. Therefore, I would not rely on this as your main control method. It may be useful for an initial knockdown or in conjunction with other chemicals. Frequent insecticidal soap applications can be effective for control of spider mites . Personally, I don’t like using insecticidal soap in flower.
- Any brand will work fine; I recommend Safer Brand insecticidal soap 3 in 1 because it also contains sulfur which increases the efficacy and protects the plant for longer, or Safer Brand Pyrethrin & Insecticidal Soap concentrate.
Example IPM Program:
Simple, Cheap, Preventative:
- Veg: Weekly application of neem oil product or sulfur product or essential oil product. If using neem or essential oils, can be combined with predatory mite release. Azadirachtin products may also replace neem oil or be mixed with neem/essential oils.
- Flower: Weekly application of citric acid product such as Nuke Em or Plant Therapy or monthly release of predatory mites
- Veg: Weekly application of neem oil or essential oil product (such as Trifecta or Zero Tolerance). These may be rotated on a biweekly basis and may be mixed with an azadirachtin product such as Azamax.
- Monthly release of predatory mites
- You can include a citric acid product in rotation, but only before mite release and it must be allowed to dry. You can increase time in between neem oil sprays under low pressure up to 14 days.
- Day 1: spray citric acid product such as plant therapy
- Day 2: release predatory mites
- Day 8: Spray Neem or essential oils (avoid purified azadirachtin)
- Day 15: Spray neem/essential oils or phosphorous acid
- Day 22: Spray neem/essential oils
- Day 29: Spray citric acid product
- Day 30: Release predatory mites
- Flip to Flower: Potassium salts of phosphorous acid spray
- Flower: Weekly application citric acid product such as Plant Therapy or Nuke Em or rotation of citric acid product and phosphorous acid product.
- Example of using predators, citric acid, and phosphorous acid:
- Day 1: spray citric acid product. Allow to dry.
- Day 2: release predatory mites
- Day 15: Spray phosphorous acid product (time between application may be decreased to 7-10 days if pressure is high)
- Day 29: spray citric acid product. Allow to dry.
- Day 30: Release predatory mites
- Example of using predators, citric acid, and phosphorous acid:
The following program is aggressive for if an infestation occurs. It requires a diverse range of products but will be highly effective assuming proper environmental conditions for biologicals:
Day 1: Citric acid product knockdown (optional but recommended)
Nuke Em: citric acid, also contains insecticidal soap
Plant Therapy: citric acid, isopropyl alcohol, oils
Day 2 or 3 (when dry) second knockdown: Veg options: All are good choices
- Azera (pyrethrins+azadirachtin)
- Mighty (pyrethrins+ canola oil)
- Ortho tree & shrub fruit tree spray (pyrethrins+neem oil+piperonyl butoxide)
- Garden safe 3-in-1 (sulfur+insecticidal soaps)
- Safer brand pyrethrins & insecticidal soap concentrate
- Pyganic gardening (pyrethrins)
- Bonide tomato and vegetable conc. (sulfur+pyrethrins)
- Sulfur may be tank mixed with pyrethrins & insecticidal soaps,
- Botanigard Maxx (Pyrethrins & B. bassiana)
Flower Options– Only use pyrethrins if you are still early in flowering. Otherwise, it is best to stick to food grade contact killer such as citric acid.
- Safer brand pyrethrins & insecticidal soap concentrate
- Pyganic gardening (pyrethrins)
- Botanigard Maxx
Day 5 or 6: Second pyrethrin application, begin biological control with B. bassiana or I. fumosorosea and mites
- Botanigard Maxx
- Pyganic + Botanigard/Mycotrol
- Pyganic+ PFR-97
Day 6 or 7: Release predator mites when previous spray is dry
Day 10: Grandevo or Venerate spray
Day 15: PFR-97 or Mycotrol/Botanigard spray (diversifying from last living fungus spray is recommended, but requires multiple products).
Assuming the infestation is severe, I would rotate between day 1 and day 2 applications for a longer period of time (1-2 weeks) before implementing more persistent biological control methods.
*If ambient humidity is low (<40% RH), I would rotate between Grandevo, Venerate, and Citric Acid (Nuke Em or Plant Therapy) instead of using living fungi*
Day 20: Grandevo or Venerate spray (Diversifying from last heat-killed bacteria is recommended)
Day 27: PFR-97 or Mycotrol/Botanigard
Day 35: Grandevo or Venerate
Day 36: Release predatory mites
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- Luypaert, G., Witters, J., Van Huylenbroeck, J., Maes, M., De Riek, J., & De Clercq, P. (2014). Temperature-dependent development of the broad mite Polyphagotarsonemus latus (Acari: Tarsonemidae) on Rhododendron simsii. Experimental & Applied Acarology, 63(3), 389–400. https://doi.org/10.1007/s10493-014-9777-z
- Luypaert, G., Witters, J., Van Huylenbroeck, J., Maes, M., De Riek, J., & De Clercq, P. (2014). Temperature-dependent development of the broad mite Polyphagotarsonemus latus (Acari: Tarsonemidae) on Rhododendron simsii. Experimental & Applied Acarology, 63(3), 389–400. https://doi.org/10.1007/s10493-014-9777-z
- Neoseiulus californicus (McGregor). (n.d.). Retrieved April 8, 2020, from http://entnemdept.ufl.edu/creatures/beneficial/Neoseiulus_californicus.htm
- swirski mite – Amblyseius swirskii Athias-Henriot. (n.d.). Retrieved April 8, 2020, from http://entnemdept.ufl.edu/creatures/BENEFICIAL/swirksi_mite.htm
- Biological Control of Two- Spotted Spider Mites – CT Integrated Pest Management Program. (n.d.). Retrieved April 8, 2020, from http://ipm.uconn.edu/documents/raw2/html/664.php?aid=664
- Neem Oil General Fact Sheet. (n.d.). Retrieved February 7, 2020, from http://npic.orst.edu/factsheets/neemgen.html
- Dahmardeh, A., Latifi, M., & Riseh, R. S. (2018). Effect of induced resistance in bean plants on Tetranychus urticae life table parameters. Systematic and Applied Acarology, 23(8), 1627–1640.
- Wu, S., Xie, H., Li, M., Xu, X., & Lei, Z. (2016). Highly virulent Beauveria bassiana strains against the two-spotted spider mite, Tetranychus urticae, show no pathogenicity against five phytoseiid mite species. Experimental & Applied Acarology, 70(4), 421–435. https://doi.org/10.1007/s10493-016-0090-x
- Zanolli, P. (2015). Effect of natural derived products on the tomato russet mite, Aculops lycopersici.
- Nugroho, I., & Ibrahim, Y. Bin. (2004). Laboratory bioassay of some entomopathogenic fungi against broad mite (Polyphagotarsonemus latus Bank). International Journal of Agriculture and Biology (Pakistan).
- Mishra, S., Kumar, P., & Malik, A. (2015). Effect of temperature and humidity on pathogenicity of native Beauveria bassiana isolate against Musca domestica L. Journal of Parasitic Diseases : Official Organ of the Indian Society for Parasitology, 39(4), 697–704. https://doi.org/10.1007/s12639-013-0408-0
- BioWorks ® BotaniGard ® MAXX SAFE. PROVEN. EFFECTIVE. (n.d.). Retrieved April 9, 2020, from http://www.bioworkscannabis.com
- Cordova-Kreylos, A. L., Fernandez, L. E., Koivunen, M., Yang, A., Flor-Weiler, L., & Marrone, P. G. (2013). Isolation and characterization of Burkholderia rinojensis sp. nov., a non-Burkholderia cepacia complex soil bacterium with insecticidal and miticidal activities. Applied and Environmental Microbiology, 79(24), 7669–7678. https://doi.org/10.1128/AEM.02365-13
- Stenseth, C. (1979). Effect of temperature and humidity on the development ofPhytoseiulus Persimilis and its ability to regulate populations ofTetranychus urticae [Acarina: Phytoseiidae. Tetranychidae]. Entomophaga, 24(3), 311–317. https://doi.org/10.1007/BF02374246
- Phytoseiulus persimilis. (n.d.). Retrieved April 9, 2020, from https://biocontrol.entomology.cornell.edu/predators/Phytoseiulus.php
- Zemek, R., Kopačka, M., & Šimáčková, K. (2016). Evaluation of Isaria fumosorosea efficacy for the control of spider mites.
- Cervantes, J., & Will. (2016). Grow Marijuana: Kill Hemp Russet Mites & Broad Mites on Cannabis. https://www.youtube.com/watch?v=T33kgEnoydo
- PFR-97 WDG Microbial Insecticide. (n.d.). Retrieved April 9, 2020, from https://www.arbico-organics.com/product/5749/caterpillar-control-deciduous-fruit-trees?gclid=Cj0KCQjwj7v0BRDOARIsAGh37io_2mnNVpRE0I4Ei1rXD9we9L2iDlR3hSLwJWVDz2tpY7Acb73K2H0aAgG0EALw_wcB
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Anyone involved in plant pathology can tell you that the genus Fusarium is one of the most damaging group of fungi to crops. There are many species of Fusarium that cause disease on different crops. Some infections can cause devastating root rots and vascular diseases, some can cause cankers on branches and stems, and some can even infect foliage. Yield losses can be dramatic in some circumstances. For instance, in 1999 in northern Great Plains and central USA, Fusarium head blight of winter wheat alone suffered $2.7 billion losses . In tomato, when disease is severe, crop losses can reach 80% .
In Cannabis, two formae speciales of F. oxysporum have been described as causing Fusarium wilt: Fusarium oxysporum f. sp. vasinfectum (FOV) and Fusarium oxysporum f. sp. cannabis (FOC) [2, 21]. Furthermore, Fusarium solani has been found to be prevalent in hydroponic Cannabis grown in Canada . Furthermore, F. brachygibbosum and F. equiseti have been isolated (in addition to F. oxysporum and F. solani) from symptomatic field-grown Cannabis plants in Northern CA . F. oxysporum has been isolated from wilted Cannabis plants that does not match with either of the formae speciales cannabis or vasinfectum .
Fusarium oxysporum species complex
Fusarium oxysporum is a diverse species; some species are harmless soil inhabitants, while some are plant pathogens. Arguments have been presented that there may be at least two phylogenetically distinct species based on DNA sequencing, and that most plant pathogens belong two one of these groups (PS2) .
Forma specialis is not a phylogenetically recognized categorization; it is a way for plant pathologists to discuss particular isolates of Fusarium species that attack particular plant species. A forma specialis is generally named after the diseased plant that it was isolated from, this is why one of the isolate groups that infect Cannabis is called F. oxysporum f. sp. cannabis. The two formae speciales that infect Cannabis can be distinguished by their host range. FOC only infects Cannabis, whereas FOV has a wider host range and can infect cotton, coffee, and other plants. F. oxysporum has at least 100 different species-specific isolates. Sexual reproduction has not been observed in this species, but horizontal gene transfer likely had an important role in the evolution of this organism . all observed spores from F. oxysporum are asexual in nature.
Infections begin with the germination of spores or growth of mycelium into plant roots through injured areas or sites of lateral root emergence. The filamentous mycelium penetrates into the xylem vessels and begins colonizing the plant’s vasculature. It becomes systemic and can form sporulating structures known as sporodochia on aerial parts of the plant. The sporodochia produce conidia (2 types, microconidia and macroconidia [macroconidia is larger, multinucleate, and multiseptate]). The conidia is carried by wind and air. When it comes to overwintering, Fusarium can survive in the infected crop residues, but it can also make overwintering asexual spores known as chlamydospores. Chlamydospores don’t require special structures to form, they can form at the terminal ends of fungal hyphae or within the hyphae (intercalary). They are generally thick walled, melanized, and multicellular spores.
This disease of Cannabis has been amplified through human activity. Fusarium oxysporum is a deadly pathogen and it has been foolishly used as a mycoherbicide all over the world in order to try to kill ‘illicit’ Cannabis plants [2, 3]. All cultivars that have been tested are susceptible to the disease. In native ecosystems, F. oxysporum is not known to be a major disease risk. It seems that through intensive agriculture and monocropping, more pathogenic and virulent isolates have been able to evolve and amplify their populations clonally .
Fusarium solani species complex 
F. solani, much like F. oxysporum, was previously divided into formae speciales based on the host range. However, recent phylogenetic studies have determined that different formae speciales are really unique species, and the F. solani species complex (FSSC) is divided into at least 60 unique species. Some of these species have been renamed, but many are still unnamed and are referred to by ‘haplotype number’, which is basically just a number that represents certain genotypes. The FSSC has a wide host range, and even particular species within the FSSC can have broad host ranges. Unlike in the FOSC, sexual reproduction has been observed in some species in the FSSC.
The life cycles, infection techniques, and symptoms are very similar between FOSC and FSSC, so I when I mention Fusarium from here on out, I will be referring to all Fusarium sepecies capable of causing root rots of Cannabis.
Chlamydospores require a conducive environment to germinate and cause disease. In soil, this generally means the presence of root exudates . Because of this, only spores in very close proximity to roots actually pose a disease risk. The rhizosphere (which I will define as the area of soil directly affected by the root exudates) is generally very small (<1 mm) from the plant root [9, 10]. The actual volume of soil that falls within this distance from roots is typically under 35%, even for plants with extensive roots and highly active exudation . However, evidence of targeted growth of germinated spores towards roots is lacking (i.e. lacking evidence for chemotaxis) . Infections may fail to establish, especially in the case of a rapidly growing root (spore germinates in response to root exudate, if the germ tube reaches the root, chances are the root tip has already advanced and the fungus is now encountering more differentiated plant tissue more capable of defensive responses) .
Flower and Seedling Infection
Along with Pythium, Fusarium can cause damping off of seedlings. Infection can begin in roots or the hypocotyl and can quickly invade the vasculature .
Fusarium usually begins its infection cycle from chlamydospores in the soil. However, as the infection progresses, sporodochia form on the crown and lower stem that produce conidia. Conidia can become airborne and can infect aerial portions of the plant. In particular, it can readily form flower infections and cause bud rot. Flower-infecting species include F. solani, F. oxysporum and F. equiseti . F. solani appears to be the most aggressive species. It appears that the F. oxysporum that has been isolated from flowers is the same type involved in root infections . These Fusarium species can directly infect the bracts and pistils of flowers.
In hydroponics, Fusarium can be particularly aggressive . In fact, researches generally use aqueous spore suspensions in experiments to guarantee that the plant is inoculated. First of all, the spores are essentially in suspension and circulate around the water, almost guaranteeing that the spores will come in direct contact with the roots. Furthermore, the spores can directly adhere to the root tips, foregoing the need for germ tubes to find their way through soil to the plant roots and decreasing the chance that the plant root can grow faster than it takes for the fungal mycelium to reach the root. This allows easy access for the fungus to susceptible meristematic tissue. Root tip infection does not occur for all plant species, but spore adhesion to roots certainly does raise disease risk for any plant species.
Once the fungal mycelium contacts a root, the fungus proliferates into a hyphal network to maximize points of contact. They likely utilize cell wall degrading enzymes to form an opening, and the mycelium can then penetrate directly through epidermal cells or may grow in between cells . Either way, growth advances towards the root cortex.
Necrotroph or Biotroph?
For those who have read my articles on two other major Cannabis pathogens, bud rot and powdery mildew, you may be aware that pathogenic fungi can have a variety of different strategies. A biotroph requires the host cells to be alive and extracts nutrients from the living host cells (a true parasite, such as PM [manipulates host immune responses]), whereas a necrotroph such as the bud rot pathogen Botrytis cinerea induces or causes cell death to overcome plant resistance responses and have dead organic matter to feed on (it may be argued that there is a brief biotrophic phase in bud rot but in general can be viewed as a necrotroph).
Strictly speaking, Fusarium is necrotrophic because even isolates that do not cause any visible damage to a given plant (nonpathogenic) are observed to grow intracellularly and cause cell death on a microscopic level . However, as mentioned, there are many cases of F. oxysporum isolates not causing any really visible disease or crop losses, or even examples of isolates that cause disease symptoms on some plant species but can colonize the roots of other plant species without causing visible disease symptoms. In these cases, though necrotrophy is visible on a microscopic level, F. oxysporum may be considered an endophyte, and the complexity of the relationship between endophytic Fusarium isolates and their plant hosts are not fully understood [1, 15, 16, 17, 18]. In fact, F. oxysporum can even be isolated as an endophyte from nonsymptomatic Cannabis plants .
There have been some conflicting reports as to how the wilt disease progresses (this will mostly focus on studies done with F. oxysporum in flax), but the differences might be attributable to environmental differences between studies, differences in how microscopic images were interpreted, or it may even be evidence that different isolates within a given forma specialis may span a spectrum of necrotrophic and biotrophic lifestyles.
Disease Cycle Proposition 1: The extended biotrophic phase 
- The fungus has an extended biotrophic phase in which infected cells remain viable and the fungus can continually be isolated from seemingly disease-free root tips.
- After entering the xylem vessels, the fungus grows in the vessels and feeds on the nutrients carried within the xylem. It continues to grow until the vessels become occluded (blocked), either through the accumulation of fungal biomass or through plant responses such as forming tyloses.
- After xylem occlusion and plant wilting/death, the fungus then grows out of the vasculature and begins a necrotrophic phase in which is begins killing and feeding on all other plant tissues.
Disease cycle proposition 2: The true necrotroph 
- No biotrophic phase observed, cell death is common among all cells the fungus comes in contact with.
- Infection of roots leads to root cell death and necrosis before the fungus even reaches xylem vessels (i.e. root rot can precede systemic vascular infection)
- Fungus aggressively colonizes both vasculature and other tissues
- Initial symptoms can look similar to Nitrogen deficiencies. Chlorosis of lower leaves and slight wilting becomes evident. Plant stunting is common, especially in the case of F. solani infection
- The crown region of the plants become darkly discolored and sunken. Discoloration of the vasculature can extend up to 15cm from the soil surface.
- In hydroponics, roots become discolored
- When xylem vessels become occluded, whole plants can begin to wilt.
- In this hydroponic system, the Fusarium wilt ended up killing the plant.
- Sporodochia form on the necrotic stem and the spores can become airborne, infecting surrounding plants. In humid conditions, mycelium can grow out of the stem
- Fusarium can cause damping off in seedlings and clones as seen in the following tray of clones:
- Fusarium oxysporum can cause bud rot! When inoculated on flowers, they can cause necrosis of the buds very similar to Botrytis cinerea. The mycelium is usually much more white than the mycelium from B. cinerea.
- Depending on where the infection occurs, wilting can be evident on some branches/colas but not others.
What Factors Favor Fusarium Development?
For F. oxysporum f. sp. lycopersici (the forma specialis that infects tomato), the following factors favor wilt development (28):
- Soil and air temperatures of 28°C (Too warm (34°C) or too cool (17-20°C) will inhibit development)
- Low nitrogen and phosphorus, high potassium
- Low soil pH
- Short day day length
- Low light
- use of ammonium nitrogen
Root exudates appear to stimulate spore germination and drive plant infection. However, certain techniques based on manipulating the soil microbiome may be beneficial in controlling the severity of Fusarium wilt.
- Amending soil with organic matter to promote microbial activity may make soils more disease-resistant .
- Soil treatments aimed at reducing the number of viable fungal propagules in the soil such as anaerobic soil disinfestation (ASD)  and solarization .
- ASD is a process of flooding a field and covering with a plastic ‘mulch’. Anaerobic bacteria multiply and gasses from these bacteria accumulate under the plastic mulch.
- Solarization is the process of putting a black plastic over a field during hot seasons in direct sun to raise soil temperatures.
- Certain bacteria or microbial groups may contribute to how conducive a soil is to disease development
- For instance, a species of Arthrobacter in suppressive soils was associated with greater levels of lysis of fungal germ tubes from soil chlamydospores .
- Soils that are more disease suppressive are sometimes associated with certain microbial groups in the soil microbiome 
- For instance, a species of Arthrobacter in suppressive soils was associated with greater levels of lysis of fungal germ tubes from soil chlamydospores .
Resistant strains undoubdtedly can be bred for. In hemp, SF and CF cultivars appear to be more resistant than the cultivar Iran . I am not sure what strains are best in regards to marijuana cultivars with resistance to Fusarium, and I am struggling to find information on this. Comments with relevant information on this would be appreciated.
I will list some approved spray/soil drench control methods, but can not promise the effectiveness of any method, much cannot be found in literature.
- In Canada, possible biocontrol agents for Fusarium infections in foliage and flowers include Prestop WP (Gliocladium catenulatum strain J1446) and Rootshield WP (Trichoderma harzianum Rifai strain RRL-AG2) .
- These microbes will be counted on CFU testing, so should not be applied late in locations that test using this method.
- In Canada, approved biocontrol agents for root-infecting pathogens are Rootshield WP (Trichoderma harzianum Rifai strain RRL-AG2) and Prestop WP (Gliocladium catenulatum strain J1446) .
- In California, Gliocladium virens, Trichoderma harzianum, and Bacillus amyloliquefaciens strain D747 are approved biofungicides .
- Other possible biocontrol biocontrol agents include Rhapsody (Bacillus subtilis strain QST 713) and Mycostop (Streptomyces griseoviridis strain K61) .
