Aphids Affecting Cannabis

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 [2]. 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 [3]. 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 [2], including viruses confirmed in Cannabis such as Cucumber Mosaic Virus (CMV) and Alfalfa Mosaic Virus (AMV) [3].
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 [3].

What Aphid Species Affect Cannabis?

At least six species have been reported to infest Cannabis plants [3]. 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 [14]. 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.

Female adult green peach aphids, Myzus persicae (Sulzer), with immatures. — Image From http://entnemdept.ufl.edu/creatures/veg/aphid/green_peach_aphid.htm

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.

Aphids May 2010-3.jpg — Image from https://en.wikipedia.org/wiki/Black_bean_aphid

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.

Image from https://influentialpoints.com/Gallery/Phorodon_cannabis_hemp_aphid.htm

P. humuli

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 [3].

Phorodon humuli — Image from https://bugguide.net/node/view/637027

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.

Nymphs (mixed ages) and dark form of wingless adult of melon aphids, Aphis gossypii Glover. — Image from http://entnemdept.ufl.edu/creatures/veg/aphid/melon_aphid.htm

Abstrusomyzus phloxae

An aphid with a fairly large host range that has been reported in California in 2018 [31]. There is little information on this aphid, but it appears to be primarily asexual, with females producing parthenogenetically [32]. I am unsure how aggressive of pests they are, but they have been reported from both indoor and outdoor grows.

Image from https://aphidtrek.org/?page_id=32

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 [3]. 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.

Image from https://www.leafly.com/news/science-tech/the-curious-case-of-the-cannabis-aphid

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.

Control

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.

Biological Control

Microbial Products

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.

Monterey LG6332 Bacillus Thuringiensis (B.t.) Worm & Caterpillar Killer Insecticide/Pesticide Treatment Concentrate, 16 oz

Safer Brand 5163 Caterpillar Killer II Concentrate, 16 oz

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) [5].

Marrone Bio Innovations Grandevo WDG Bioinsecticide Miticide OMRI Listed – 6 lbs – 2019 Reformulated

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 [4]. 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.

Venerate CG Bioinsecticide Insecticide for Mites, Thrips, aphids, borers, whitefly, leafhoppers on Grapes, Strawberries, Potatoes, Citrus and More (1, Quart)

Marrone Bio Innovations Venerate CG Gallon

Beauveria bassiana

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 [6], and even endophytic B. bassiana may negatively impact aphid populations on the colonized host plant [7]. 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) [10]. 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%.

BotaniGard 22WP Biological Insecticide 1lb

Isaria fumosorosea

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 [11], M. persicae [12]. 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 [13].

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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.

Green Lacewing Life Cycle — Image from https://biocontrol.entomology.cornell.edu/predators/Chrysoperla.php

Nature’s Good Guys Green Lacewing 1,000 Eggs – Slow Release Hanging Bag

Green Lacewing 5,000 Eggs – Organic Natural Aphid Control

Green Lacewing 10,000 Eggs – Organic Natural Aphid Control

Lady Bugs/Beeltes

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.

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Parasitoid Wasps

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].

Aphidius colemani

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).

A. ervi

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 [27]. 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 [28]. 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 [29].

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.

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Chemical Control

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 [17]. 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.

Organic Neem Bliss 100% Pure Cold Pressed Neem Seed Oil – (16 oz) High Azadirachtin Content – OMRI Listed for Organic Use

Organic Neem Bliss 100% Pure Cold Pressed Neem Seed Oil 32 oz – OMRI Listed for Organic Use

General Hydroponics GH2045 AzaMax, 4 Ounce

General Hydroponics AzaMax, 16 oz GH2007

Insecticidal Soaps

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 [18]. 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.

Safer Brand 5118-6 Insect Killing Soap Concentrate 16oz

Natria 706230A Insecticidal Soap Organic Miticide, 24 oz, Ready-to-Use

Safer Brand 5110-6 Insect Killing Soap, 32 oz.