In California extract of Giant Knotweed (Reynoutria sachalinensis) REGALIA® Rx Biofungicide is an approved fungicide.Marrone Bio Innovations Regalia Biofungicide Fungicide inhibits fungal and Bacterial Disease Boosting Yield, 0-Day PHI, 4 Hour REI, OMRI Listed (1 Gallon)
Kelp extracts (contain arachadonic acid) and crab meal/insect frass (contain chitin) may be useful soil amendments for priming plant resistance to soil borne fungal pathogens.Liquid Kelp Extract Seaweed 32 Ounce Fertilizer Concentrate
In Oregon, potassium phosphite such as Agri-Fos, (which happens to also be a good source of potassium and phosphorus in flower) is also approved as a plant protectant and fungicideMonterey Agri-Fos Disease Control Fungicide – Pint LG3340 Quest Reliant Systemic Fungicide (Agri-Fos/Garden Phos) 1 Gallon
- Control and prevention should include efforts to reduce inoculum loads. For growers using hydroponics (including coco) and/or indoor grows: ultraviolet light in ducting and even the grow area (which also may increase cannabinoid production if used correctly), ozonation of the grow area (too high of a level may have negative effects on plant and human health), chlorination of water used in hydroponics, hydrogen peroxide flushes of the root zone (or products such as Zerotol which also contains peroxyacetic acid), heat pasteurization and/or mechanical filtration of water .DPD ZeroTol 2.0 2.5GAL
- It is a good idea to remove wilted plants to prevent aerial spore transfer and quickly remove any infected flowers or branches, especially in environments of high humidity.
- In hydroponics, keeping nutrient solution at temperatures between 17℃ and 22℃ is ideal for preventing pathogens, promoting water oxygenation, and preventing growth retardation of the plants.
Active Aqua AACH10HP Water Chiller Cooling System, 1/10 HP, Rated per hour: 1,020 BTU, User-Friendly
- Always sterilize your tools in between cuts, wear proper PPE to avoid introducing inoculum.
- Despite common conceptions that Fusarium grows best in flooded soils, many Fusarium species actually grow best in aerobic, well-draining soil . Another study similarly found F. oxysporum f. sp. lycopersici to not grow in saturated soils. However, plants were actually resistant to infection at soil moisture contents of 13%-19% .
- In short, it is good to let your soil dry between waterings (not to the point of plant wilting though). Fusarium grows best in aerobic (well-draining), and moist but not flooded soils (i.e. most coir or peat based media).
- Anaerobic soil disinfestation (ASD) may be a good way to reduce soil inoculum levels between grows in no-till systems.
The most important factor in preventing flower infections from Fusarium is probably humidity . Flower infection relies on airborne conidia released from the sporodochia (spore-bearing structures) on aerial tissue of the plant. Humidity needs to be high in order to successfully form these sporodochia. A different Fusarium species, F. graminearum requires humidity of over 85% RH to form perithecia (sexual spore-bearing structure of this species) .
For Fusarium oxysporum f. sp. erythroxyli (this paper is unfortunately discussing the possible use of this Fusarium species to kill the ‘illicit narcotic’ coca plant), the isolate was found to sporulate at relative humidities (RHs) between 75% and 100% . Fusarium‘s primary route of infection is through the roots in soil, and it is a bit easier to control for aerial infections than soil infections in Cannabis.
General humidity control aiming to correlate with vapor pressure deficit conditions or slightly lower for IPM reasons (around 60% RH in veg, 50% in flower down to 40% the last couple weeks of flower) should be good enough to prevent a lot of aerial sporulation. Good airflow and ciculation is definitely recommended to reduce high-humidity microclimates.
- Gordon, T. R. (2017). Fusarium oxysporum and the Fusarium Wilt Syndrome. Annual Review of Phytopathology, 55(1), 23–39. https://doi.org/10.1146/annurev-phyto-080615-095919
- McPartland, J. M., & Hillig, K. W. (2004). CANNABIS CLINIC Fusarium Wilt. Journal of Industrial Hemp, 9(2), 67–77. https://doi.org/10.1300/J237v09n02_07
- Council, N. R. (2011). Feasibility of Using Mycoherbicides for Controlling Illicit Drug Crops. The National Academies Press. https://doi.org/10.17226/13278
- Bonanomi G, Antignani V, Capodilupo M, Scala F. 2010. Identifying the characteristics of organic soil amendments that suppress soilborne plant diseases. Soil Biol. Biochem. 42:136–44
- Hewavitharana SS, Mazzola M. 2016. Carbon source–dependent effects of anaerobic soil disinfestation on soil microbiome and suppression of Rhizoctonia solani AG-5 and Pratylenchus penetrans. Phytopathology 106:1015–28
- Greenberger A, Yogev A, Katan J. 1987. Induced suppressiveness in solarized soils. Phytopathology 77:1663–67
- Smith SN. 1977. Comparison of germination of pathogenic Fusarium oxysporum chlamydospores in host rhizosphere soils conducive and suppressive to wilts. Phytopathology 67:502–10
- Mazzola M. 2004. Assessment and management of soil microbial community structure for disease suppression. Annu. Rev. Phytopathol. 42:35–59
- Huisman OC. 1982. Interrelations of root growth dynamics to epidemiology of root-invading fungi. Annu. Rev. Phytopathol. 20:303–27
- Rovira AD. 1969. Plant root exudates. Bot. Rev. 35:35–57
- Olivain C, Humbert C, Nahalkova J, Fatehi J, L’Haridon F, et al. 2006. Colonization of tomato root by pathogenic and nonpathogenic Fusarium oxysporum strains inoculated together and separately into the soil. Appl. Environ. Microbiol. 72(2):1523–31
- Beckman CH. 1987. The Nature of Wilt Diseases of Plants. St. Paul, MN: Am. Phytopathol. Soc. 175 pp
- Recorbet G, Alabouvette C. 1997. Adhesion of Fusarium oxysporum conidia to tomato roots. Lett. Appl. Microbiol. 25:375–79
- Olivain C, Alabouvette C. 1997. Colonization of tomato root by a non-pathogenic strain of Fusarium oxysporum. New Phytol. 137:481–94
- Correll JC, Puhalla JE, Schneider RW. 1986. Vegetative compatibility groups among nonpathogenic root-colonizing strains of Fusarium oxysporum. Can. J. Bot. 64:2358–61
- Gordon TR, Okamoto D, Jacobson DJ. 1989. Colonization of muskmelon and nonsusceptible crops by Fusarium oxysporum f. sp. melonis and other species of Fusarium. Phytopathology 79:1095–100
- Katan J. 1971. Symptomless carriers of the tomato Fusarium wilt pathogen. Phytopathology 61:1213–17
- Scott JC, McRoberts DN, Gordon TR. 2014. Colonization of lettuce cultivars and rotation crops by Fusarium oxysporum f. sp. lactucae, the cause of Fusarium wilt of lettuce. Plant Pathol. 63:548–53
- Turlier M-F, Eparvier A, Alabouvette C. 1994. Early dynamic interactions between Fusarium oxysporum f. sp. lini and the roots of Linum usitatissimum as revealed by transgenic GUS-marked hyphae. Can. J. Bot. 72:1605–12
- Kroes GMLW, Baayen RP, Lange W. 1998. Histology of root rot of flax seedlings (Linum usitatissimum) infected by Fusarium oxysporum f. sp. lini. Eur. J. Plant Pathol. 104:725–36
- Punja, Z. K., & Rodriguez, G. (2018). Fusarium and Pythium species infecting roots of hydroponically grown marijuana (Cannabis sativa L.) plants. Canadian Journal of Plant Pathology, 40(4), 498–513. https://doi.org/10.1080/07060661.2018.1535466
- Coleman, J. J. (2016). The Fusarium solani species complex: ubiquitous pathogens of agricultural importance. Molecular Plant Pathology, 17(2), 146–158. https://doi.org/10.1111/mpp.12289
- Punja, Z., Scott, C., & Chen, S. (2018). Root and crown rot pathogens causing wilt symptoms on field-grown marijuana ( Cannabis sativa L.) plants. Canadian Journal of Plant Pathology, 40. https://doi.org/10.1080/07060661.2018.1535470
- Punja, Z. K., Collyer, D., Scott, C., Lung, S., Holmes, J., & Sutton, D. (2019). Pathogens and Molds Affecting Production and Quality of Cannabis sativa L. Frontiers in Plant Science, 10, 1120. https://doi.org/10.3389/fpls.2019.01120
- Punja, Z. K. (2018). Flower and foliage-infecting pathogens of marijuana (Cannabis sativa L.) plants. Canadian Journal of Plant Pathology, 40(4), 514–527. https://doi.org/10.1080/07060661.2018.1535467
- Department of Pesticide Regulation, C. (n.d.). CANNABIS PESTICIDES THAT ARE LEGAL TO USE. Retrieved March 28, 2020, from http://www.cdpr.ca.gov/cannabis
- Manstretta, V., & Rossi, V. (2015). Effects of Temperature and Moisture on Development of Fusarium graminearum Perithecia in Maize Stalk Residues. Applied and Environmental Microbiology, 82(1), 184–191. https://doi.org/10.1128/AEM.02436-15
- Fusarium oxysporum f. sp. lycopersici. (n.d.). Retrieved March 28, 2020, from https://projects.ncsu.edu/cals/course/pp728/Fusarium/Fusarium_oxysporum.htm
- Gracia-Garza, J. A., & Fravel, D. R. (1998). Effect of Relative Humidity on Sporulation of Fusarium oxysporum in Various Formulations and Effect of Water on Spore Movement Through Soil. PhytopathologyTM, 88(6), 544–549. https://doi.org/10.1094/PHYTO.19184.108.40.2064
- Stover, R. H. (1953). THE EFFECT OF SOIL MOISTURE ON FUSARIUM SPECIES. Canadian Journal of Botany, 31(5), 693–697. https://doi.org/10.1139/b53-050
- Clayton, E. (1923). The Relation of Soil Moisture to the Fusarium Wilt of the Tomato. American Journal of Botany, 10(3), 133-147. Retrieved March 29, 2020, from http://www.jstor.org/stable/2435361
- Goswami, R. S., & Kistler, H. C. (2004). Heading for disaster: Fusarium graminearum on cereal crops. Molecular Plant Pathology, 5(6), 515–525. https://doi.org/10.1111/j.1364-3703.2004.00252.x
- Fusarium Wilt in Processing Tomatoes – Seminis. (n.d.). Retrieved March 29, 2020, from https://seminis-us.com/resources/agronomic-spotlights/fusarium-wilt-in-processing-tomatoes/
- Akhter, A., Hage-Ahmed, K., Soja, G., & Steinkellner, S. (2016). Potential of Fusarium wilt-inducing chlamydospores, in vitro behaviour in root exudates and physiology of tomato in biochar and compost amended soil. Plant and Soil, 406(1), 425–440. https://doi.org/10.1007/s11104-016-2948-4
*A special thank you to Feedspot for including Indica Info on its list of top 200 marijuana blogs at https://blog.feedspot.com/marijuana_blogs/ *
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The main draw for me for this growing style is the reduction of salt based nutrients which ultimately end up leaching in to waterways. Nitrates in particular can lead to ecological problems in marine ecosystems, and they are very energy intensive to produce. Using this method, I don’t water until runoff and I never flush. I was inspired to try it myself when I saw a YouTube video on the channel ‘Mendo Dope’ with Minnesota Nice about growing indoors with a no-till soil. Brownguy420 on YouTube was also a good resource.
There are some misconceptions in their videos. I think they grow great plants, but don’t have a great understanding of some of terminology they use. For instance, Minnesota Nice frequently refers to his potting soil as ‘very loam’. Loam soil refers specifically to the mineral composition of the soils and tends to be fairly equal in its amounts of clay, silt, and sand (silt and sand are usually composed of quartz, don’t provide much in terms of plant nutrients, and are generally not found in any potting mixes). What he really meant by that is that his soil had good structure, was not overly compacted, and had a good amount of aggregation of organic matter.
They frequently also talk about forming a humus layer at the top of their soils, which is not really possible in potting mixes. I will discuss this more later, but a humus layer specifically refers to well-decomposed organic matter in the very top of the topsoil and sits on top of more mineral-heavy layers. In potting soils that are mostly organic matter, this layer of well decomposed organic matter will actually accumulate more evenly throughout the pot (more at the bottom of the pot; generally takes multiple years). There is now a lot of controversy over whether a humus layer as it has been historically defined even exists in soil. Potting soils certainly don’t act much like natural soils or have similar compositions.
What Does It Mean To Be No-Till?
I think in regards to indoor growing, it would be more aptly-named no-dig. We are not really ’tilling’ our pots like a farmer would to a field. However, the concept of leaving the soil undisturbed is similar. Living soils contain a wide array of living organisms including earthworms, arthropods, bacteria, fungi, actinomycetes, protozoa, nematodes, and algae. All of these organisms interact with each other in the soil, and may work syngergistically in breaking down organic matter. They may feed on the waste products of another organism, or may even feed on another organism in the soil. In normal soils, the highest amount of microbial activity is in the topsoil. This is because in most soils, the topsoil contains the highest amount of organic matter. Indoor potting soil mixes, however, are mostly organic matter and are usually well-mixed. Therefore, microbial activity is probably much more evenly distributed throughout a potting soil mix. If using fabric pots, the oxygen levels are likely more evenly distributed as well, promoting microbial activity.
Can potting mixes really be treated the same as a no-till system in natural soils?
This leads to the question of if a potting soil can really be used indefinitely like a mineral soil. Can a potting soil that is probably at least 90% organic matter retain good soil structure and nutrient content by using techniques such as cover cropping/intercropping with legumes, grasses, and brassicaceous plants, top dressing with composts and manures, and adding mulches?
Well first off, unlike natural soils, potting mixes are limited by the volume that can be fit in a container. Treating a soil with no-till methods requires adding more and more organic matter to the soil over time with the goal of having a net positive amount of carbon and nutrients accumulating in the soils. In pots, this will eventually lead to a lack of space and the inability to keep adding more and more materials. I believe this is the main limiting factor in treating potting soil with no-till methods.
Changes to potting mixes over time
Aside from the space issue, there are physical changes to the soil over time. Large organic materials will break down into smaller and smaller molecules. Peat moss, coconut coir, composts, manures, and other plant residues will break down and lose their macro structures as they are digested by earthworms, arthropods, fungi, and bacteria. Eventually, what is left is a material that has historically been referred to as ‘humus’. Humus is a dark brown or black material that is the byproduct of the breakdown of organic matter.
Historical Understanding of Humus
Humus has been thought to be an amorphous material made of large organic molecules. It lacks the presence of cellular structure seen in living organisms. It generally has very few available plant nutrients and is quite stable over time. It resists further breakdown by microbes, it has various phenolic, furan, alcohol, and carboxylic acid functional groups in large molecules bound together by ionic interactions. These molecules are generally classified as humins, humic acids, and fulvic acids. Humins are considered insoluble, fulvic acids are soluble at any pH, and humic acids are most readily soluble in very high pH solution.
Fulvic acids and humic acids are chelators that bind to positively charged ions including nutrients such as ammonium, calcium, iron, and any other positively charged nutrient. They have also been proposed as chelators of heavy metals that help protect plants from toxicity. Thus, they increase the cation exchange capacity of soils allowing for better retention of nutrients. Furthermore, negatively charged fulvic acids that are soluble may deliver bound nutrients directly to the root zone. Fulvic acids and humic acids have also been proposed to positively effect plant growth and plant health through stimulating plant hormone and reactive oxygen species pathways leading to improved growth, nutrient uptake and transport [1, 14].
Criticisms of the Old Model of Humus
If humic substances accumulate in soils and resist further breakdown, why does the humus layer not keep growing indefinitely in size? Well, new research has found that these large humic molecules that resist further breakdown don’t actually exist naturally in soils. In the past, humic acids were characterized by first performing soil extractions with a highly alkaline sodium hydroxide solution. The extracts were then fractionated and hydrolyzed with acid solutions and the molecules that were soluble at low pHs were determined to be fulvic acids, whereas the molecules that remained only soluble in high pH solutions were classified as humic acids.
Recent studies have shown that without the alkali extraction, these molecules do not exist naturally in soil, and therefore, humic acids and fulvic acids are actually created during the extraction process. The humus layers in soil do not actually have these novel stable compounds that have been reported for so long. In fact, the humus layer is really just organic matter in various stages of decomposition [2, 3].
The main takeaway here is that it is not true that you need to purchase ‘humic and fulvic acid’ products before your soil has developed a ‘humus layer’. Observed vigor when plants are grown in soils with a ‘humic layer’ is likely due to a higher amount of well decomposed organic matter that is more readily mineralized by microbes than larger organic matter. Adding organic matter in various stages of decay (such as adding compost or vermicompost) into a potting soil with larger organic matter will more closely resemble this situation and establish a continuum of organic matter in various stages of decay. Humic and fulvic acids still have benefits to plants, just understand that they are synthetic and will not be naturally produced in your soil over time.
Of course, potting soil will break down over time and undergo humification, but I believe they can indeed be sustainably reused over time (until a space capacity is reached). It is important to realize that as the organic matter breaks down, there may be compaction that needs to be dealt with.
How to Deal With Soil Compaction and Decomposition
There are a few good tools to deal with this issue in potting soil mixes. In my opinion, these are the best ways to keep your potting mixes sustainable:
- Earthworms- Help with soil porosity by boring through the soil and forming tunnels. Water infiltration can be increased by 4 to 10 fold when earthworms are actively forming tunnels . In addition, they can help homogenize a no-till system by consuming fresh organic matter at the soil surface and redistributing the organic matter throughout the pot through their castings. Earthworms also greatly increase the nutrient availability of the compounds in the organic matter. Earthworm populations do not do well with tillage, and soil tillage (digging) can greatly reduce the population of earthworms in the topsoil. They prefer to have organic litter on top of the soil to feed on. Furthermore, soils with active earthworms appear to have fewer pathogens .
- Cover Crops/Intercropping - Cover crops such as crimson clover, hairy vetch, ryegrass, and rapeseed can be used to increase soil infiltration. Rapeseed tends to have a large taproot that can easily drive down to the bottom of a pot and help provide aeration and inject fresh organic matter deep into the soil. Hairy vetch and crimson clover are legumes that are useful for nitrogen fixation, and of course their roots also provide organic matter and infiltration. Grasses such as ryegrass typically do not have a large taproot, but do have very prolific root systems and are fantastic green mulches for increasing fungal activity in the soil.
- Microbial Activity (Fungi and Bacteria)- Both fungi and bacteria are very important for the formation of soil aggregates. A soil aggregate is essentially matter that is ‘glued’ together and remains stable when watered. Channels that allow for water infiltration form in between aggregates.
What is a soil aggregate? 
Soil molecules are brought into contact with each other through physical processes and may weakly associate with each other through ionic interactions. Aggregates can become stable through the filamentous growth that binds aggregates together such as plant roots and fungal mycelium. Earthworms can help form aggregates through their excrement. However, microbial activity is probably the most important. Aside from fungal mycelium physically holding aggregates together, fungi and bacteria can release metabolites into the soil such as gums, waxes, polysaccharides, and glycoproteins. For instance, Glomeromycetes form mycorrhizal associations with plant roots and secrete a glycoprotein known as glomalin into the soil that helps stabilize aggregates. Glomalin activity appears to be reduced in heavily tilled soils.
What Kind of Earthworms Should I Add?
I recommend adding a variety of species of earthworms. Some species of worms may be better at feeding at different depths of the soil. For instance, red wigglers are thought to be good at feeding on organic matter near the soil surface, whereas nightcrawlers are better at feeding on more decomposed matter further down in the soil . I have added both of these species of worm to my pots as well as a native species of earthworm that I collected from my yard. I added about 5 nightcrawlers, 15 red wigglers, and 15 native earthworms to each 10 gallons of soil. I am not really sure what to expect from the amount I added, and I am not sure if one species will do best and outcompete the others or if they will find their niches in the soil.Uncle Jim’s Worm Farm 500 Count Red Wiggler Live Composting Worms
What about the nematodes and the arthropods in the soil food web?
Nematodes are basically small worms. Numerically, they are the most abundant animals on the face of the earth, and there are tens of thousands of different species that occupy a vast number of ecological niches. In soils, different species may feed on fungi, bacteria, insects, other nematodes, and some are parasitic to plants. Therefore, they play important roles in nutrient cycling, disease control, and a balanced soil ecosystem . Many nematodes are naturally introduced in to your soils through the use of composts. However, if you are having insect problems in your soil, three beneficial nematodes are commonly used as additives: Heterorhabditis bacteriaphora, Steinernema feltiae and Steinernema carpocapsae. In particular, S. feltiae is particularly useful for helping control fungus gnats and thrips .
I normally don’t add insects to my soils. Though insects such as millipedes, pill bugs, or sowbugs may assist in breaking down organic matter, they really aren’t necessary and may do damage to young, tender transplants. I rely mostly on worms for helping to break down the larger organic matter. In regards to smaller arthropods such as mites, many soil mites will be inoculated in to your soil with the compost you add. They are useful as well for breaking down decaying organic matter . Similar to beneficial nematodes, there are predatory mites that can be added to your plants and soils if you are having issues with certain arthropods, specifically whiteflies, thrips, fungus gnats, and mites such as spider mites and russett mites. There are different species of predatory mites that are commonly added for pest control, and each one has different lifestyles and does best under different environmental conditions. I recommend looking at the following site to help determine what kind of mite will best help you with the pest you are dealing with in your particular environmental conditions: https://www.chascience.com/predatory-insects
What Cover Crops Should I Plant?
For the reasons talked about earlier, my cover crop mixture contains crimson clover, hairy vetch, rapeseed, and ryegrass.
Normally, a cover crop such as this would be planted in the winter, after harvest of an outdoor crop. It would be allowed to grow until the next planting season, at which point it would be cut down and a mulch or compost would be placed over the crop residue.
They can also be treated similar to an intercrop. For instance, I am growing indoors and wanted to begin growing soon after preparing the soil, so I decided to grow my cover plants at the same time as my Cannabis plants. I transplanted the clones to the center of the pots and seeded everywhere except for a 4 inch perimeter around the clone. I then covered the unseeded area in compost.
It can take up to 6 weeks for legumes to begin fixing Nitrogen , so it is important to leave them planted throughout the life cycle of the Cannabis to contribute a net gain of Nitrogen to the soil (or at least 2 months). To avoid intercropping, have pots that you cycle between for your Cannabis, and plant the cover crop mixture on pots you are not currently growing in.
How Do I Stimulate Soil Microbe Activity?
The number one way to stimulate microbial activity is by adding organic material to the soil. Earthworm castings, green manure, compost, mulches etc. Having active soil biology including worms and arthropods can help with keeping microbes active in the soil. However, I also tend to use microbial additives including compost teas and prepackaged microbe inoculants.
What’s the Deal with Compost Tea?
There is some debate regarding the value of compost tea. Undoubtedly, there are small amounts of soluble nutrients in the compost tea, but the main ‘selling point’ on compost tea is the claim that it inoculates soil with microbes and helps feed the microbes already in the soil. Compost teas can be made a variety of ways, but the only compost teas I have ever made are aerated. Healthy soils are not rich in anaerobic bacteria, so I don’t see a lot of value in making anaerobic compost teas. Many soil bacteria are facultative anaerobes, meaning they can grow in environments with and without oxygen, so unaerated teas may be useful in growing these bacteria, but these same bacteria can grow in aerated teas.
Of course, adding compost directly to the soil will add the same bacteria to the soil, but when you grow in containers and are limited in space, I think that compost teas are a good way to continually inoculate your soil with microbes. In my opinion, just steeping compost in water is not the best way to make a tea, as there isn’t enough simple carbon and nutrients for vigorous microbial growth. I view compost as the source of inoculum in the compost tea, and then I add amendments to support rapid growth of microbes.
What do I put in my compost teas?
- Compost– I think it is best to use a high quality, local compost made with a variety of organic matter. It should at least have some sort of animal manure such as rabbit, chicken, cow, horse etc.
- Personally, I don’t make my own compost and to be honest, the compost I use is not of the highest quality. I use cedar grove compost because it is very cheap and available at a store near me. It contains no animal manures unfortunately, and is made from kitchen waste, yard trimmings, and food products. It also tends to have little pieces of trash because it is made from peoples’ waste. I use a few handfuls of this in a mesh strainer bag for 5 gallons of actively aerated compost tea (AACT).
If you want to buy a high-quality compost online because you don’t have access to a good local compost, the following compost has good ingredients:Charlie’s Compost 10 lb
- Vermicompost or vermicompost extract- Many consider vermicompost to be the ideal compost for plant growth. I have recently established a worm bin to get a consistent source of earthworms and earthworm castings, but in the meantime I have been using vermicompost extract from Terra Vesco. I don’t measure it, I just splash a small amount into the tea. Recommended dilution rates are 5%-10% which would work out to 4-8 cups in a 5 gallon tea. However, I am not using this as my only nutrient and microbe source, so I use less. I would estimate that I usually end up adding about 1 cup of it into my teas. I will stop using this once I get a consistent source of vermicompost, at which point I will add the vermicompost to another strainer bag in my aeration bucket.
- Fish and Kelp Blend– This liquid additive is a 2-3-1 organic liquid fertilizer. I only add a few tablespoons of this to my 5 gallon tea. Kelps such as Ascophyllum nodosum have been demonstrated to enhance plant growth hormone levels in plant tissues. They also contain fatty acids such as arachadonic acid that act as plant systemic acquired resistance (SAR) activators [11, 12]. Fish hydrolysate is enzymatically digested fish that contains lots of proteins, amino acids, fatty acids, and available nutrients. Fish hydrolysate promotes fungal growth in compost teas, leading to a more balanced tea that is not dominated completely by bacteria. While it has available plant nutrients, the microbial growth in the teas may assimilate some of these available nutrients into more complex organic matter.
- Insect Frass- I also add the frass (castings and exoskeleton remains) of black soldier fly larvae to my compost teas. The frass I use is rated as a 4-2-2.5 fertilizer and has various other benefits. Insect frass contains chitin due to insect exoskeleton remains. Chitin is also found in crustacean shells and the cell walls of fungi. Plants have receptors in their cell membranes that bind to chitin and trigger expression of disease resistance genes, so insect frass may help prime plants for disease resistance. Furthermore, insect frass is rich in microbes including bacteria and protozoa and provides a good source of food for fungi. I add about a tablespoon of this to a 5 gallon AACT.
- Down To Earth Acid Mix– I don’t tend to use much of this in my soils because it contains cottonseed meal which can lower soil pH some. However, it contains the following ingredients: Cottonseed Meal, Fish Bone Meal, Langbeinite, Rock Phosphate and Kelp Meal. It also contains humic acids. Rock phosphate and kelp meal are common additives for fungal dominant compost tea recipes, but the other ingredients certainly don’t hurt anything in terms of providing a more diverse source of food for microbes. Humic acids are also effective chelators. I only add about a tablespoon of this to the 5 gallon bucket (I add it into the pouch with the compost so the actual material doesn’t go in to the soil.
- Unsulfured Blackstrap Molasses– This is added as a simple sugar source to help amplify the amount of microbes (mostly bacteria) in the compost tea. Molasses contains very small amounts of available plant nutrients and is not very useful as a fertilizer. I am of the opinion that any source of sugar can replace molasses, but many people swear by only using molasses, especially people that don’t want to use anything too refined or not organic. I do not use much of the molasses; I usually use 1-2 tablespoons in the 5 gallons of tea. Using sugars such as molasses tends to favor a more bacterial tea whereas more complex organic matter such as kelp, fish, or insect frass tend to favor a more fungal dominant tea. I don’t believe it is well understood what the ‘ideal’ compost tea for Cannabis is, but I tend to try to make my teas with a variety of different food sources for microbes to try to diversify the microbial life in my teas as much as possible. However, if I was to leave out one food source from my teas, I would leave out the molasses. I believe that the more complex organic matter that I add also supports some amount of bacterial growth. However, I like to make a relatively balanced tea, and I like to add the molasses to have good bacterial proliferation as well as fungal growth. I have never quantified the microbes in my teas, though.
- Soluble Silica– This additive is not very important in my opinion. I mostly add it just because I have excess of it but I don’t plan on adding it after flipping to flowering light schedules. Silicon is important for stress tolerance and strong structure, but dicotyledonous plants such as Cannabis generally accumulate less silicon than monocots . I use straw mulches and rice hulls in my soils, so I should have more than enough silicon in my soil already. However, it likely benefits the ryegrass that I plant as a cover crop as well.
In addition to compost teas and compost mulches, I use microbial amendments. My go-to additive is Recharge. It is a microbial inoculant that contains arbuscular mycorrhizal fungal species, Trichoderma fungal species, and Bacillus bacterial species. Bacillus and Trichoderma species act as decomposers as well as biocontrol agents of pathogens, and mycorrhizal fungi form associations with plant roots to help provide nutrients (primarily phosphorus) to plants. Initially, during soil preparation, I sprinkle recharge into my soil mixture. I also sprinkle some onto the root ball of the clone or seedling I am transplanting into the soil. Finally, I sprinkle a small pinch of it onto the soil surface before each watering or feeding.Real Growers Recharge (8oz)
There are comparable brands to recharge including Great White and Mikrobs (both of which actually have more diverse arrays of microbes in the mixture). In fact, I would probably recommend Great White just for the amount of microbial diversity it has, but it is a bit expensiveGreat White PRPSGW04 100049823 4 oz Mycorrhizae, 4 Ounce Mikrobs – Microbial Plant Food. Great Root Booster. OMRI Listed Certified Organic (8 Oz)
Other recommended microbial products include Tribus and Mammoth P. Both Tribus and Mammoth P are marketed as bacteria blends that are useful for Phosphorous cycling. Tribus has three Bacillus species, but all of them can also be found in Great White and two of them can be found in Recharge and Mikrobs. Mammoth P, however, is a formulation that has unique bacterial species not found in other microbial mixes: Enterobacter cloacae, Citrobacter freundii, Pseudomonas putida and Comamonas testosteroni. These bacteria were chosen for their high phosphorous cycling ability in soil. Therefore, I think Mammoth P would be a good addition to Recharge, Mikrobs, or Great White.Mammoth P Bloom Booster, 250 Milliliter Tribus Original, Bloom Booster Super Concentrated – 250 ml
I think that microbial amendments can be useful in promoting root growth, nutrient uptake, and biocontrol, but they certainly aren’t necessary for good plant growth and you should not break the bank buying them. I think that Recharge and Mikrobs are both relatively affordable and a little bit can go a long way. You could even get away with doing a one-time application at transplant, you don’t need to apply them as frequently as I do. I don’t personally use Mammoth P, but I think it would be worth experimenting with and seeing if the benefits you get from it are worth the price tag.
You can get a microbially active soil just from compost, these additives are not necessary, but there are a lot of positive reviews and anecdotal evidence supporting the use of these additives, especially those with mycorrhizae.
How Do I Prepare a Potting Soil for No-Till Systems?
You can make this as simple or as complex as you would like. I like to try to include organic matter in various levels of decomposition, a good amount of aeration, microbial-rich materials, mineral amendments, and some nutrient-rich organic matter. First start off with a good base mix. One simple mix I like is: 20% Canadian sphagnum peat moss, 20% high-quality coco coir, 25% high-quality compost (can be compost, vermicompost, or a mixture), 15% rice hulls (provides aeration and organic matter as well as silica), and 20% small black lava rock or pumice (I don’t recommend perlite in no-till mixes because of its propensity to get pulverized over time).
Next up, I like to add amendments besides the compost that may add microbes to the soil.
- Sprinkle a thin layer of Recharge or a powdered microbial additive of your choice over the soil mixture. It is not necessary to add a large amount; the microbes will multiply by themselves, plus you will likely do extra additions throughout the grow. If you are using Tribus, Mammoth P, or another liquid microbial additive, wait until your first watering to add it.
- Add about 1/2 cup of insect frass for every cubic foot of potting mix. This will add more microbes and will also provide a good amount of plant nutrients and organic matter.
Since the potting mix has very little mineral content, it is important to add minerals to your soils. If you are trying to save money, it may be good to choose one of the two following materials. If you want to maximize mineral diversity, it may be a good idea to mix both of them into your soil.
- Azomite- Azomite is volcanic ash mined from a seabed. It has an NPK rating of 0-0-0.2, but its main value to soils is it contains over 70 trace minerals and elements. Add about 2/3 of a pound of azomite (1 cup) for each cubic foot of potting mix.
- Greensand– Derived from the mineral glauconite, greensand has a similar NPK ratio and trace mineral content as Azomite. However, relative ratios of micronutrients are surely different than that of Azomite. For instance, greensand has a higher amount of total potash and iron than Azomite. You can add 2/3 of a pound (1 cup) of greensand for each cubic foot of potting soil.
- If you are using both greensand and azomite, add 1/3 of a pound (1/2 cup) of each
For more long term phosphorus and calcium, I recommend adding some soft rock phosphate. Be aware that this is different than hard rock phosphate. It still contains elements that need to be mineralized by microbes over time, but it can be broken down within a reasonable timeframe for no-till potting mixes.
- Soft rock phosphate has an NPK ratio of 0-3-0, with 3% soluble phosphorus available to the plant. However, it contains up to 20% total phosphate that will be released over time through the activity of microbes such as mycorrhizae or phosphorous cycling bacteria such as those found in Mammoth P (and naturally in soils).
- Soft rock phosphate also has approximately 20% total calcium
- Add about 1-1.25 pounds per cubic foot of potting soil (around 1.75 cups)
Langbeinite is a great addition to help balance out the rock phosphate. Langbeinite is calcium magnesium sulfate.
- Langbeinite has an NPK ratio of 0-0-22. It also helps with magnesium to balance out the calcium from rock phosphate. The sulfur in this mineral is important for making particular amino acids. Because of this, sulfur is necessary to make many enzymes responsible for the production of molecules such as chlorophyll.
- It is a common claim that sulfur helps with the production of terpenes or other odors in Cannabis. I have not been able to find any publications on this topic, but I suspect that sulfur is more responsible for odors such as thiols or mercaptans  or are used in enzymes responsible for secondary metabolite production.
- Add langbeinite at a rate of 1/2 pound per cubic foot of potting mix (about 1/2 cup)
Now that we have amended our potting mixes with plenty of minerals with micronutrients, phosphorous, and potassium, we need to add organic amendments with plenty of Nitrogen and organic matter for the soil microbes to feed on.
- Add 1/2 cup per cubic foot of potting mix.
- Ascophyllum nodosum kelp meal has an NPK ratio of 1-0.1-2. Besides containing organic matter for microbes to feed on and some available plant nutrients, it may contain fatty acids that stimulate plant disease resistance and also may stimulate plant production of growth-promoting phytohormones.
Fish Bone Meal and Fish Meal
- Fish bone meal is what it sounds like, ground up fish bones.
- It has an NPK ratio of 4-12-0 but it also has about 14% calcium
- Add at a rate of 1/4 cup per cubic foot of potting mix
- Fish meal has an NPK ratio of 8-6-0. It provides great food for microbes and has a good amount of Nitrogen
- Add at a rate of 1/4 cup per cubic foot of potting soil
Neem Seed Meal
- Neem seed meal is the ground up byproduct of neem seeds after the cold press extraction of neem oil.
- Neem seed meal has an NPK ratio of 6-1-2
- Neem seed meal is one of my favorite additions to soil because it acts as an antifeedant and growth cycle disruptor for various insect pests that complete part of their life cycle in the soil such as fungus gnats 
- Add 1/2 cup per cubic foot of soil mixture
- Amazon Links:
Feather Meal and Blood Meal
- Feather meal has an NPK ratio of 12-0-0. It is a great source of nitrogen that releases over time through microbe activity
- Add feather meal at a rate of 1/4 cup per cubic foot of potting mix
- Blood meal also has an NPK ratio of 12-0-0, but the nitrogen from blood meal releases more quickly than the Nitrogen in feather meal.
- Add blood meal at a rate of 1/4 cup per cubic foot of potting mix
- Crab meal has an NPK ratio of 4-3-0. It also has a calcium content of 14%
- Crab meal also has chitin, similar to insect frass and may help prime plants for disease resistance.
- Add 1/2 cup per cubic foot of soil
- Crab meal also contains calcium carbonate which can help buffer the soil pH
- Alfalfa meal is a well-rounded organic amendment that has all of the micronutrients and macronutrients required for plant growth. It is a great food source for microbes.
- Alfalfa Meal has an NPK ratio of 2.5-0.5-2.5. Add alfalfa meal at a rate of 1 cup per cubic foot of soil.
- Composed mostly of calcium carbonate, carbonate ions help buffer the soil pH
- Oyster shell also improves soil tilth and drainage
- It provides slow release calcium
- Add 1 cup per cubic foot of potting mix.
For a cheaper potting mix with fewer ingredients, I recommend using the following amendments (not including the microbial amendments mentioned):
- Azomite- at previously mentioned application amounts
- Rock Phosphate- at previously mentioned application amounts
- Langbeinite- at previously mentioned application amounts
- Neem Seed Meal (Mostly for the insect control)- Add at 1/2 cup per cubic foot of soil
- Bio-Live from Down to Earth Amendments that contains: Fish Bone Meal, Fish Meal, Alfalfa Meal, Crab Meal, Shrimp Meal, Langbeinite, and Kelp Meal– Add 1 pound (3.5 cups) per cubic foot of soil mixture
- Crushed Oyster Shell- For buffering the soil pH. Add 1 cup per cubic foot of potting mix.
After mixing your potting mix with the base, microbial amendments, mineral amendments, and organic amendments together and filling your pots, I recommend top dressing your containers with a thin layer of compost (1/4-1/2 inch) and adding your earthworms to your pots.
- If you are growing outdoors and it is after harvest/before planting season, I recommend planting a cover crop mixture to grow over the winter. You can find a variety of mixtures online, but I recommend having a mixture of legumes, brassicaceous plants, and grasses.
- Following this, I recommend applying a mulch on top that the sprouts can grow through such as a layer of straw about an inch thick and not too dense.
- After winter, before planting, chop down the cover crop, smother the residue in another layer of compost, sprinkle a small amount (around 1/4 cup per cubic foot of soil) of Bio Live on top, transplant your Cannabis plants, and add another layer of straw mulch on top.
- If you are growing indoors or want to plant immediately, You can try an intercropping technique.
- Transplant your Cannabis plants into the pot. Seed the pot with your cover crop except for a 4 inch radius around your plant (larger radius is better if you have the space) Apply a straw mulch on the soil covering the seeds and the soil under the transplant.
- Of course, the intercrop will use some of the nutrients in the soil, but there should be a net gain of Nitrogen due to the legumes after a period of time. They should also benefit the soil structure and will support microbes in the soil.
- Don’t forget to add a straw mulch. If your legumes have been growing for at least 2 months, you can consider chopping them down and smothering plant residues with compost to reduce nutrient competition during flowering. If your vegetative cycle is short (under 2 months), it may be a good idea to thin the legumes but leave them through flower.
In addition to cover crops and compost, it is important to follow a fertilizer regimen. After transplanting my Cannabis plant, I add at least one additive every watering
- Seed Sprout Tea- Seed sprout tea has become quite popular recently. Claims are frequently made that these teas provide active amounts of plant growth hormones and digestive enzymes. Unfortunately, most of the claims that I see don’t have citations of where they got this information.
- Of course, growth hormones have been isolated from various plant seeds and/or sprouts (such as zeatin from corn kernels), and have even been shown to be biologically active at certain concentrations
- For instance, corn grain extract has been shown to be useful at promoting root growth of banana propagules and potato seed [7, 8].
- Seed sprouts also have high levels of enzymes necessary for producing energy from starch reserves in the endosperm, and there is some evidence that enzymes can be stable in soil. The main things that will deactivate enzymes in soils are extreme pH, high temperatures, and protease enzymes from microbes [9, 10].
- I only use corn because I can get organic corn kernels from the grocery store for very cheap. I sprout 2 oz. of dry kernels by soaking them for 12 hours in water and then putting them in a jar with a breathable lid after draining the water. It can take up to 2 weeks to get usable sprouts. Next, I blend the sprouts in a couple of cups of water, and dilute it to 2 gallons of water. I also add in 1 tablespoon of kelp meal. I don’t filter out the seed material, but you can if you are worried about saving space in the pot.
- I add this tea every third watering.
- Other seeds such as barley or alfalfa may produce sprouts with other beneficial enzymes and hormones.
- Compost Tea
- I outlined my compost tea recipe earlier, I also apply this every third watering, but not at the same time as the seed sprout tea.
- Coconut Water
- Contains mincronutrients including potassium, calcium, and magnesium. Also contains phytohormones such as cytokinins.
- Add 50 mL per gallon of water
- I add this every third watering as well, not on the same days as the compost teas or seed sprout teas.
- Dry Amendments
- During Flower, I do 2 top dresses. I add 1/2 cup of Down to Earth’s Rose and Flower mix at first signs of flower and add another 1/2 cup 4 weeks after this.
- I also reapply Bio Live (1/2-3/4 cup per 10 gallons of soil) to the soil surface after transplanting new clones in to the soil. If you are transplanting a new clone or seedling in directly after harvest, do just 1/2 cup. If you grew a cover crop all winter, add up to 1 cup.
- After harvest, top dress with a small amount of compost and azomite, plant your cover crop, and begin the cycle again.
With all of the additions, it should be obvious that you should start with a large pot with plenty of empty space. I recommend starting with at least 15 gallon fabric pots and filling them up with 10 gallons of starting mix. This will last you many years of no-till growing without ever having to take the soil out of your pots.
I believe the ideal pot size is 20 gallons for indoor growing with 15 gallons of starting mix, but the absolute minimum starting amount is 10 gallons of soil. When you eventually run out of room or your soil has been mostly humified, I do recommend taking the soil out of the pots, adding more rice hulls and minerals, and splitting the soil up in to more pots or taking excess soil and adding it to a garden.
For outdoor growing in pots and very long growing seasons, you benefit by using up to 1,000 gallons of soil, but you can grow smaller plants in as little as 20 gallons. However, I believe too many outdoor growers don’t take advantage of growing directly in-ground. A lot of money can be saved by amending planting holes with aeration, dry amendments, and compost without paying a ton of money for coir and peat based soils. After amending the holes, you can treat the area with no-till practices and keep your holes healthy with cover crops, composts, mulches, and dry amendment top dresses. Plant roots won’t be restricted by volume, water usage is more efficient, and it is better for the environment than relying on shipping in tons of potting mix.
- García, A. C., Olaetxea, M., Santos, L. A., Mora, V., Baigorri, R., Fuentes, M., Zamarreño, A. M., Berbara, R. L. L., & Garcia-Mina, J. M. (2016). Involvement of Hormone- and ROS-Signaling Pathways in the Beneficial Action of Humic Substances on Plants Growing under Normal and Stressing Conditions. BioMed Research International, 2016, 3747501. https://doi.org/10.1155/2016/3747501
- Baveye, P. C., & Wander, M. (2019). The (Bio)Chemistry of Soil Humus and Humic Substances: Why Is the “New View” Still Considered Novel After More Than 80 Years? . In Frontiers in Environmental Science (Vol. 7, p. 27). https://www.frontiersin.org/article/10.3389/fenvs.2019.00027
- Lehmann, J., & Kleber, M. (2015). The contentious nature of soil organic matter. Nature, 528(7580), 60–68. https://doi.org/10.1038/nature16069
- Lehmann, A., Zheng, W., & Rillig, M. C. (2017). Soil biota contributions to soil aggregation. Nature Ecology & Evolution, 1(12), 1828–1835. https://doi.org/10.1038/s41559-017-0344-y
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- Rice, S., & Koziel, J. A. (2015). Characterizing the Smell of Marijuana by Odor Impact of Volatile Compounds: An Application of Simultaneous Chemical and Sensory Analysis. PloS One, 10(12), e0144160–e0144160. https://doi.org/10.1371/journal.pone.0144160
- AndiIlham Latunra, Baharuddin, M. T. (2016). In Vitro Effects of Young Corn Extract on Propagules Stage of Musa acuminataColla. International Journal of Recent Engineering Research and Development, 1(5).
- Ulfa, F., Sengin, E. L., Baharuddin, Syaiful, S. A., Sennang, N. R., Rafiuddin, Nurfaida, & Ifayanti. (2013). Potential of Plant Extracts as Growth Exogenous Regulators of Potato Seeds. International Journal of Agriculture System, 1(2), 98–103. http://pasca.unhas.ac.id/ijas/pdf/2) IJAS Vol. 1 Issue 2 December 2013.pdf
- Frankenberger, W. T., & Johanson, J. B. (1982). Effect of pH on enzyme stability in soils. Soil Biology and Biochemistry, 14(5), 433–437. https://doi.org/https://doi.org/10.1016/0038-0717(82)90101-8
- Kandeler, E. (2015). Chapter 7 – Physiological and Biochemical Methods for Studying Soil Biota and Their Functions (E. A. B. T.-S. M. Paul Ecology and Biochemistry (Fourth Edition) (Ed.); pp. 187–222). Academic Press. https://doi.org/https://doi.org/10.1016/B978-0-12-415955-6.00007-4
- Dedyukhina, E. G., Kamzolova, S. V, & Vainshtein, M. B. (2014). Arachidonic acid as an elicitor of the plant defense response to phytopathogens. Chemical and Biological Technologies in Agriculture, 1(1), 18. https://doi.org/10.1186/s40538-014-0018-9
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What is a plant virus?
A virus is an infectious nucleic acid-based pathogen that is parasitic to the host. Essentially, it is a non-host genome that ‘hijacks’ the replication machinery of the host cell in order to amplify its genome. It also requires host enzymes to translate the viral transcripts into proteins that the virus uses to form a protective coat and move within the host/spread to alternate hosts such as insects. A virion is a viral genome encapsulated in a protein coat. Some viruses will also have a fatty membrane surrounding the protein capsid.
There is some argument as to whether viruses can really be classified as ‘alive’. Though they have a genome, they do not have their own metabolism or replication abilities. They can really be thought of as ‘selfish genes’, meaning that they are a group of genes that evolved together in order to perpetually reproduce. They are truly an insight into the evolution of life: how replication is always selected for, seemingly without purpose in the case of a virus.
Since they are obligate parasites, viruses generally have evolved to not kill their native host before they can reproduce enough. In the case of insect vectored viruses, this may mean interacting with the plant in such a way that the metabolism of the plant becomes more attractive to the insect vector. In the case of seed-transmitted viruses, it may mean inducing early flowering to shorten the period of time that the virus is restricted to the host, one can think of many ways that a virus can affect the phenotype of a plant to favor its spread while harming the agricultural value of the crop.
The very first virus to ever be discovered was actually a plant virus, and one that has had some amount of interest within the Cannabis community, tobacco mosaic virus (TMV). We will discuss TMV further, but for now I will just say there is little evidence of it causing disease symptoms in Cannabis. Different plant viruses are transmitted through different means. Plant viruses are usually spread by an insect/nematode vector, through seed, through pollen, or are mechanically transmitted (usually in the setting of human agriculture).
- Some plant viruses are limited to the plant phloem and cannot infect epidermal/mesophyll cells.. These viruses generally cause ‘yellows’ symptoms, a faily even chlorosis
- Sap-transmissible viruses can infect epidermal/mesophyll cells and generally cause mosaic and mottling symptoms
Generally, the phloem limited viruses are semi persistent (travel to the insect vector’s foregut) or persistent viruses (travel to the insect vector’s haemolymph and salivary glands) that are vectored exclusively by phloem-feeding insects such as aphids, whiteflies, or leafhoppers. Sap-transmissible viruses commonly cause mosaic and mottling symptoms on host tissue, whereas phloem-limited viruses tend to be ‘yellowing’ type viruses with more uniform symptoms. Many viruses are multipartite, meaning they have segmented genomes that are encapsulated separately but must all be present in the host in order to cause disease.
Persistent Circulative plant viruses are able to enter the haemolymph (analagous to blood) and salivary glands of their insect vectors/seconday hosts, but they do not replicate within the host cell. Peristent Propagative viruses are able to actively reproduce within the insect and also infect the insect haemolymph and salivary glands. Circulative viruses remain viable within for long periods of time (often the lifespan of the insect).
Semi-persistant viruses do not enter the haemolymph of the insect vectors, but do travel to the foregut. Some yellows and some mosaic viruses are semi-persistent. They do not take as long to acquire for the insect vector compared to circulative viruses, and the latent period is very short. They remain viable for inoculation for a few days.
Non persistent viruses
Nonpersistent viruses are only carried on the stylets of sucking/probing insects and remain viable for a few hours and must be quickly transmitted to a new host.
What is a plant viroid?
All known viroids infect plants, and most are pathogenic. Viroids are very similar to viruses except that they do not have a protein coat, they are simply self-replicating and transmissible nucleic acid pathogens. Viroids are usually spread through aphids or mechanical transmission.
What is a plant phytoplasma?
Phytoplasmas are very small mollicutes (bacteria that lack a cell wall). They are not much like viruses biologically, but I am including them in this particular article because the symptoms many phytoplasmas cause can be similar to those caused by viruses. In addition they are unculturable and are insect-transmissible (mostly by leafhoppers, planthoppers, and psyllids). Phytoplasmas act very similarly to circulative viruses within the insect vector, they enter the insect haemolymph and colonize the salivary glands.
What viruses have been reported in Cannabis?
In my opinion, viruses are significantly underdiagnosed in Cannabis and are the least understood of all pathogen classes in the species. There appears to be more research into Cannabis diseases recently, but up until the recent past, only 5 viruses were reported as commonly and naturally causing problems in commercial Cannabis production. These viruses are: Hemp streak virus (HSV), alfalfa mosaic virus (AMV), cucumber mosaic virus (CMV), arabis mosaic virus (ArMV), and hemp mosaic virus (HMV). As far as I am aware, HSV has only been reported on fiber cultivars in Europe. HMV, on the other hand, has been reported in some drug cultivars in Pakistan. AMV, CMV, and ArMV have been reported on European hemp. There is next to no information in regards to infection of modern North American drug cultivars by these viruses
In 1971, Hartowicz et al. screened 22 common plant viruses and found that over half of them were able to infect Cannabis, but only 8 actually caused symptoms. Keep in mind that these were all manual innoculations and may not actually be vectored to Cannabis in nature. Of the manually inoculated viruses, 5 of them caused severe mosaic symptoms and plant stunting: Tobacco Ringspot Virus (TRSV), Tomato Ringspot Virus (TomRSV), Tobacco Streak Virus (TSV), and Cucumber Mosaic Virus (CMV). Two of the viruses caused mosaic symptoms but did not cause heavy stunting: Alfalfa Mosaic Virus (AMV) and Eunoymous Rinspot Virus (ERSV). Elm Mosaic Virus (EMV) also caused some symptoms of necrotic flecking.
Most recently (2020), Beet Curly Top Virus (BCTV) was found to be a common and naturally occurring infectious agent on plants in Colorado . In 2019, Lettuce Chlorosis Virus (LCV) was found in Cannabis grows in Israel .
BCTV symptoms were first noticed in a field in CO in 2015. Leaves would begin yellowing in a mosaic pattern from the petiole to the tips of leaves. Plants with advanced symptoms displayed stunted growth, malformation of new leaves, leaf chlorosis, and necrotic flecking. BCTV was confirmed to be the causal agent by utilizing next generation total active RNA sequencing and using control plants to find the viral genome present. It was confirmed through PCR as well. This has been the method of discovering viruses in high value crops such as grapevine stock, and it appears to be utilized more for discovering diseases of Cannabis. BCTV is a Curtovirus within the Geminiviridae family. It is fairly unique because its genome is ssDNA instead of RNA. This means that it needs to utilize the host plant’s transcription and translation mechanisms. It has a monopartite genome that is encapsulated in a dual icosahedral capsule. It is the only virus in Cannabis that is transmitted by a leafhopper, Neoaliturus tenellus, native to the western USA. BCTV is a circulative nonpropagative virus, has a very broad host range, and can have significant impacts on yield. In some species , it can be transmitted by seeds, but this is unknown for Cannabis.
HSV is reportedly one of the most common viral diseases, at least in commercial hemp in Europe. However, researchers have been unable to identify any causal agent for the symptoms associated with Hemp Streak Virus. It has long been assumed that HSV is viral and transmissible, but more recent molecular studies suggest that an abiotic factor may be at play because no pathogenic viruses were found in symptomatic Cannabis plants from screening with targeted PCR reactions and RNA sequencing . It has been suggested that if this is indeed a viral disease, that it may also be responsible for certain leaf curling symptoms found on Hemp in Hungary in the late 1990s . It is also reportedly vectored by aphids and seed, but this has not been demostrated experimentally. Insect vectoring would certainly point towards HSV being a vectored pathogen .
I wonder if HSV symptoms may have been misdiagnosed mite damage, such as broad mites or russet mites which produce symptoms very similar to those described of HSV. Some mites may even be vectored by insects by hitching a ride The following image is taken from a plant with broad mites, and displays symptoms very similar to those described from HSV:
***Please be aware that there are many online sources that believe that Sunn-Hemp Mosaic Virus is the same as HMV and claim that Sunn-Hemp Mosaic can infect Cannabis. There is absolutely no evidence of HMV being caused by SHMV, and this misidentification does not even align with reports of HMV being only an insect-vectored disease. Sunn-hemp is a legume and is a completely different plant than hemp, but the name seems to be confusing for some. Reputable websites including Dinafem’s website seem to equate Sunn-Hemp virus to Hemp Mosaic Virus without justification.***
The causal agent of HMV is also unidentified, but has been suggested to be a Cucumovirus or a Nepovirus. It is reportedly vectored by aphids, but I have seen one report that it was vectored by onion thrips, which doesn’t make sense as no Cucumoviruses or Nepoviruses are vectored by thrips. In one experiment, an Argentine sunflower virus was inoculated onto hemp and the plant contracted HMV-like symptoms and could be transmitted by aphids .
HMV causes chlorotic leaf lesions that expand, become necrotic, and can kill leaves. HMV may cause leaf enation, leaf curl, bunchy top, and reduced leaf size. Severe symptoms may look something like this:
This is simply a plant with symptoms consistent of those with HMV, it is not a confirmed diagnosis.
Cannabis cryptic virus
Another virus known as Cannabis cryptic virus has been identified to be ubiquitously present in Cannabis without causing any disease symptoms [8, 9]. Virions were visualized and sequences were obtained, but viral presence does not seem to be correlated to symptoms. The cryptic virus appears to be a Partitivirus which is likely seed transmissible .
Lettuce Chlorosis Virus
Recently (2019), a new virus was reported in Cannabis. LCV is a Crinivirus within the Closteroviridae. Lettuce Chlorosis Virus (LCV) was reported as causing interveinal chlorosis, leaf brittleness, and necrotic spots in licensed Cannabis grows in Israel. The virus was shown to be transmissible by the whilefly species Bemisia tabici . LCV was not found to be seed transmissible, but can be transmitted by vegetative propagation.
AMV is an Alfamovirus within the Bromoviridae. AMV was first identified on hemp in Germany through sap transmission tests. It is most commonly spread through aphid and seed/clone transmission, but can also be spread through dodder or root grafts. It is a ssRNA virus and the genome is split between 4 virions.
Symptoms include leaf chlorosis in a mosaic or mottle pattern, sometimes presenting as a gray mosaic. Young leaves may have strange morphology (puckering).
An example of AMV on tobacco is shown below, demonstrating the leaf puckering and chlorotic mosaic:
The following Cannabis leaf shows some very minor puckering as well as mosaic symptoms:
ArMV is a Nepovirus within the Secoviridae family. Both CMV and ArMV are tripartite viruses that cause similar symptoms. ArMV has been described as causing chlorotic spots and stripes, and has also been described as displaying symptoms of chlorotic ‘check mark’ shapes . It appears to have a negative impact on the growth of the plant as well. ArMV has a broad host range. It infects many vegetables, and also infects hops. It is primarily transmitted by nematodes, but may also be seed transmissible in many species and can be transmitted by vegetative propagation (cloning).
CMV is a tripartite Cucumovirus. It is within the Bromoviridae like AMV. It has a very broad host range and has been found on dicots and monocots including various vegetable, ornamental, and grass crops . It is vectored by aphids and is also seed transmissible in many species. It will also be transmitted by cloning. This will be difficult to distinguish from other mosaic viruses, but has been described as having light green ‘check mark’ chlorosis, similar to that described for ArMV . An image of such symptoms on Cannabis is shown below, but I am not sure which virus is causing this:
Can TMV Infect Cannabis?
In Hartowicz et al., they also mention that TMV is indeed able to infect Cannabis, but it did not cause any noticeable symptoms [2, 3]; rather, Cannabis appears to act as a carrier for the virus. They confirmed the presence of TMV in nonsymptomatic Cannabis by back-inoculating to indicator plants. It seems that in almost every Cannabis forum site or popular media site, it is commonly reported that TMV can cause symptoms in Cannabis. However, these claims are usually uncited and unsubstantiated. In fact, in the cases of people actually posting serological test results of plants with mosaic symptoms, I have never seen a positive result for TMV. Of course, different cultivars of Cannabis may react differently to infection by TMV and some may actually produce noticeable symptoms that were not seen in experiments conducted on hemp cultivars. While it may be possible for TMV to cause symptoms in Cannabis, based on published information, it is more likely that TMV is symptomless in Cannabis and that mosaic symptoms are caused by viruses that have been reported to affect the health of Cannabis plants.
A farmer once reported that he infected a Cannabis plant with a tobacco virus that caused stunting and mosaic symptoms, but it is speculated that virus transferred was Tomato ringspot virus, Tobacco rinspot virus, or Tobacco streak virus as these can all infect Cannabis in cases of mechanical inoculation and are sap transmissible .
What Insects Vector Viruses in Cannabis?
According to Ceapoiu (1958), the worst vectors of Cannabis viruses are bhang aphids (Phorodon cannabis), greenhouse whiteflies (Trialeudodes vaporariorum), onion thrips (Thrips tabaci) and green peach aphids (Myzus persicae). P. cannabis has been shown to vector at least 2 viruses to Cannabis: CMV and AMV . P. cannabis has also been shown to vector Pea Mosaic Virus in the lab , but I am unaware of any cases of natural infection of PMV in Cannabis. In 1955, P. cannabis was also reported to be the vector for HSV , though as mentioned earlier, it is still unknown if HSV is actually a viral disease. P. cannabis has also been reported to vector Hemp Mosaic Virus (HMV), though molecular evidence of this is lacking . Much like HSV, the causal agent of HMV has not been identified and confirmation of insect vectoring has not occurred .
Despite T. vaporariorum being reported as a vector for Cannabis viruses, there is no information in the literature as to which Cannabis diseases it may vector. The only viruses shown to be vectored by T. vaporariorum in plants are within the Crinivirus genus. The only confirmed crinivirus in Cannabis is the recently reported Lettuce Chlorosis Virus, which was reported B. tabaci as a likely vector, but T. vaporariorum may be able to vector this virus as well.
In regards to thrips, the two main species affecting Cannabis are onion thrips (Thrips tabaci) and western flower thrips (Frankliniella
occidentalis) . Again, there is no evidence here of whether or not any viruses are actually vectored by thrips in Cannabis. The only viruses known to be vectored by thrips are within the Tospovirus genus, none of which have been reported in Cannabis. Of course, this does not mean that tospoviruses don’t affect Cannabis or that thrips do not transmit viruses, just that there is not yet evidence of this.
Green peach aphids (Myzus persicae) have also been reported as vectors in Cannabis. They are at least capable of transmitting AMV, CMV [11, 12], but it is unknown what other viruses they may vector.
In regards to ArMV, it appears that it is most readily vectored by dagger nematodes within the genus Xiphinema . However, no Xiphenema species have every been reported on Cannabis. Despite an uncited claim on Wikipedia, ArMV has not been demonstrated to be vectored by insects such as aphids or whiteflies, but may be able to be transmitted by other genera of nematodes such as needle nematodes withing the Longidorus genus .
BCTV is the only Cannabis virus to be vectored by a leafhopper (specifically Neoaliturus tenellus, at least that is the only reported vector of BCTV on the wide range of hosts it can infect.
What to do if you suspect your plant(s) have a virus
First off, there is no easy answer to this. There is no ‘cure’ to viral diseases; there is no spray that will eliminate the problem for you. Growers have to be on top of insect control, proper sanitation, and culling of infected plants in order to prevent infection in the first place. Of course, sourcing virus-free growing stock (seeds or clones) is of upmost importance.
Second, it is important to have an IPM program in place to control insects that may be vectors for viruses. I would say that aphids and whiteflies are the two biggest threats as insect vectors.
For all pathogens covered in this article including viruses, viroids, and phytoplasmas, one of the simplest things you can do to prevent spread is to practice sanitation such as sterilizing your tools with 70% alcohol between each cut or plant, to sterilize your indoor facility after each grow, and to practice cleanliness in your grow areas and what you wear in your grow areas. It is never a bad idea to shower before going into your grow area, wear scrubs that you wash frequently, and have dedicated boots that your sterilize the soles of frequently.
What Viroids Infect Cannabis?
As of now, the only viroid that has been reported in Cannabis is the Hop Latent Viroid (HpLVd) . It is a 256 bp circular RNA within the Cocadviroid family. It was identified by Dr. Jeremy Warren at Dark Heart Nursery through total RNA sequencing of symptomatic plants. It was confirmed as the disease-causing agent through development of infectious RNA constructs and RT-PCR of infected plants. In Cannabis, the disease is known Cannabis Dudding. Plants with Dudding may have reduced vigor, smaller size, more stretch and weaker branching, and small leaves in vegetative growth:
In flower, buds lack trichome development, terpenes are reduced, bud is more leafy and more airy (larfy), buds can be irregularly shaped, chlorosis and leaf death may occur, and buds appear behind schedule. In the following picture, the cola on the left is from a healthy plant, and the cola on the right is from a dudded plant.:
The infected bud does look frosty in this picture, but the lack of density and high amount of vegetative growth is evident. The following picture shows dried buds from the same plants:
Dark Heart nursery has been able to provide clean stock by doing tissue culture to produce mother plants. This is a process where cells from the very tip of the apical meristem are cultured and generated into a new plant, and many viruses and viroids have not yet been able to infect the newest growth of plants. HpLVd does not appear to be insect-vectored, but is transmitted through mechanical means, usually by using non-sterile shears and tools on plants. It may be transmissible by seed at low rates as well.
To prevent HpLVd, it is important to begin with clean stock and use proper cultural controls in your grow including spraying tools with 70% ethanol in between each plant.
What Phytoplasmas Infect Cannabis?
Cannabis is susceptible to a wide range of phytoplasmas.
- In 2007, hemp witches’ broom in China was identified as a phytoplasma in the Elms Yellows (EY) group .
- In 2011, hemp witches’ broom in Iran was confirmed to be caused by a phytoplasma in the stolbur group. .
- In 2015, Cannabis sativa was identified as a host for the asteris group of phytoplasma in India .
- In 2019, C. sativa was identified as a host for the trifolii group of phytoplasma in NV, U.S.A. .
All of these phytoplasmas were identified through nested PCR using conserved phytoplasma primers followed by sequencing.
Phytoplasma symptoms in Cannabis can include:
- A high amount of branch proliferation from a branch node
- Shortened internode spacing
- Small leaves or leaf dieback
- Phyllody (abnormal development of flowers as leafy structures.
What vectors phytoplasmas in Cannabis?
The primary vectors of phytoplasmas are leafhoppers, planthoppers, and psyllids .In India, the leafhopper Hishimonas phycitis was found to vector the asteris phytoplasma and is the putative vector in Cannabis . Elms yellows diseases are usually transmitted via leafhoppers. Stolbur group phytoplasmas are frequently transmitted by planthopper species. Trifolii group phytoplasmas are frequently transmitted by leafhopper species.
Generally, Phytoplasma diseases are controlled by preventing infection through insect control. Some phytoplasmas can be seed transmissible, but this has not been demonstrated in Cannabis. Phytoplasma diseases may be able to be controlled through application of antibiotics such as tetracycline and rifampicin , but these are not approved for commercial production anywhere that I am aware of. A plant genotype can be recovered through meristematic tissue culture, but once infected, a plant will not produce well or have marketable bud.
In short, viruses, phytoplasmas, and viroids are mostly controlled through prevention and include virus free seed, tissue cultured clones, and insect control. One a plant has one of these problems, it will not be able to be treated and will not be worth producing with. In fact, it is important to cull any plants you find with these symptoms unless you want to preserve the plant’s genetics, in which it may be worth having the plant go through tissue culture.
I hope this article shed light on the confusion surrounding these types of diseases in Cannabis and addressed some of the misinformation commonly spread on forums and message boards.
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Bud rot is one of the most devastating things a grower can deal with. This day and age, when a positive microbial test can cause a grower to lose their crop, there is zero tolerance for fungal pathogens. Bud rot can be especially devastating indoors. If there are conducive conditions and susceptible varieties in close proximity, spores can quickly and easily spread throughout the entire grow groom.
What causes bud rot?
Bud rot is cause by Botrytis cinerea. which is the name for the anamorph (asexual form) of this fungus; sexual reproduction is rare in this species. It is an aggressive necrotrophic plant pathogen that causes disease on over 1,000 crops . It is responsible for up to $100 billion in annual crop losses worldwide . B. cinerea can infect a wide range of tissue types including fruits, flowers, leaves, stems, and storage tissues. It is a serious issue for both preharvest and postharvest (i.e. that characteristic fuzzy gray mold that grows on your strawberries that you get from the supermarket). However, sometimes this fungus can be desirable. Some grape growers utilize the fungus on wine grapes to produce ‘noble rot’ wines. These high value grapes have enhanced ripening responses and may have greater levels of sugars and flavor compounds. In Cannabis, bud rot is never desirable.
Luckily for us, B. cinerea is one of the best studies fungal organisms in the history of plant pathology. It is often used as the model organism for studying necrotrophic plant pathogens. B. cinerea is not just a mold that grows passively on your buds like Penicillium does on bread, it employs mechanisms to both suppress plant defenses and kill plant tissues.
Powdery Mildew and Bud Rot are Both Labeled as ‘Mold’. How do they Differ?
Let us compare B. cinerea to one of the other most damaging fungal disease of Cannabis: powdery mildew (PM). PM is a biotrophic fungus, meaning that it is an obligate parasite that requires living cells to extract nutrients from. Because of this, PM species are required to evolve strategies to specifically overcome the defenses of particular plant species and manipulate plant immune responses (this leads to relatively narrow host ranges for any given species of PM fungi). B. cinerea on the other hand, is much less precise in its pathogenicity strategies.
It tends to rely much more heavily on the secretion of plant toxins and enzymes to kill and digest plant tissues as opposed to suppressing cellular immune responses and maintaining host viability. The fungus can then feed on the dead tissue without having to defend itself against further attack by the plants. This partly explains the much wider host range of B. cinerea, as many of the toxins and cell wall degrading enzymes will ubiquitously kill a wide variety of plant tissues from a wide variety of plant species. As a rule of thumb, biotrophic plant pathogens rely most heavily on secreted effector proteins that manipulate the host plant’s defense response, while necrotrophic plant pathogens rely most heavily on phytotoxins, cell wall degrading enzymes, and other extracellular enzymes.
How does B. cinerea infect plants?
Though B. cinerea has long been considered a necrotroph, newer literature is beginning to show that B. cinerea might be better classified as a hemibiotroph, meaning that it begins its life as a parasite and later transitions to a necrotrophic lifestyle . In order to succeed as a necrotroph, a fungus first has to gain a foothold in the plant, and it does this by suppressing plant immune responses for a short period of time until enough fungal biomass has accumulated in the host plant to successfully switch to a necrotrophic lifestyle and kill surrounding plant cells. For Botrytis, this is not a long time, and the very first symptoms of infection by Botrytis are necrotic lesions that begin spreading, showing that the parasitic phase is quite short-lived indeed.
So how do we know that there is a parasitic phase? As described earlier, necrotrophs must kill plant cells to feed on. However, in initial infection events, Botrytis actually interferes with the plant’s programmed cell death immune function known as the hypersensitive response (HR). The hypersensitive response is a common way for plants to defend against pathogen invasion by killing cells under attack and bolstering the defenses of neighboring cells. One might think that this is beneficial to necrotrophs, but it appears that in order to initially establish itself in the host, Botrytis must suppress PCD by secreting effector proteins and small RNAs that interfere with normal plant responses . Secreted small RNAs from Botrytis ‘hijack’ the RNA interference system in plants and help silence genes involved in plant immunity . Another factor that challenges the notion of Botrytis as an obligate necrotroph is the fact that it can sometimes colonize plants asymptomatically, although I am not aware of this being demonstrated yet in Cannabis . This raises the question of whether B. cinerea is a fairly ubiquitous endophyte that only causes serious disease when both the environment and host are conducive to disease development (there may be cases where a necrotrophic phase is not tiggered).
What is going on biochemically during plant infection?
After accumulating biomass within the host, Botrytis can switch to a necrotrophic lifecycle, and it employs a variety of pathogenicity tactics, including inducing the HR response in plant cells rather than suppressing it. For instance, it begins to produce macromolecular toxins that can induce plant cell death [6, 7]. One such metabolite, oxalic acid, may induce these responses through acidification of plant tissues, leading to production and activation of various fungal enzymes including pectinases (pectin degrading enzymes), laccases (lignin-degrading enzymes), and proteases (protein degrading enzymes) [8, 10]. Furthermore, oxalic acid can weaken cell walls by chelating calcium ions, which is necessary for forming calcium pectate in plant cell walls . In a closely related necrotrophic species, Sclerotinia sclerotiorum, oxalic acid has been shown to trigger host HR response . Botrytis has also been found to produce plant hormones and/or induce plant hormone changes including ethylene and abscisic acid levels, both of which have been found to make plants more susceptible to Botrytis infection [11, 12, 13, 14]. Furthermore, different effector proteins are expressed at different points of infection. BcSpl1 is one such protein that is more abundant in later infection stages and is an important virulence factor for B. cinerea, inducing cell death . Another effector, BcIEB1, may help protect the fungus from a host-produced antifungal, osmotin . Most putative effector genes in Botrytis are not well characterized, and some effectors may not contribute to virulence due to host plants evolving ways to recognize the effectors and trigger defense responses, such as is the case with BcNEP1 and BcNEP2 .
What is the life cycle of Botrytis?
As previously mentioned, Botrytis usually functions as an asexual fungus. In the spring, fungal mycelium and sclerotia (defined later) begin to grow/germinate and make conidiophores that produce asexual spores known as conidia. Spores are usually dispersed through wind and water. In the presence of moisture (unlike PM, Botrytis spores require free water to germinate), the conidia (asexual spores) germinate and produce structures known as appressoria to penetrate plant cuticles through immense pressure buildup. The invasive hyphae can then grow within the extracellular space of plant tissues.
As previously discussed, they initially suppress plant responses and attempt to remain undetected while accumulating biomass. After a short period of growth, the fungus switches to a necrotrophic phase and begins to kill and rot proximal cells. After this initial foothold is established, the hyphae can continue to invade the host tissues and the fungus can produce more conidiophores on the plant tissue surface. This acts a secondary inoculum source that amplifies the spread of the disease within a given season (polycylclic disease). Come winter, the fungus begins to produce structures called sclerotia that are dense survival structures composed of highly melanized mycelium. Furthermore, mycelium can survive within dead plant tissue through the winter and produce conidia the following spring. The sexual phase is rare, but occurs when the sclerotia produce a fruiting body known as an ascocarp (analagous to a mushroom) that releases sexual spores known as ascospores.
What does bud rot look like in Cannabis? How do I diagnose it?
This disease displays both symptoms (visible effects on plant tissues) as well as signs (seeing the actual fungal tissue). However, long before you see a tuft of gray mold coming out of your buds, you will notice flagging of plant colas. This means that the tops of your colas will begin to dieback, leaving brown tissue as shown below:
You might also first notice symptoms on foliage. If your plant is not showing signs of nutrient burn or natural senescence of late flower, it can be easy to spot random leaves that have turned brown and dry, such as below:
The picture above shows signs of visible mycelium within the bud. However, if bud rot continues to progress, aerial mycelium will begin to grow as tufts out of your buds such as this:
I personlly lost a plant to bud rot when I first started growing because I attempted to grow in an unvented tent placed in a closet indoors without a humidifier, so I know how devastating it can be to see something like this on what you have worked so hard to produce.
How Do I Prevent Getting Bud Rot?
As with all plant diseases, it is important to keep in mind the disease triangle, which states that for a disease to develop, there needs to be a susceptible host, a conducive environment, and a virulent pathogen.
Unless you are in a perfectly enclosed environment, you have to assume that Botrytis spores are fairly ubiquitous in the environment. If you are within a contained grow, you have a bit of control over this factor, and you can do a few things to try to keep potential inoculum levels low.
- UV lamps: Using supplemental UV lighting is recommended for increasing secondary metabolite production in plants (for more info on this, check out my post on grow lights under the Home Growing Made Easy page). It also has the added benefit of helping reduce spore inoculum levels. Beyond your supplemental UV lights, you can add UV-B lamps within the ducting in your sealed room to help sanitize the air.
- HEPA filters: Using HEPA filters in your grow room will help remove spores from the air through active filtration.
- Ozone: Ozone generators may be helpful in reducing the growth of Botrytis. One study found reduced growth rates in air with 1.5 uL/L of ozone . However, ozone may have human health risks and may have negative effects on your plants at high concentrations.
- Use dedicated clothing for your grow space can help prevent you from bringing in spores from the clothes you wear out in public. If you really want to take an extra step, you can invest in some Painters’ coveralls.
- Have good cultural practices including sanitizing hands, tools, shoes etc. regularly. Always sterilize your tools frequently and sterilize your entire grow area after each harvest using techniques such as burning sulfur, spraying with 10% bleach, or Quat soaps. If you have infected tissues, do not mulch it into your soil at the end of the grow. cut off infected buds as soon as possible.
Most people do not select their plants based on their resistance to bud rot, though it can certainly be a factor. Growers (particularly indoor growers) generally choose what they are growing based on the market demand of what consumers want to smoke, and they rely on other practices to try to prevent their crop from getting bud rot. However, despite the lack of studies of this disease in Cannabis, one can look to community forums for discussion of strains that may show promise of being relatively resistant to bud rot.
In general, it is useful to consider what environments favor bud rot development. Landrace strains from regions of the world that have favorable conditions for bud rot likely experience the most pressure from Botrytis and are the most likely to have evolved resistance to the pathogen. Based on this, it is likely that Cannabis landrace strains from equatorial, moderately warm, humid, and wet regions of the world have developed greater resistance to bud rot than landrace strains from cold, drier climates. This is basically opposite of strains with higher levels of PM resistance. For instance, strains with Afghani heritage generally are more PM resistant than many equitorial sativa strains, whereas these equitorial strains generally are more resistant to bud rot than Afghani Cannabis plants.
One reason that I would recommend sativa strains that evolved in wet, moderate-warm climates is obvious from their plant structure: sativa buds are not as dense, internodal spacing is greater than indica plants, leaves are much thinner, and in general, the structure of these plants is conducive to high amounts of airflow and avoids wet microclimates within dense buds. Beyond the macro structure of the plant, there are likely more complex molecular interactions going on that contribute to a genotype’s resistance to bud rot.
It certainly is not a guarantee that growing a particular strain ensures bud rot resistance. It is also important to know that there can be genetic variability within a strain when it comes to disease resistance, and a bad enough environment may still be able to overcome resistant strains.
Examples of landrace strains with putative resistance to bud rot include:
- Durban Poison is a 100% inbred sativa plant from a wam, wet region of South Africa
- Brazil Amazonia is a landrace strain from one of the most conducive environments in the world to bud rot- the Amazon Jungle. However, it somewhat breaks the stereotype of what plants do best in these climates: it has an indica-like phenotype with denser buds and quicker flowering times than other strains on this list.
- Colombian Gold- Another landrace sativa from the tropics. Colombia is one of the most ecologically diverse regions in the world.
- Thai Sativa- from one of the most conducive regions in the world to bud rot, the Thai sativa is a very long-flowering strain with a classic sativa phenotype: lanky, thin leaves, large internodal spacing, buds not particularly dense.
These particular genetics are not really viable in a modern Cannabis market, aside from the occasional old-school stoner or a connoisseur of unique phenotypes. They are generally difficult to grow, low-yielding, and low-potency. However, there are many modern hybrid strains that have been bred from these landrace genetics.
Modern, commercially viable sativa strains:
- Ghost Train Haze
- Bruce Banner
- Amnesia Haze
- Hawaiian Maui Wowie
- Sour Diesel
- Laughing Buddha
- Bay 11
- G13 Haze
- Casey Jones
- Super Lemon Haze
- Red Dragon
- Cannalope Haze
- Jamaican Dream
- Pineapple Express
- Jack Herer
Again, I am not making the claim that you will not get bud rot if you grow with these strains, but it certainly can make a difference to pick a strain with genetic history of sativa landrace from wet climates. Strain selection is most important for those growing outdoors in a moderate temperature, wet climate. If you have a dry environment or can control your environment, it should be possible to prevent bud rot with proper defoliation, training, and grow area design/IPM)
As mentioned previously, B. cinerea thrives in moderate temperature, humid, and wet environments. If moisture from dew or rain accumulates on plant buds, it is likely that there is a microclimate that develops in your buds with up to 100% relative humidity and free water that can stimulate Botrytis conidia germination. While Botrytis can infect foliage and stems, causing lesions, it is far less common than flower infections. This is likely due to the conducive microclimate in buds as compared to the microclimates surrounding leaves and stems, though floral tissues may also have less defensive abilities than these other tissue types.
There are a few basic tips to help prevent bud rot.
- Have high amounts of air circulation in your growing area. Not only will it help with drying out wet tissues quickly, it will help disrupt microclimates that may form where there is stagnant air, reducing local spikes in humidity and temperature. Have a good amount of fans, and do proper training and defoliation in order to maintain that breezes can reach every part of your plant.
- In my opinion, temperature can be important, but it is less important than controlling humidity, moisture and airflow. For instance, if you are in flowering stage and are supplementing CO2 in a commercial grow, it is important to maximize your yield and to make the most of high light and CO2 levels, a temperature around 80F is ideal for plant growth while maintaining most of the monoterpenes in your buds. This temperature is conducive to bud rot, but a lower temperature may be even more conducive. It seems that for Botrytis, moderate temperatures around 60-75F are most conducive for plant infection . However, in greenhouse tomatoes, it was found that flower infections increased at higher temperatures (77F was the highest used in the study), whereas stem infections decreased. Based on this, it may be that Botrytis can thrive in Cannabis flowers at higher temperatures. I feel comfortable recommending that grow areas be kept at 77-82F during flower for the sake of plant evapotranspiration and growth, and I would not worry too much about temperatures dropping at night or reducing day temperatures in late flower to induce color changes and/or preserve some monoterpenes.
- Humidity: For those familiar with VPD (vapor pressure deficit), it is a figure that indicates the rate of evapotranspiration from plants dependent upon leaf temperature and local humidity. It is a good guide for grow room environmental settings to maximize plant growth, and a chart showing the ideal humidity level for different leaf temperatures is shown below:
After switching to flower, at 27C (around 80F), VPD recommendations would be to keep humidity around 55% until late flower, when it can be dropped to around 42%. For grapes, these recommendations should be sufficient to keep B. cinerea from taking hold. 65% RH or lower seems to be sufficient to prevent Botrytis in grape . In a study done one rose petals, an RH of 92% at 15C was required to cause lesions within 24 hours . However, it is imporant to keep in mid that Cannabis buds have a unique structure that can have very humid microclimates in the buds compared to these other plants.
In tomato, mild disease symptoms were found at humidity as low as 56% RH. Unlike in tomato, Cannabis should have a zero tolerance policy for bud rot. When inhaling spores, certain immunosuppressed individuals can actually develop infections from inhaling Botrytis spores. Furthermore, the microclimates formed within Cannabis buds can significantly raise the humidity of the air in the flowers above that found in the grow space. While it is not ideal on the VPD chart, I recommend keeping humidity at 50% RH during early flower, and 40% RH during the last few weeks of flower. It is not worth trying to gain a marginal yield increase while risking your entire crop to bud rot. I believe the quality/yield is still maintained at this RH, but if you would like to keep it higher, 50% RH is likely okay. I tend to recommend staying on the safe side when it comes to avoiding mold issues altogether.
Application-Based Control Methods
Given that there are few approved fungicides with a targeted mode of action, my recommendations will mostly be based on approved fungicides in California.
I am not a big fan of using neem oil in mid-late flower, mostly because of certain health risks associated with neem. However, I certainly believe in a good regimen of spraying neem in vegetative growth and early flower. In addition to spraying about every 10 days during vegetative growth, I recommend doing one application when you flip your light cycles to 12/12 and doing another application at around 2 weeks of flower. I would not apply neem after this point. However, one could continue using other oils including triglycerides such as cottonseed oil or soybean oil, or even a product such as Trifecta crop control that utilizes corn oil and essential oils from plants such as garlic, thyme, peppermint, and rosemary. It also contains citric acid which may help control fungi.
Personally, I prefer to stop using all oils about 2-3 weeks into flower depending on the strain, and I begin alternating a biofungicide such as Stargus or Serenade with a potassium bicarbonate product such as Green Cure. Every 5 days I spray one of the products, alternating between the two. However, for the last 2-3 weeks, I also stop applying the biofungicide and only spray potassium bicarbonate once per week excluding the last week.
I believe that biofungicides are good to use for prevention of bud rot. They are not toxic and can be sprayed up to the day of harvest. We focus on soil health and the microbial community in plants’ rhizospheres, but we tend to not pay as much attention to the phylosphere of plants. In regards to premade products, I recommend spraying Bacillus amyloliquefaciens products such as Double Nickel 55 or Stargus during flower. I only say this because it is the only approved Bacillus spray to use in California, but Bacillus subtilis sprays such as Serenade are good. I would do the first spray at first sign of flower and reapply every 10 days for 3-5 applications. Essentially, this bacteria will be able to colonize the aerial portions of the plant and help prevent initial colonization from Botrytis.
I believe that all Cannabis growers should have a good spray schedule of either a citric acid-based product (i.e. Nuke Em by Flying Skull or Plant Therapy by Lost Coast) or a potassium bicarbonate product (i.e. Green Cure). These products help inhibit fungal spore germination by altering the surface pH of plant tissues. I would not use both products, since citric acid works by acidifying the surface while bicarbonate works by basifying the surface. Both citric acid and potassium bicarbonate have been found to inhibit Botrytis spore germination . There is no research in Cannabis comparing these products, but both appear to be inhibitory in vitro. However, we also know that Botrytis functions partly by acidifying the plant tissues. We also have some evidence in postharvest rot of kiwi fruit, that citric acid may make disease incidence worse, while potassium bicarbonate is effective for control [23, 24]. For these reasons, I recommend using a potassium bicarbonate spray such as green cure weekly during flower, even up to the day of harvest.
- Fillinger, S., & Elad, Y. (2016). Botrytis: the fungus, the pathogen and its management in agricultural systems. Springer.
- Hua, L., Yong, C., Zhanquan, Z., Boqiang, L., Guozheng, Q., & Shiping, T. (2018). Pathogenic mechanisms and control strategies of Botrytis cinerea causing post-harvest decay in fruits and vegetables. Food Quality and Safety, 2(3), 111–119. https://doi.org/10.1093/fqsafe/fyy016
- Veloso, J., & van Kan, J. A. L. (2018). Many shades of grey in Botrytis–host plant interactions. Trends in Plant Science, 23(7), 613–622.
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- Ngah, N., Thomas, R. L., Shaw, M. W., & Fellowes, M. D. E. (2018). Asymptomatic Host Plant Infection by the Widespread Pathogen Botrytis cinerea Alters the Life Histories, Behaviors, and Interactions of an Aphid and Its Natural Enemies. Insects, 9(3), 80. https://doi.org/10.3390/insects9030080
- Huo, D., Wu, J., Kong, Q., Zhang, G. B., Wang, Y. Y., & Yang, H. Y. (2018). Macromolecular Toxins Secreted by Botrytis cinerea Induce Programmed Cell Death in Arabidopsis Leaves. Russian Journal of Plant Physiology, 65(4), 579–587. https://doi.org/10.1134/S1021443718040131
- Govrin, E. M., Rachmilevitch, S., Tiwari, B. S., Solomon, M., & Levine, A. (2006). An elicitor from Botrytis cinerea induces the hypersensitive response in Arabidopsis thaliana and other plants and promotes the gray mold disease. Phytopathology, 96(3), 299–307.
- Manteau, S., Abouna, S., Lambert, B., & Legendre, L. (2003). Differential regulation by ambient pH of putative virulence factor secretion by the phytopathogenic fungus Botrytis cinerea. FEMS Microbiology Ecology, 43(3), 359–366.
- Kim, K. S., Min, J.-Y., & Dickman, M. B. (2008). Oxalic acid is an elicitor of plant programmed cell death during Sclerotinia sclerotiorum disease development. Molecular Plant-Microbe Interactions : MPMI, 21(5), 605–612. https://doi.org/10.1094/MPMI-21-5-0605
- Petrasch, S., Knapp, S. J., van Kan, J. A. L., & Blanco-Ulate, B. (2019). Grey mould of strawberry, a devastating disease caused by the ubiquitous necrotrophic fungal pathogen Botrytis cinerea. Molecular Plant Pathology, 20(6), 877–892. https://doi.org/10.1111/mpp.12794
- Takino, J., Kozaki, T., Ozaki, T., Liu, C., Minami, A., & Oikawa, H. (2019). Elucidation of biosynthetic pathway of a plant hormone abscisic acid in phytopathogenic fungi. Bioscience, Biotechnology, and Biochemistry, 83(9), 1642–1649. https://doi.org/10.1080/09168451.2019.1618700
- Blanco-Ulate, B., Vincenti, E., Powell, A. L. T., & Cantu, D. (2013). Tomato transcriptome and mutant analyses suggest a role for plant stress hormones in the interaction between fruit and Botrytis cinerea. Frontiers in Plant Science, 4, 142.
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I am not sponsored by any companies that make the products I recommend. However, as an Amazon associate, I earn from qualifying purchases made through provided links.
I also recommend checking out Growbuds USA for a wide selection of high-quality LED grow lights at a bargain.
First off, let me say that there is not grow light on the market that will compete with the cost efficiency and the spectrum of sunlight. Of course, there is some argument as to whether indoor grown Cannabis is a higher quality than outdoor Cannabis, which in some cases, may be true. I believe in these cases, it is not the spectrum of the sunlight that is to blame, it is usually other environmental factors. If you are lucky enough to grow outdoors or have access to greenhouse growing in a good environment, I recommend you take full advantage of that. The purpose of this article is to educate you on indoor lighting and supplemental greenhouse lighting for Cannabis, it is not advocating that growing under synthetic light is the only proper way to do it.
There are a wide range of lighting options available to purchase including light emitting diode (LED) lights, high intensity discharge (HID) lights such as high pressure sodium (HPS), metal halide (MH), ceramic metal halide (CMH) lights, or fluorescent lights such as T5 lights. A beginning grower can be quite overwhelmed by the options, and many would just appreciate a recommendation of what kind of light to buy for their purposes. In this article, I will attempt to educate you on the pros and cons of each type of light and provide some recommendations of lights for different purposes and price points.
Just a disclaimer: I have only ever grown using MH/HPS lights. For me, the reason for this is simply the low up-front cost and an okay level of efficiency. If I had more money to spend, I would likely upgrade to a CMH bulb with supplemental red light and UV bulbs or a full spectrum high-quality LED with supplemental UV light.
Terms you need to know
Color Warmth– This term is only relevant for broad spectrum grow lights that have red, green, and blue light represented in the spectrum; ‘blurple’ (red and blue diodes) grow lights are not really represented in this measurement. While it is more useful to actually see the wavelength spectrum of your grow light, the color warmth measurement can give you an idea of what the spectrum is like. Color warmth is a description of how red or blue your eyes perceive the light emitted from your grow light. It is denoted by a number followed by the letter ‘K’, and is on a scale from 1,000K to 10,000K. The ‘K’ stands for Kelvin, but is not related to the unit of temperature.
- 2000K-3000K: Warm white, illumination appears yellowish-orangish (the spectrum has relatively more red light than other wavelengths; this is the range most broad spectrum grow lights fall in)
- 3100K-4500K: Cool white, illumination appears fairly neutral
- 4600K-6500K: Appears more similar to daylight and is more blueish than the previous two.
PPFD (Photosynthetic Photon Flux Density)
This is a spot measurement of how many photons are falling on a particular area withing a given time. It is measured in units of micromoles of photons per square meter per second. Though the units are in square meters, the measurement is usually taken with a small light meter, and so it is best to take multiple measurements at various locations around your grow area and average them together. An ideal lighting environment would have equal PPFD all over your grow area, but this is simply unattainable and different lights have different light hotspots and distributions. In general, Cannabis can tolerate a very high level of PPFD. Certain studies have shown that Cannabis photosythesis rates can be maximized at PPFD levels of 1500-2000 PPFD [8, 9] at normal CO2 levels. However, many also seem to agree that indoors at temperatures around 25C, PPFD over 1000 provides a negligible yield increase unless CO2 is also supplemented. It is hard to know for sure without repeating previous experiments.
Based on common ‘rule of thumb’ knowledge:
- Most indoor growers should shoot for PPFD levels between 700-1000 across their grow space to maximize the photosynthetic rates of their flowering plants (though many have less).
- 700 PPFD is generally sufficient for a grow area with a good spectrum, but many growers like to have PPFD levels of 800+ across their grow area
- Seedlings and clones grow well with PPFD levels of 100-200 PPFD
- Established plants in the vegetative stage do well with PPFD levels of 300-600. However, I often provide my vegetative plants with the same amount of light as my flowering plants (around 800 PPFD), though I would recommend around 600 PPFD for cost efficiency purposes. I think vegetative plants can utilize a good amount of light and it helps them acclimate to flowering intensities.
PPFD is a measurement of PAR
PAR (Photosynthetically Active Radiation)
This is the amount of light available for photosynthesis, which lies between 400nm and 700nm, as described by the McCree curve shown below. This is essentially a measurement of energy harnessed by the plant in relation to the wavelength of the photon hitting the plant.
However, the long-held beliefs about PAR are being challenged, particularly by Professor Bugbee at Utah State University, proposing that far-red (700-780nm) light is more photosynthetically active than previously thought, and can have an effect on plant morphology (stetching, elongation, cell enlargement), and may also have a role in helping flowers initiate more quickly.
For quite a while, most LED grow lights were comprised of an array of only two different colored lights, red and blue. The was because the lights were designed to target the absorption spectrum of chlorophyll, shown below:
However, as can be seen by the McCree curve, all visible wavelengths are also photosynthetically active. Some pigments called carotenoids have been characterized as photosynthetically active for wavelengths up to 550nm. While known photosynthetically active pigments may not absorb green light very well, it appears that chlorophyll a and b can utilize green light in small amounts.
Green light is particularly effective at penetrating leaf tissues. Up to 80% of green light passes through a chloroplast, and it is able to penetrate far into the mesophyll cell layers . In whole, a leaf is able to absorb up to 85% of incident green light as the light is refracted within a large area of leaf tissue. Up to 10% of green light is able to penetrate through leaves to the lower canopy as well .
At very high PPFD levels of broad-spectrum white light, geen light can actually stimulate photosynthesis more efficiently than red light. This is because at high light levels (which Cannabis is normally grown under), chloroplasts in the upper mesophyll cells can become ‘saturated’. As red light continues to be absorbed, excess energy can actually be dissipated as heat. However, green light is absorbed less efficiently than red light by a given chloroplast, and so it penetrates further and is able to be absorbed by chloroplasts in lower mesophyll cells that are not saturated, .
It has also been noted that light wavelength has a direct effect on plant morphology. For instance, it is common for growers to use metal halide lights during vegetative growth because it has a more blue spectrum than high pressure sodium lamps, and blue light tends to promote shorter, more bushy growth patterns. HPS lamps are generally used in flower because the more red spectrum is more photosynthetic efficient and may help in promoting flowering.
HPS lights are more efficient to run than MH lights, and some hypothesize that red light may help stimulate flowering. While it has not been shown in Cannabis, red light has been known to accelerate flowering in some other species . Green light may act as a shade signal when in a high enough ratio to other wavelengths, resulting in plants that may stretch or induce flowering early . In short, all wavelengths seem to have some effect on morphology of the plant, and sometimes different wavelengths can act antagonistically or synergistically, meaning that it is not fully understood how different types of lights are affecting the phenotype of the plant both morphologically and metabolically. The spectrum of the sun is amazingly even, and so the different wavelength ratios of different grow lights likely have phenotypic effects that are not fully understood. A representation of the spectrum of sunlight is shown below:
PPE- Photosynthetic Photon Efficiency
This is a way to measure how efficient your grow light is at converting input energy to photosynthetically active radiation, measured in micromoles of PAR photons per Joule of energy. Some of the newest LEDs have an efficiency of over 2.0. A lot of the high quality LEDs from around 2014 were about 1.7 PPE, which at the time was comparable efficiency to double ended HPS lamps. A lot of LEDs of the time were actually less efficient than HPS lamps. I still have a couple blurple Mars 300 lights from around 2014 that are probably quite low on the efficiency scale.
How Do Different Grow Lights Work?
Fluorescent lights are made of a glass tube under mild pressure. The tube is filled with an inert gas such as Argon and a small amount of liquid mercury. Electrodes at either end of the tube produce a voltage that sends electrons through the Argon gas from the cathode to the anode. This vaporizes the liquid mercury into the arc. Some of the electrons hit the mercury atoms, causing electrons in the mercury atoms to become excited to higher energy states. As the electrons drop back down to lower energy states, photons are emitted at UV wavelengths. Fluorescent tubes are coated in phosphor that shifts the wavelengths of the photons, resulting in a distribution of different wavelengths and a fairly broad light spectrum.
Fluorescent lights are the least useful lights on this list in my opinion. I would not use CFLs or T5s for anything other than rooting cuttings and/or small seedlings or maintenance of mother plants. They are not a bad choice for vegetative growth, but I believe metal halide and LED lights perform better and may be more efficient.
Most cheap LEDs that you would use to replace fluorescents for seedlings also tend to be composed of only red and blue diodes, which creates a very unnatural purple light. If you operate a dispensary keeping clones in a showroom for sale, the balanced spectrum from the fluorescents might be the better choice simply because it’s a bit more appealing to customers and the plants can be seen much better.
Because of the lower light output by fluorescents, they generally need to be kept pretty close to the top of your plants (around 6-10 inches).
This paragraph will get a bit technical: LEDs (Light Emitting Diodes) are semiconductor light sources. This means that the conductivity of the material is in between that of a pure conductor and an insulator. Generally, there are three layers in an LED. The n-type semiconductor has an excess of electrons and the p-type semiconductor has an excess of ‘holes’, which are regions where electrons are absent in a crystalline lattice. These semiconductor types are made by adding different impurities to the materials. In between the two layers is an active layer that does not have an excess of electrons or holes. When a voltage is applied, excess electrons and holes from the n and p layers flow into the active layer, and when the electrons recombine with electron holes, they drop in energy states from the conduction band to the valence band. When this drop occurs, the difference in energy is released as a photon with a particular wavelength depending on the energy difference between the conduction and valence bands. The energy difference can be altered by changing the alloy composition. For instance, if the active layer is composed of indium gallium nitride, altering the ratio of indium nitride to gallium nitride can tune the bandgap of this material to different wavelengths of light. As you can likely guess, this means that each active layer can only emit a particular wavelength of light, which is why many grow lights will use a number of different colored diodes in order to provide light of different wavelengths to your plants. Broad spectrum LEDs can be made by coating blue LEDs with different colored phosphors, similar to fluorescent lights. This can shift the wavelength of blue photons, making emitted light come out in a broad spectrum of wavelengths. The typical phorphor coated LED spectrum looks something like this:
Two major LED types are used in LED grow lights: SMD and COB.
SMD- Surface mounted diodes
large 5x5mm SMD LEDs are capable of housing up to 3 LEDs per module. These are diodes that do not require wires. Instead, they are soldered directly to a circuit board. Sizes vary greatly, from under 1mm squared to over 25mm squared. Most SMD circuit boards are 3W or 5W and are composed of 0.5W modules, although 10W SMD circuit boards do exist. Each diode in an SMD module has its own anode and cathode. Due to these properties, a variety of spectrums can be produced by SMD panels that can span the PAR spectrum.
COB- Circuit on Board
COB LEDs allow for multiple diodes (at least 9, but can be much higher) to be attached to a single substrate with a single circuit (one cathode and one anode). Diodes can be placed quite close to one another and produce a high light intensity for a small space. Most COB grow lights will have a mixture of warm white and cool white (3000K-4000K) diodes, resulting in a wide and benefiial spectrum for plants. Both COB and SMD LEDs are quite efficient, though a panel of 3W SMD LEDs is generally slightly more efficient than COB LEDs. COBs tend to have a much more focused light than SMDs, meaning that for the light spread to be useful over a large area, a lens usually has to be placed to increase the area that the light can reach.
HID stands for high intensity discharge. Like fluorescent lights, HID lights are arc lights. They work by forming an electric arc between two tungsten electrodes that are housed in an arc tube. The arc tube contains a noble gas that ignites when the circuit is active. The gas begins to heat up and eventually vaporizes metals within the arc tube. Light is produced when excited electrons drop in energy levels, releasing photons. The spectrum of the HID light is largely determined by the metal and impurities that are being vaporized in the arc tube. There are three main types of HID lights used in plant cultivation: HPS (High Pressure Sodium), MH (Metal Halide), and CMH (Ceramic Metal Halide).
As you can probably guess, HPS lamps contain sodium as the primary metal in the arc tube. Other impurities such as mercury are added to alter the spectrum of the lamp to have more blue wavelengths. HPS lights are dominated by orange, yellow, and red wavelengths, while still containing small amounts of blue and green wavelengths. The composition of the materials in HPS bulbs for horticulture has changed over time to try to make a more balanced spectrum. An HPS spectrum looks something like this (although each lamp is slightly different):
Double ended HPS bulbs began to be popular for horticulture around 2014. Double ended HPS bulbs are slightly different because they don’t screw into a socket, they have electrodes at either end of the bulb similar to fluorescent tubes. DE HPS lamps, as mentioned previously, are more efficient than their single ended counterparts. For SE HPS bulbs, there are 4 common wattages, 250W, 400W, 600W, and 1000W. 600W bubs are the most efficient, while 1000W bulbs are second most efficient. 400W is less efficient still and 250 W bubls are the least efficient. As many long-time indoor growers know, HPS bulbs used to be the only way to grow indoors with high intensity light and they have been the gold standard of horticultural lighting for decades. Even today, HPS bulbs are some of the best flowering lights you can buy, though modern LEDs are quite competetive and arguably better. Because of the high amount of red light, HPS bulbs are generally only used for flowering. The red light of the HPS affects the structure of cannabis (during vegetative growth) causing it to become more lanky and weak in stem and branch growth.
MH (Metal Halide) MH bulbs also contain mercury, though in a higher concentration than HPS bulbs. However, MH bulbs have metal halides instead of elemental sodium as the light source. Metal halides are metals that are compounded with bromine or iodine. A MH spectrum might look something like this:
As you can see, there is very little red and far-red light in this spectrum as compared to the HPS bulb. This not only reduces the photosynthetic efficiency, it has an effect on plant structure, causing the plant to be more compact than it would under a red-light heavy spectrum. There is also very little far-red light.
CMH- Ceramic Metal Halide
CMH lights are the newest HID lights used in horticulture. They are more efficient than MH bulbs and also have more red in the spectrum. In fact, the CMH spectrum is fairly well balanced and could be compared to a mixture of MH and HPS bulbs. For this reason, they can be quite effective at both vegetative growth and flowering, growers don’t need to worry about switching out bulbs or even replacing bulbs as frequently. An example spectrum from a 3100K CMH bulb is shown below:
CMH bulbs also have a much longer lifespan than MH and HPS bulbs, and they produce far less heat than HPS bulbs. However, they are significantly more expensive than other HID options. Also, there is no 1000W CMH option; most of the CMH bulbs on the market are 315W or 630W double ended bulbs. Many believe that CMH bulbs may not be as effective as DE HPS bulbs in flower, but this is hotly debated. It is also important to look at the quality of the flower produced. CMH bulbs produce relatively more UV radiation and blue light, for example, which has been shown to be effective at stimulating the production of secondary metabolites such as cannabinoids and terpenes . In the end, MH/HPS bulbs and CMH bulbs are both great choices, and each option has its positives and negatives. In fact, some growers will use both strategies in their grow area in order to take advantage of the broad spectrum of the CMH while also adding in supplemental blue-dominant and red-dominant light at different growing stages. One could also consider using HPS bulbs and supplementing with LED lights with a cooler color temperature to try to get a broader spectrum than HPS could provide by itself.
So, now that we have covered the basics of the different types of horticultural lights, how do you determine what to buy? Well, there is no right answer. If you spend any time on internet forums, you will soon realize that many different people feel quite strongly about their choice in light. My personal opinion is that it is best to mix different types of lights in order to reach the PPFD levels that you need at a spectrum that is acceptable. If you have a light or light mixture with a truly broad spectrum like a CMH bulb or broad spectrum LED, you don’t have to switch out lights for flowering. If you do not, it is best to stick to blue-heavy spectra for vegetating and red-heavy spectra for flowering.
How Expensive are Different Lights?
Cheapest (<$200/ sq. ft.)
If you are on a tight budget, I recommend a single-ended HID setup using a digital ballast that can support both MH and HPS bulbs (MH for veg, HPS for flower). This can cost less than $200 to light up 3 ft x 3 ft grow area, and you can even light up a 4 ft x 4 ft area using HID for less than $200, though I would not be comfortable doing the same with LEDs at this price point.
If you have a moderate budget ($200-$400)/ 10 square feet, and don’t want to have to worry about switching bulbs out, CMH lights are a very good option. DE HPS is a very good choice for flowering as well, and there are fixtures that can also run DE MH bulbs for veg. While CMH lamps can be quite good, some argue that they need some supplemental red light to help get the most out of flowering. CMH lights may be more efficient overall than a MH/HPS system since MH lamps tend to be less efficient than CMH and HPS lamps.
This price range also allows for some nice LED choices. For instance, the Mars Hydro TS3000W light is a good price for the light, but it uses some cheaper materials and designs for the fixture. It covers a 4 ft x 4 ft quite well and has a broad spectrum with some supplemental red light (SMD chips) and can be purchased for $480.000, giving an average cost of $27.5/1 square foot. There are also some cheaper COB and COB/SMD combo grow lights that can be purchased within this price range, though the COB LEDs used in these cheaper lights are usually either not a high quality full spectrum COB or they are not as powerful over a large area compared to SMD panels. A good 100W COB LED can be purchased at this price point, but it will not light up a 10 sq. food area well at all.
Big Budget ($400+ for 10 sq. feet)
This is the price range in which LED really shines. As I said, there are a lot of opinions out there about if LEDs really can outperform HID lights. However, a lot of people simply don’t have physics on their side. There is no magic to HID lights that make them the be-all-end-all of grow lights. If you have LED lighting with the exact same PPFD, the exact same distribution, and the exact same spectrum as an HID light, they will perform exactly the same. HID lights do not ‘penetrate’ better. Photons travel at the same speed regardless of what their source is, and in fact, the most important wavelengths for canopy penetration are green wavelengths. A well-designed full spectrum LED light will have more green light than HPS lights. Therefore, it is actually modern LEDs that have better penetration than HPS lights (although CMH lights tend to have good amounts of green light as well). Furthermore, no grow lights on the market can match the efficiency in PAR/Joule of the top tier LED grow lights.
For instance, for $1,000.00, you could purchase a Chilled logic 660 LED grow light, With an efficiency of around 2.4 PAR/Joule, an even distribution, and a broad spectrum with supplemental red wavelengths that looks like this:
it is hard for HID lights to compete. The measured PPFD is similar to HID lights as well. The PPFD measurements over a 4×4 area are shown below:
I have not used this light, it is just one that I saw mentioned recently that I am using as an example of the kind of performance one can achieve with modern LED lights.
Finally, it is important to consider whether or not you want to include UV light in your grow. For instance, UVB has been shown to increase cannabinoid levels in Cannabis , and many CBD and marijuana commercial growers are beginning to add UV light to their grows (specifically UV-B and UV-A).
UV-B (290nm-320nm) light appears to be sensed by the UVR8 photoreceptor and plays a stimulating role in producing secondary metabolites such as cannabinoids and terpenoids [3,4,5]. Essentially, UVR8 induces upregulation of genes involved in producing metabolites involved in defense against various environmental and biotic factors including sunlight. It is difficult to achieve the extremely high 30% THC levels found in some buds today without supplemental UV lighting. UV-A light has also been shown to increase cannabinoid levels , and does not cause DNA damage to cells. In fact, UV-A and far-blue light can activate DNA repair mechanisms through a process known as photreactivation that can mitigate some damage from UV-B . UV-A is sensed through different proteins than UV-B including cryptochromes and phototropins and acts to repair DNA damage by upregulating the expression of photolyase proteins . In the proper ratios, UV-A exposure can enhance plant response to UV-B light. Though UV-A exposure may help with secondary metabolite production, stimulation of the UVR8 pathway is very important to maximize the effects of UV light. In actual sunlight, UV-A is present in greater amounts than UV-B, and obtaining the proper ratio will be important for optimizing plant UV responses for the purpose of cannabinoid production. Most UV lights on the market do not have the same ratio as sunlight, but I believe it is important to at least make sure that the lights chosen have both UV-B and UV-A represented in the spectrum.
UV-B light is damaging to cells, and cannabinoids act as a suncreen, absorbing UV light in trichomes before the light reaches plant cells. This explains why UVR8 may participate in upregulating cannabinoid production. The best way to currently supplement plants with UV light economically is with UV T5 fluorescent bulbs, which can be purchased quite cheaply, and will provide both UV-A and UV-B light. Reptile UV lights and similar short-tube designs are not strong enough, and UV LED lights are too expensive simply for a supplemental light.
It is difficult to suggest how long to leave UV lamps on or when to start supplementing, as many people suggest pulsing the UV light for short periods of time, using them every other day, or only using during the last couple weeks of flower. However, many growers also advocate using them daily for multiple hours a day, even during vegetative growth (sometimes up to 12 hours per day). For instance, Lydon et al. 1987 found increased cannabinoid production after 40 days of exposure, and cannabinoid concentrations increased with dosage. In nature, UV radiation is present whenever visible radiation is, so it makes sense that the UV-B and UV-A radiation from a T5 won’t be too much for the plant to handle. I believe more optimization experiments are needed here, but I would say that my current recommendation would be to use daily, at least during the flowering cycle, for at least 4 hours per day, and up to 12 hours per day should be okay but the returns from longer time periods are likely diminishing. The one time I might cut back on UV exposure is during the last couple weeks of flower, after which cannabinoid production won’t increase by a large amount, but UV radiation might contribute to cannabinoid degradation. I would probably limit the expose to about 2 hours per day during the last couple weeks of flowering.
Product Recommendations (In no particular order): I will provide recommendations based on reviews and tests I have watched and read online, not based on testing all of these products personally. Recommendations are not ranked.
T5- With T5s, one can get a variety of fixtures and bulbs of different lengths, color temperatures, and number of bulbs.
- Complete Kit (value): DuroLux T5 Grow Light (4ft 4lamps) DL844s Ho Fluorescent Bulbs- ($92 on Amazon)
- Improved Spectrum LED T5s full kit: Active Grow with 8 x 24W T5 HO 4FT LED Tubes ($385)
- While this is better than standard T5s, it is overpriced compared to SMD or COB LEDs at similar watts in my opinion
- Recommended fluorescent bulb for highest performance: Hortilux PowerVEG Full Spectrum with UV 54w – 4ft T5 HO Bulb ($28)
For UV supplementation to any grow light system, I recommend using AgroMax Pure UV T5 bulbs.
- 2 ft.- $22
- 4 ft.- $28
Alternatively, the Hortilux PowerVEG proveds plenty of cool-white PAR as well as UV that would be a good supplement to HPS bulbs. ($28/bulb)
In regards to HID fixtures, I think Vivosun and iPower are both good products that are a good value. I have had the iPower 600W ballast for around 6 years, and it is still running well.
HPS/MH– Keep in mind that different bulbs may have different color temperatures. For CMH, 3100K is a good color temperature in my opinion.
DE HPS/MH Fixtures (good values, good performance)
- iPower ($259)
- Vivosun ($226)
DE HPS Bulbs:
- Best Value- Ushio brand ($74)
- High Performance- Eye Hortilux ($94)
- I would recommend the same brands for MH bulbs
- iPower 600W setup with wing reflector fixture ($120)- This is not air cooled, meaning that it will significantly heat your growing space, but will have greater efficiency since light does not pass through a layer of glass.
- iPower 600W setup with air-cooled hood fixture ($150)
- Performance: Eye Hortilux 600W HPS bulb- ($78)
- Best Value: Ushio 600W HPS bulb Optired- ($55)This is the bulb I have. I like it quite a bit and lasts quite well
1000W HID Fixtures and Bulbs:iPower 1000 Watt HPS MH Digital Dimmable Grow Light System Kits Air Cooled Reflector Hood Set VIVOSUN Hydroponic 1000 Watt HPS MH Grow Light Air Cooled Reflector Kit – Easy to Set up, High Stability & Compatibility (Enhanced Version) Ushio Enhanced Performance 1000W HPS Lamp Ushio HiLUX GRO MH 1000W opti-blue Bulb (for HPS Ballast) Hortilux 1000 Watt Super HPS Hortilux Universal Eye Hortilux Metal Halide Bulb, 1000W
600W Fixtures and Bulbs:iPower 600 Watt HPS MH Digital Dimmable Grow Light System Kits Wing Reflector Set with Timer VIVOSUN Hydroponic 600 Watt HPS MH Grow Light Air Cooled Reflector Kit – Easy to Set up, High Stability & Compatibility (Enhanced Version) (6) Eye Hortilux 600W Super HPS Hydroponics Enhanced Spectrum Grow Light Bulbs Ushio Enhanced Performance HPS Lamps 600 WATT OPTI RED LAMP 902905 Ushio US5001675 Conversion Lamp, 600-watt, Opti Blue,Clear Hortilux Metal Halide Blue 600 by Hortilux
- HydroCrunch 630W DE CMH setup ($355)
- Eye Hortilux 315W CMH setup ($463)- high quality and has a higher UV ratio in the spectrum than many CMH bulbs.
- Vivosun 630W CMH fixture($230)- fits 2 315W bulbs
- Eye Hortilux 315W CMH bulb ($92)
- Ushio HiLux Grow 315W bulb ($82)
- I mentioned the Chilled Logic 660 earlier, and I would recommend this light for a 4×4 space, but it will work in a 5×5 space, especially for a home grower. It uses SMD LEDs.
- The Lumatek Zeus 600W is a fantastic SMD-based light with an extraordinarilly high efficiency of 2.3 umol/J. Like the Chilled logic, I recommend it for a 4×4 area with CO2 supplementation, but for a home grower it will perform great in a 5×5 area.
- The California Lightworks 500W SolarXtreme is a fantastic broad spectrum COB-based fixture with a high-quality fixture. I recommend it over their red/blue SMD light, the SolarSystem 550
- The Mammoth Lighting SMD 800W strip-lighting fixture can easily flower a 5×5 area with an efficiency of around 2.12 umol/J and costs around $1,000.
- The HLG 550 V2 R-Spec quantum board can easily flower a 4×4 area and has a claimed efficacy of 2.6 umol PAR/J, though I am sure the actual PPFD/J is lower than the PAR/J figure. It will cost around $850
- The Fluence SPYDR 2x and 2i lights are well known among commercial cultivators as being high quality and well designed for rack mounting. They have a claimed PAR efficiency of 2.7 umol/J, which again, likely doesn’t count the actual PAR landing on the plants.
Cheap but good
- The Mars TS3000 is a great deal for home growers looking to light up a 4×4 area well and a 5×5 area decetly. It will run you about $440. The Efficiency is claimed to be around 2.2 umol/J, though the PPFD/J is likely lower than this.
- The Spider Farmer SF 4000 will supposedly flower a 5×5 area quite well, has a claimed efficiency of 2.7 umol/J, and will run you about $570.
- For a 2×2 area or supplemental lighting for HID lighting, a 100W CANAGROW CREE CXB3590 is a good choice and is lensed to give a wider range. It will run you around $150.
Of course, there are many other products that will work well. These are just products I have seen reviews on that are well-received.
It is never a bad idea to mix and match lights in order to improve the light spectrum your plants are exposed to. Some combinations might look like this:
- HPS bulbs for flower, supplemental cool temperature LED lights to increase blue and green light, and UV T5 bulbs
- CMH bulbs for flower, supplemental red warm temperature LEDs to help with flowering, and UV T5 bulbs
- Broad Spectrum LED lights with UV T5 bulbs
If you are doing combinations, you will want to slightly increase the spacing of your main light fixtures and to install supplemental lights in between your main fixtures. Make sure to space your lights so that your PPFD in any given area is not too high (wasting energy). Do not change spacing if only adding UV bulbs as these do not contribute to PAR.
Hopefully this has answered some questions!
- Terashima, I., Fujita, T., Inoue, T., Chow, W. S., & Oguchi, R. (2009). Green Light Drives Leaf Photosynthesis More Efficiently than Red Light in Strong White Light: Revisiting the Enigmatic Question of Why Leaves are Green. Plant and Cell Physiology, 50(4), 684–697. https://doi.org/10.1093/pcp/pcp034
- Growing Plants with Green Light – Greenhouse Product News. (n.d.). Retrieved February 22, 2020, from https://gpnmag.com/article/growing-plants-with-green-light/
- Zavala, J., & Ravetta, D. (2002). The effect of solar UV-B radiation on terpenes and biomass production in Grindelia chiloensis (Asteraceae), a woody perennial of Patagonia, Argentina. Plant Ecology, 161, 185–191. https://doi.org/10.1023/A:1020314706567
- Lydon, J., Teramura, A. H., & Coffman, C. B. (1987). UV-B RADIATION EFFECTS ON PHOTOSYNTHESIS, GROWTH and CANNABINOID PRODUCTION OF TWO Cannabis sativa CHEMOTYPES. Photochemistry and Photobiology, 46(2), 201–206. https://doi.org/10.1111/j.1751-1097.1987.tb04757.x
- Liu, H., Cao, X., Liu, X., Xin, R., Wang, J., Gao, J., Wu, B., Gao, L., Xu, C., Zhang, B., Grierson, D., & Chen, K. (2017). UV-B irradiation differentially regulates terpene synthases and terpene content of peach. Plant, Cell & Environment, 40(10), 2261–2275. https://doi.org/10.1111/pce.13029
- What Effect Does Red Light have on Plants? | Ursa Lighting. (n.d.). Retrieved February 23, 2020, from http://ursalighting.com/effect-red-light-plants/
- Zhang, T., Maruhnich, S. A., & Folta, K. M. (2011). Green Light Induces Shade Avoidance Symptoms. Plant Physiology, 157(3), 1528–1536. https://doi.org/10.1104/pp.111.180661
- Chandra, S., Lata, H., Khan, I. A., & Elsohly, M. A. (2008). Photosynthetic response of Cannabis sativa L. to variations in photosynthetic photon flux densities, temperature and CO. Physiol. Mol. Biol. Plants, 14, 4.
- Chandra, S., Lata, H., Mehmedic, Z., Khan, I. A., & ElSohly, M. A. (2015). Light dependence of photosynthesis and water vapor exchange characteristics in different high Δ9-THC yielding varieties of Cannabis sativa L. Journal of Applied Research on Medicinal and Aromatic Plants, 2(2), 39–47. https://doi.org/https://doi.org/10.1016/j.jarmap.2015.03.002
- Magagnini, G., Grassi, G., & Kotiranta, S. (2018). The Effect of Light Spectrum on the Morphology and Cannabinoid Content of Cannabis sativa L. Medical Cannabis and Cannabinoids, 1(1), 19–27. https://doi.org/10.1159/000489030
- Britt, A. B. (1995). Repair of D N A Damage lnduced by Ultraviolet Radiation. In Plant Physiol (Vol. 108). http://www.plantphysiol.org
- Aphalo, P., Albert, A., Björn, L., Mcleod, A., Robson, T., & Rosenqvist, E. (2012). Beyond the Visible, A Handbook of Best Practice in Plant UV Photobiology.
Powdery mildew is usually first observed as small white circular spots on Cannabis leaves. They can be faint, but they begin to cover entire leaves if left unchecked and can begin to grow on buds as well.
For powdery mildew, the conditions that favor the host, also favor the pathogen. Dry leaves, warm temperatures, and moderate to high humidity. It can tolerate low humidity as well
Many websites out there will tell you that powdery mildew in Cannabis prefers cool temperatures and low humidity. Powdery mildew can be one of the most difficult diseases to control because it grows best in the same conditions as your Cannbis plants. However, PM can tolerate a wide range of RH levels, and simply lowering your RH levels will not eliminate PM risk, though it does help. In fact, low humidity can favor the spread of the disease, but high humidity can favor spore germination (although liquid water in contact with spores will inhibit germination).
Powdery Mildew on Cannabis: The Summary
What PM Species Affect Cannabis? What Environmental Conditions help control PM on Cannabis?
Much of the information on popular websites for Cannabis Powdery Mildew conflicts with the information in the scientific literature. I will report what has been published in scientific literature. Powdery mildew fungi often have a narrow host range. They are known as biotrophs, meaning that they can only live and reproduce on living hosts. Coincidentally, they also cannot be cultured in vitro because they require the living host to survive. There is a closer evolutionary relationship between the host and the parasitic fungus than that of necrotrophic fungi such as B cineria, the causal agent of bud rot.
Many species have historically been identified as capable of infecting Cannabis. In the early 1990s, McPartland reported at least 2 different species (Leveillula taurica and Sphaerotheca macularis) [1i, 2i]. In 2018, a Canadian publication described the causal agent of Cannabis Powdery Mildew in samples from drug Cannabis as belonging to the Golovinomyces cichoracearum species complex from looking at ITS sequences, which may include other species such as G. spadiceus or G. ambrosiae . Subsequent studies show that G. spadecius is a common PM pathogen on Cannabis species [2, 3,]. Cannabis can also be infected by PM from closely related plant species such as hop PM, Podosphaera macularis .
Golovinomyces spadiceus grows best in warm, low humidity climates. It is commonly found on wild plant species such as wild sunflower  or plants within the same tribe, such as Zinnia flowers and various other plants [5, 6]. Cannabis can get a decent amount of infection on the flowers, and this can lead to unmarketability in the private and medical sectors. It is not recommended to consume bud infected with PM, though I do not believe there is any research as to the health effects of consuming Cannabis infected with PM. It can certainly destroy trichomes and affect the flavor and odor of your buds is you have a heavy infection.
When temperatures drop, the relative humidity of your grow area will go up because cooler air holds less water and condenses water easily . The tomato-infecting PM species, which has also been reported as infecting Cannabis, is signifiantly reduced at low humidity levels (20-40% RH) , and this is a common recommendation for Cannabis growers. For G. spadiceus, the pathogen is presumed to act similarly, and keeping humidity under 50%, and preferably lower is ideal for controlling Powdery Mildew. For instance, one report of G. spadiceus in Cannabis says that G. spadiceus thrives in warm temperatures and moderate to high humidity .
The Biology of Powdery Mildew
Powdery mildew are ascomycete fungi in the order Erysiphales. As mentioned, they are obligate biotrophs. The life cycle of PM is shown below. This diagram is for grape powdery mildew, though the life cycle is the same for PM on most plants
Cleistothecia are structures produced in the late summer that are known as ‘overwintering structures’, meaning they help protect the sexual spores (ascospores) inside until they are released in the Spring. They are far more resilient than the conidia spores that are asexually reproduced in the disease’s main reproduction cycle that provides secondary infections during the Spring, Summer, and Fall. Ascospores are sexually produced spores, meaning that they undergo sexual recombination, whereas conidial spores have the same genotype as the isolates that formed them. Conidia begin to be produced quickly, usually within a week of initial infection, leading to rapid disease spread and exponential growth of the pathogen.
PM fungi, despite what some Cannabis websites claim, are not systemic. For a pathogen to be systemic, is has to be able to spread via the vasculature of plants. Of course, it can spread to distant parts of the plant through spread of spores or growth of mycelium over susceptible tissue. I would hypothesize that websites that claim that asymptomatic tissues are testing positive despite showing symptoms are simply identifying spores or initial infections that are not yet visible to the naked eye.
PM fungi only grows on the surface of plants, obtaining nutrients from living epidermal cells from specialized structures known as haustoria that penetrate the cell walls and invaginate the cell membranes. PM fungi use the haustoria to obtain nutrients from living cells, but also to modulate the host plant’s defense responses. All plants have an immune system, and fungi that feed on living hosts use proteins known as effectors in order to inhibit host defense responses.
As mentioned, it is important to control temperature and humidity. It is recommended to keep humidity low to control PM (20-40%RH). If PM is not such a big problem, keeping RH so low may not be recommended. I tend to try to keep RH in my grow tents around 40% RH. In addition to humidity in your grow area, it is important to have good airflow and ventilation. Every plant makes a microclimate around leaves . The trichomes, hydrophobic leaf cuticle, and leaf transpiration create a small layer of air around leaves that is relatively still and higher humidity than surrounding air. Since this is the climate that PM spores and mycelium actually grows and reproduces in, it is important to disturb this microclimate with fans that gently disturb the leaves as well as to have high levels of air exchange in your grow area.
It is difficult to use temperature as a control method, as temperatures between 50 and 90 Farenheit (10-32 Celsius) can be conducive to PM growth and spread. Unfortunately, the high and low temperatures that may inhibit PM are also very stressful to Cannabis plants. I would recommend maintaining your normal temperatures (70-85 Farenheit or so), and focusing on humidity, circulation, and fresh air exchange in terms of climate control.
One thing to consider is that the longer time a plant takes to achieve maturity, the more time PM has to develop and spread. Choosing plants with short flowering periods such as indica-heavy hybrids or short total growth periods such as autoflowering plants may be a good choice in reducing the risk of harvesting buds infected with PM.
PM disease resistance can indeed be bred for. Unless a major resistance gene is identified, it is likely that all resistance is multigene, quantitative resistance. Unfortunately there is not public research available as to which strains have high PM resistance. Research that has been done is likely by private companies and breeders screening commercial strains for PM resistance and keeping information in-house for a commercial advantage. Unfortunately, the strain selection here must be based on knowledge spread around the growing community on forums and growing websites such as this. It is important to note that these are anecdotes and have not been verified with any experiments.
Besides indica heavy strains being a better choice due to flowering time, it also appears that many strains with Afghani heritage are more resistant to PM than most strains. In general, landrace sativa strains from equitorial regions have higher mold (bud rot and fusarium) resistance than Afghanis. However, these strains also appear to be more susceptible to PM than many Afghani-dominant strains.
Here is a list of strains that I see popping up a lot when people discuss PM-resistant strains:
- Bubba Kush
- L.A. Confidential
- Purple Punch
- Northern Lights
- White Widow
- Super Skunk
- White Russian
- Grape Ape
- Purple Kush
The one thing in common with all of these strains? Afghani Heritage. In fact, some of the most resistant strains also tend to have the most Afghani genetics. If you are selecting strains for PM resistance, a good rule of thumb is to maximize the Afghani landrance genetics by choose strains that are not just polyhybrids, but have had recent crosses with Afghani strains. Pairing proper selection of genetics with proper environmental controls will likely prevent you from having to deal with PM outbreaks. However, if you are still dealing with problems, you will have to move on to chemical control methods.
A few tips can help prevent a wide range of pest issues. If you are so able, isolate your grow area and use HEPA filters in your grow room, preferrably in your air intake. This will prevent spores from accessing your plants. UV lamps can also be installed in your ducting that are effective at killing airborne spores. All tools that you use for trimming, defoliating, or any plant manipulation should be soaked in a 70% alcohol solution before every use. After each cut, it should also be sprayed with the same solution and wiped dry. If possible, keep a box of nitrile or latex gloves close by and always wear them when entering the grow area. It would benefit you to not wear your outside clothes in your grow room. Have a pair of dedicated clothes for your grow area that you wash regularly.
Finally, sterilize your grow area after each harvest. Using 10% bleach can be one good way, but I would recommend cleaning your grow room once with a bleach solution, wipe it dry, and then do a second cleaning with a ‘Quat’ soap. If you follow the cultural, environmental, and strain selection guidelines I have put forth, you will likely not need any targeted fungicides (which are not approved for use in Cannabis at least at a commercial level, due to bureaucratic and legal reasons). However, I do recommend putting together a pest control spray program for your plants as well, for use in vegetative growth and even early flowering.
Unfortunately for commercial growers, there are very few registered fungicides that can be used on Cannabis. On the federal level, no pesticides have been approved for drug Cannabis. However, states have approved certain fungicides that the EPA has approved for hemp, which became federally legal in 2018.
If Cannabis is being sold on the legal markets and tests positive for an unapproved pesticide, it cannot be sold. Most of the fungicides on the market for Cannabis are not nearly as effective as fungicides used, for instance, in grape production for control of PM. The approved fungicides are generally untargeted, broad spectrum, and diversity of FRAC groups are not represented. Below is the list of approved fungicides in California:
• Bacillus amyloliquefaciens strain D747
• Cloves and clove oil
• Corn oil
• Cottonseed oil
• Gliocladium virens
• Neem oil
• Peppermint and peppermint oil
• Potassium bicarbonate
• Potassium silicate
• Rosemary and rosemary oil
• Sodium bicarbonate
• Reynoutria sachalinensis extract
• Trichoderma harzianum
As you can tell, there are various fungi and bacteria that can be sprayed as biocontrol agents, plant extracts and oils, and some basic salts that can be sprayed. .
Compare what is available to be used on Cannabis with what is approved to be used for PM on another smokeable crop, tobacco, and you will see how handicapped the Cannabis industry is in terms of pest control options: Mancozeb, Terramaster, Azoxystrobin, Copper based fungicides, Actigard, Agri-Mycin, Manzate, Orondis, Aliette. It is likely that similar pesticides will eventually be approved for use in Cannabis on a federal level, though not until it is federally legal and goes through rigorous studies for pesticide safety. The EPA will not approve any pesticides for Cannabis if it remains a scheduled drug.
What do I use to address a PM outbreak and help prevent PM infection?
First, I want to address one common home remedy: milk. A lot of people swear by using diluted milk as a control method. In my experience, this is the least effective method out there. If you want to use something that basifies your leaf surface, I recommend using a baking soda (sodium bicarbonate) solution.
Many homegrowers swear by using baking soda or other bicarbonates (such as potassium bicarbonate which may be more effective than Sodium Bicarbonate). I believe that when used as a preventative, it is far more effective than to stop an outbreak. In my experience, once an outbreak starts, it may behoove you to go with more aggressive measures. Bicarbonate ions work by increasing the pH of the leaf surface which inhibits fungal growth/spore germination. It is best to include a spreader/sticker to your baking soda solution such as vegetable oil and dish soap without antibiotics.
- 3 tbsp baking soda
- 1 tbsp vegetable oil
- a few drops of dish soap without antibiotics to emulsify the mixture.
- *If you can get a nonionic surfactant such as Yucca extract or CocoWet, I would recommend that over dish soap which may have some amount of phytotoxicity. I would also recommend potassium bicarbonate over baking soda*
Spray liberally on your leaves, coating the tops and bottoms. This can be used on buds through harvest as well, though I would recommend rinsing your buds with water before harvesting.
Apply once per week as a preventative.
Neem Oil Products:
I would recommend to only use these during vegetative growth, pull back on use when buds begin to visibly form. Some have reported allergies to this product, and it may pose other health risks if ingested [11, 12].
- During vegetative growth, neem oil should be sprayed as a preventative every 7-14 days depending on how aggressive you are trying to be in your disease control.
- Generally it is recommended to use 2 tbsp of 70% neem oil concentrate for each gallon of water.
- If you experience negative reactions in your plants, try diluting it further to about 1 tbsp/gal.
- I like to add just a couple of drops of dishwashing soap to help emulsify the oil.
- Use a one-hand pressure sprayer to fully coat the tops and bottoms of all of the leaves of your plant.
- Spray your plants at night, just after the sun sets so that there is plenty of time for the leaves to dry
- If it is not dry by the time the sum comes out, your leaves can get sunburned quite easily, make sure you have plenty of fans moving and drying your leaves
- About 3 days after your neem oil application, rinse your leaves with water or a solution of citric acid pesticide such as Nuke Em by Flying Skull or other comparable products.
- I like to rinse just to prevent buildup of oils, but this is not necessarily required, in fact having the oil on the leaves can help deter insects. You could also just do a rinse right before your next application.
- If you are experiencing an outbreak, begin using it more frequently, up to once every 5 days.
- Purified azadirachtin products, while useful for insects, are not as useful for PM. The main ingredients in neem oil that are effective against PM are the triglycerides and terpenes that are not present in products like Azamax.
- Stop using it when you see buds beginning to form (not right when flowering starts, but you don’t want neem residue on your buds)
Some have reported that citric acid sprays are not as effective at controlling powdery mildew outbreaks as compared to other fungicides, but I believe that they can be effective when used in an IPM program with other fungicides. It is important to use what we have available since there are few targeted pesticides available for use in Cannabis.
- Some studies have shown that citric acid can significantly reduce the incidence of PM  (though there are no studies on Cannabis specifically)
- Citric acid sprays (especially solutions such as Nuke Em that also have insecticidal soaps and yeast) have the added bonus of helping to control insects including aphids, whiteflies, and arthropods such as mites.
- Finally, citric acid may be able to increase yield of your plants, including dry flower weight [14, 15].
- You can continue to use citric acid all through flower, and some people even spray it on their buds at harvest to help prevent postharvest bud rot without any noticeable change in flavor or bud quality.
- I would not use this in an IPM program along with baking soda solutions, because both affect the pH of your leaf surface in different directions.
Plant Extract Oils
There are some approved plant extracts described in my list of approved pesticides. I have never tried these for powdery mildew, but it may be worth trying. There are some products that have premixed a variety of different oils.
For instance, Trifecta crop control has the following ingredients:
However, this product, as well as neem oil, has the potential for causing some foliar stress symptoms until the plant becomes acclimated. It would be interesting to make a mix of Trifecta as well as Neem oil and replacing the pure neem spray with this mixture.
Again, I am not sure as to the efficacy of mixing this with neem. I have seen many forums of people claiming that this is a helpful product for controlling insects, though I have not seen much information for how it works on PM.
Reynoutria sachalinensis Extract
Reynoutria sachalinensis is a plant from which extracts are made. Extracts are classified as ‘plant activators’, meaning that the compounds stimulate the SAR (systemic acquired resistance) and ISR (induced systemic resistance) responses of plants. In short, this means that resistance genes in the plant are induced prior to any pathogen recognition, essentially making the plant more resistant to attack. I definitely recommend using this, and I believe it can be worked into a good IPM program. This, much like citric acid sprays, can be used up to the day of harvest.
B. subtilis Spray
Finally, I recommend adding a biofungicide (applying a spray with living microorganisms). The most common biofungicides utlize Bacillus subtilis bacteria or Trichoderma harzianum fungi. In general, B. subtilis is used as a foliar spray, and T. harzianum is used as a soil soak, mainly to control soil pathogens. However, there is evidence that using T. harzianum to the soil can actually increase the efficacy of B. subtilis foliar sprays [16,17], though I recommend sticking to B. subtilis sprays unless you are growing outdoors and concerned about soil pathogens. Such products include Serenade and Cease.
Keep in mind that B. subtilis in not labeled for Cannabis use in CA, if you want to use a Bacillus spray that is on-label, use B. amyloliquefaciens such as Revitalize or Triathlon.
Hydrogen Peroxide can be good for when an outbreak is actively occurring. If I could recommend one product for sanitation purposes, it would be Zerotol. Not only does it have hydrogen peroxide, but it also has peroxyacetic acid. Peroxyacetic acid forms when acetic acid (vinegar) reacts with hydrogen peroxide. Both of these are strong oxidizing agents that will kill the fungus on contact without harming your plants. As these products degrade, they will form water, carbon dioxide, and water. Peroxyacetic acid can be corrosive and dangerous in high concentrations, so be sure to dilute it according to producer recommendations. In the midst of an outbreak, this can be used every 3-5 days to help control the pathogen outbreak. This is a particularly effective tool for when an outbreak occurs in flower.
One preventative IPM program centered around PM might look like this:
Day 1: Neem Oil or Azadirachtin Spray/Plant Oil Extract Spray (or mixture)
Day 4: Citric Acid/Insecticidal Soap spray such as Nuke Em (to rinse neem oil) or sodium bicarbonate mixture outlined previously.
Day 7: Regalia Spray
Day 10: Restart Cycle.
Another rotation might look like:
Day 1: Neem Oil or Azadirachtin Spray/Plant Oil Extract Spray (or mixture)
Day 4: Citric Acid/Insecticidal Soap spray such as Nuke Em (to rinse neem oil) or sodium bicarbonate mixture outlined previously.
Day 7: Serenade Spray
Day 10: Restart Cycle.
You can also make tank mixes of Serenade and Regalia, or replace the neem oil with one or the other, especially after stopping using oil sprays in flower.
- Punja, Z. K. (2018). Flower and foliage-infecting pathogens of marijuana (Cannabis sativa L.) plants. Canadian Journal of Plant Pathology, 40(4), 514–527. https://doi.org/10.1080/07060661.2018.1535467
- Weldon, W. A., Ullrich, M. R., Smart, L. B., Smart, C. D., & Gadoury, D. M. (2020). Cross-Infectivity of Powdery Mildew Isolates Originating from Hemp (Cannabis sativa) and Japanese Hop (Humulus japonicus) in New York. Plant Health Progress, 47–53. https://doi.org/10.1094/PHP-09-19-0067-RS
- Szarka, D., Tymon, L., Amsden, B., Dixon, E., Judy, J., & Gauthier, N. (2019). First Report of Powdery Mildew Caused by Golovinomyces spadiceus on Industrial Hemp (Cannabis sativa) in Kentucky. Plant Disease, 103(7), 1773. https://doi.org/10.1094/PDIS-01-19-0049-PDN
- Félix-Gastélum, R., Olivas-Peraza, D. D., Quiroz-Figueroa, F. R., Leyva-Madrigal, K. Y., Peñuelas-Rubio, O., Espinosa-Matías, S., & Maldonado-Mendoza, I. E. (2019). Powdery mildew caused by Golovinomyces spadiceus on wild sunflower in Sinaloa, Mexico. Canadian Journal of Plant Pathology, 41(2), 301–309. https://doi.org/10.1080/07060661.2019.1577916
- Félix-Gastélum, R., Olivas-Peraza, D. D., Quiroz-Figueroa, F. R., Leyva-Madrigal, K. Y., Peñuelas-Rubio, O., Espinosa-Matías, S., & Maldonado-Mendoza, I. E. (2019). Powdery mildew caused by Golovinomyces spadiceus on wild sunflower in Sinaloa, Mexico. Canadian Journal of Plant Pathology, 41(2), 301–309. https://doi.org/10.1080/07060661.2019.1577916
- Dahlia – Plant Parasites of Europe. (n.d.). Retrieved February 13, 2020, from https://bladmineerders.nl/host-plants/plantae/spermatopsida/angiosperma/eudicots/superasterids/asterids/campanulids/asterales/asteraceae/asteroideae/coreopsineae/dahlia/
- Humidity | North Carolina Climate Office. (n.d.). Retrieved February 13, 2020, from https://climate.ncsu.edu/edu/Humidity
- Guzman-Plazola, R. A., Davis, R. M., & Marois, J. J. (2003). Effects of relative humidity and high temperature on spore germination and development of tomato powdery mildew (Leveillula taurica). Crop Protection, 22(10), 1157–1168. https://doi.org/https://doi.org/10.1016/S0261-2194(03)00157-1
- Sunflower (Helianthus spp.)-Powdery Mildew | Pacific Northwest Pest Management Handbooks. (n.d.). Retrieved February 14, 2020, from https://pnwhandbooks.org/node/3589/print
- Aust, H., & Hoyningen-Huene, J. V. (1986). Microclimate in Relation to Epidemics of Powdery Mildew. Annual Review of Phytopathology, 24(1), 491–510. https://doi.org/10.1146/annurev.py.24.090186.002423
- Bhaskar, M. V, Pramod, S. J., Jeevika, M. U., Chandan, P. K., & Shetteppa, G. (2010). MR imaging findings of neem oil poisoning. American Journal of Neuroradiology, 31(7), E60–E61.
- Neem Oil General Fact Sheet. (n.d.). Retrieved February 7, 2020, from http://npic.orst.edu/factsheets/neemgen.html
- Jafari, N., & Hadavi, E. (2011). Growth and essential oil yield of dill (Anethum graveolens) as affected by foliar sprays of citric acid and malic acid. I International Symposium on Medicinal, Aromatic and Nutraceutical Plants from Mountainous Areas (MAP-Mountain 2011) 955, 287–290.
- Talebi, M., Hadavi, E., & Jaafari, N. (2014). Foliar Sprays of Citric Acid and Malic Acid Modify Growth, Flowering, and Root to Shoot Ratio of Gazania (Gazania rigens L.): A Comparative Analysis by ANOVA and Structural Equations Modeling. Advances in Agriculture, 2014, 147278. https://doi.org/10.1155/2014/147278
- Ghazijahani, N., Hadavi, E., & Jeong, B. R. (2014). Foliar sprays of citric acid and salicylic acid alter the pattern of root acquisition of some minerals in sweet basil (Ocimum basilicum L.) . In Frontiers in Plant Science (Vol. 5, p. 573). https://www.frontiersin.org/article/10.3389/fpls.2014.00573
- Zaim, S., Bekkar, A. A., & Belabid, L. (2018). Efficacy of Bacillus subtilis and Trichoderma harzianum combination on chickpea Fusarium wilt caused by F. oxysporum f. sp. ciceris. Archives of Phytopathology and Plant Protection, 51(3–4), 217–226. https://doi.org/10.1080/03235408.2018.1447896
- Maketon, M., Apisitsantikul, J., & Siriraweekul, C. (2008). Greenhouse evaluation of Bacillus subtilis AP-01 and Trichoderma harzianum AP-001 in controlling tobacco diseases. Brazilian Journal of Microbiology : [Publication of the Brazilian Society for Microbiology], 39(2), 296–300. https://doi.org/10.1590/S1517-838220080002000018
1i. McPartland, J. M. (1991). Common names for diseases of Cannabis sativa L. Plant Disease, 75, 226–227.
2i. McPartland, J. M. (n.d.). A review of Cannabis diseases. Retrieved February 5, 2020, from http://druglibrary.org/olsen/hemp/iha/iha03111.html
Clones vs Seedlings
Clones and seedlings may seem very similar, but there are some differences between the two starting points. First of all, seedlings (small plants sprouted from seed) have a taproot. This is a central dominant root that tends to grow straight down and proliferate the branching root structures that explore the growing medium. Clones do not have a taproot; instead, they immediately begin producing a fibrous branching root structure. I would argue that the taproot is most important in outdoor grows due to the higher degree of anchoring and stem support that it can provide in windy weather.
Secondly, seeds all have unique genotypes while clones have the same genotype as the mother plant they were cut from. Truly stable seed lines produce plants with phenotypes so similar that they could be mistaken for clones, but usually in the Cannabis industry, a given seed pack for a strain may produce multiple different phenotypes. Sometimes this can be desirable if you are phenotype hunting for a unique plant to grow or breed with, but at least in large scale production, uniformity is usually preferred because it simplifies the growing, harvesting, and processing techniques
How Do I Germinate Seeds?
There are many ways to skin this particular cat. First of all, it is important to consider the environmental conditions required for germination.
While some small seeds without much of a starch reserve require light to germinate, it appears that light actually inhibits the germination of Cannabis seeds. This is likely due to the red light sensing system by light-sensing proteins called phytochromes. In general, far-red light can penetrate further into soil than red light due to the longer wavelength. Plants often utilize the ratio of far red light to red light as a way to sense depth in soil. For Cannabis, it appears that it requires a low far red/red ratio (no to minimal light) in order to germinate. However, pure darkness is unnecessary in my experience. In fact, it is a bit of a balancing act because after germination, your seedlings require light or they will not begin to produce chlorophyll and will continue to etiolate (grow and stretch in search of light to begin photosynthesis). Therefore, you will need to check your seedlings frequently so if you germinate in complete darkness, you can quickly introduce your newly sprouted seedlings to light.
Moisture and Humidity
Seedlings require water uptake in order to trigger germination. The media they are in contact with should be moist, and the humidity should be kept high but ventilated to help prevent microbial growth. In general, this translates to around 70-90% humidity. However, I don’t generally keep track of the humidity of the air in my germination area. Humidity is ensured to be high by enclosing the germination medium with a ziploc bag with holes cut in it.
I like to follow the rule of thumb: keep the temp in the 70s throughout the day and night. Don’t let temperatures dip below 70F and don’t let temperatures rise above 80F. In Celsius, this translates to approximately 21-27C. In practice, it is okay if it gets warmer, though I certainly would avoid letting temperatures get above 30C (86F). However, high heat can inhibit germination and encourage microbial growth. Also, dipping below 70F does not ensure failure, but may not be as efficient at germinating seeds.
I will only cover 2 germination methods. This is because in my opinion, they are simple, effective, and I have experience in both.
Method 1: Wet Paper Towel.
Get a paper towel and soak it in water (it is probably ideal if you get sterile, deionized water, though I generally use tap water and it works just fine). Squeeze out the paper towel so that it is damp but not wet. Place your seeds on the damp paper towel, and fold the paper towel one time over the seed. Place the folded paper towel in a gallon size plastic bag.
Option 1: Poke a few holes in the plastic bag (I like to use sharpened pencils, it has a good size for holes), blow into the bag to ensure it’s not collapsed on the paper towel, seal the bag, and place it in a dark, warm place. Check daily for germination and make sure to keep the paper towel moist. If it gets too dry, just use a spray bottle to spritz the inside of the bag and paper towel.
Option 2: Don’t poke any holes, exchange the air inside by sucking the air out the bag and blowing back into it to inflate it. Seal the bag, place it in a cool, dark place, and exchange the air in the bag once to twice/day and check frequently for germination.
Transplanting Germinated Seeds
After germination, I like to wait until the taproot is about an inch long. After this, pick it up by the seed coat with tweezers or a very gentle touch. Don’t touch the taproot. take your soil or growing medium, moisten it using appropriately pH’d water (around 6.5 for soil) and prepare a a hole deep enough to place the germinated seed in with the taproot facing down and the seed coat barely below the soil line. Place the seed in so that the taproot is straight down and so that the tip of the taproot is not bent or hooked when you plant it.
Plant your seed directly in a seed starter (I like to use coco coir with a bit of mycorrhizal fungi sprinkled in).
Option 1 is to buy seed starter coco coir pellets. All that is required is to wet the pellets with properly pH’d sterile water. They will expand and will have a small hole in the center for you to place your seed. After planting your seed, gently cover it up. This will provide both light and local humidity around your seed. Cover the pellet loosely with an open plastic bag to help retain moisture and leave it on a windowsill or under artificial light. This will ensure that once the seed germinates under the soil which is dark and humid, it will sprout above the ‘soil’ line, remain in a humid environment in the plastic bag , and will also be exposed to light so that the seedling can begin photosynthesis. I do not like to use peat moss or peat-based pellets such as Jiffy pellets. First of all, coco coir is far more environmentally friendly because peat is a nonrenewable resource, unlike coco coir. I stay away from rock wool for the same reason, coco coir is just a more responsible consumer choice for the environment. Secondly, peat is extremely acidic and may affect nutrient uptake early in a plant’s life as compared to coco coir. For all seed starting mixes, I like to make about a quarter of the volume perlite. Seedlings do not uptake water well and you want good soil aeration to avoid damping off and root rot diseases. If you use premade pellets, you will not have this option.
Option 2 is to fill a small, 2-3 oz plastic cup with coco coir, moisten it and make a hole for the seed yourself, and sow your seed. Follow the same directions as outlined for the pellets.
What do I do now that my seedling has germinated?
Now that you have a germinated seedling, you will notice two small ‘leaves’ that are kind of oval-shaped. These are known as cotyledons, and can actually help provide your plant with nutrients that were stored in the seed until the plant can feed on fertilizer or nutrients in soil depending on your growing style.
You will want to provide your plant with enough light to not stretch out. You can get away with using even a 60W single CFL ‘grow’ bulb for a seedling, but I tent to keep my seedlings under a 300W LED panel. I like to keep the light source about a foot from the top of my seedling.
Go ahead and keep the temperatures in the 70s (Farenheit). This is a great range for Cannabis growth and isn’t as conducive to disease development as warmer temperatures.
You will want to slowly lower the relative humidity. Keeping the same level of humidity as for germination will prove to be too conducive to disease development especially seedling damping off and root rots. Go ahead and keep the plastic bag over the seedling at first, and slowly increase the amount of time each day that it is not under the plastic bag. In general, I like to leave the bag off the seedling at night after it sprouts and during the day, reduce the time it is under the bag by an hour each day until the second set of true leaves are visible, then remove it altogether.
Moisture and Feeding
Seedlings require more moisture that mature plants but you also want to avoid root rots. Therefore, I like to use a spray bottle to mist the soil every day without soaking it. This should be done until the second set of true leaves are visible, then begin your normal watering schedule. Your seed starter mix should not have fertilizer in it. Your plant should have all the nutrients it needs from the cotyledons and all the energy it needs from photosynthesis. However, after the first true leaves are fairly large and the second set of true leaves are barely visible, I will sometimes include a Nitrogen dominant fertilizer at 1/4 strength in the spray bottle and lightly mist until slightly damp (I only ever do this once before transplanting, and only if the leaves are looking light). I tend to use liquid fertilizers (you can find conventional or organic fertilizers depending on your fancy).
Transplanting Your Seedling
By the time the second set of true leaves have grown in, your seed starter plug should be colonized by roots. Or, if you purchased a clone, it is likely already rooted in a rock wool cube. Take a pot approximately 10x the volume of your rooting medium, fill it with the planting medium of your choice (soil or soilless medium[if soilless, it is a good idea to do a light feeding (1/4 strength) as well at this point]). Water the medium in your new pot before transplanting and allow it to drain to field capacity. Make a crater in the center of your moistened medium deep enough to completely cover the seedling medium. I like the lowest nodes on the plant to be about 1-2″ above the soil line after planting. Place in your rooting plant, fill in the crater, smooth it out, and lightly pack in the planting medium around the stem of your plant. Water the pot once more to ensure the soil settles in, allow it to drain capacity.
Congratulations, you have germinated your seed and/or transplanted your clone/seedling. That was pretty easy, right?