Pyrethrins

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.

PyGanic Gardening 8oz, Botanical Insecticide Pyrethrin Concentrate for Organic Gardening

Pyrethrins can be mixed with insecticidal soaps or neem products such as azadirachtin

Safer Pyrethrin & Insecticidal Soap Concentrate, 1 Gallon

Azera Gardening 8 oz, Botanical Dual Action Azadirachtin/Pyrethrin Fast-Acting Insecticidal Concentrate for Organic Gardening.

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 [19].

Trifecta Crop Control Super Concentrate All-in-One Natural Pesticide, Fungicide, Miticide, Insecticide, Eliminate Spider Mites, Powdery Mildew, Botrytis, Mold and More on Plants Non-Toxic 4oz

Monterey LG 6299 Horticultural Oil Concentrate Insecticide/Pesticide Treatment for Control of Insects, 32 oz

IPM Concepts

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 [17]. 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 [17]. 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 [20].

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 [21]. However, spraying green lacewings directly with B. bassiana spore suspensions can negatively affect lacewing populations [22]. 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 [22].

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:

Infestation Response:

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.

Maintenance/Prevention

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 [24], and is of similar effectiveness as B. bassiana on the hop aphid (Phorodon humuli), closely related to P. cannabis [26]. 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.

Dedryver, C.-A., Le Ralec, A., & Fabre, F. (2010). The conflicting relationships between aphids and men: A review of aphid damage and control strategies. Comptes Rendus Biologies, 333(6), 539–553. https://doi.org/https://doi.org/10.1016/j.crvi.2010.03.009
APHIDS: What You Want to Know About Them and How to Organically Get Rid of an Aphid Infestation in Your Cannabis — Ed Rosenthal. (n.d.). Retrieved April 21, 2020, from https://www.edrosenthal.com/the-guru-of-ganja-blog/aphids-what-you-want-to-know-about-them-and-how-to-organically-get-rid-of-an-aphid-infestation-in-your-cannabis
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OREGON DEPARTMENT OF AGRICULTURE FACT SHEETS AND PEST ALERTS. (n.d.). Retrieved April 26, 2020, from http://www.Oregon.gov/ODA
Kim, J. J., Jeong, G., Han, J. H., & Lee, S. (2013). Biological Control of Aphid Using Fungal Culture and Culture Filtrates of Beauveria bassiana. Mycobiology, 41(4), 221–224. https://doi.org/10.5941/MYCO.2013.41.4.221
Endophytic Beauveria bassiana negatively impacts green peach aphids on strawberries – E-Journal of Entomology and Biologicals – ANR Blogs. (n.d.). Retrieved April 26, 2020, from https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=21711
Sayed, S. M., Ali, E. F., & Al-Otaibi, S. S. (2019). Efficacy of indigenous entomopathogenic fungus, Beauveria bassiana (Balsamo) Vuillemin, isolates against the rose aphid, Macrosiphum rosae L. (Hemiptera: Aphididae) in rose production. Egyptian Journal of Biological Pest Control, 29(1), 19. https://doi.org/10.1186/s41938-019-0123-y
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May 6, 2020 2

Fusarium Wilt and Fusarium Bud Rot in Cannabis (Hemp and Marijuana)

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 [32]. In tomato, when disease is severe, crop losses can reach 80% [33].

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 [21]. 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 [23]. F. oxysporum has been isolated from wilted Cannabis plants that does not match with either of the formae speciales cannabis or vasinfectum [21].

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) [1].

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 [1]. 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.

Image from Plant Pathology 5th Edition, Agrios (2005)

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 [1].

Fusarium solani species complex [22]

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.

Root Infection

Chlamydospores require a conducive environment to germinate and cause disease. In soil, this generally means the presence of root exudates [34]. 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 [1]. However, evidence of targeted growth of germinated spores towards roots is lacking (i.e. lacking evidence for chemotaxis) [11]. 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) [12].

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 [21].

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 [25]. 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 [25]. These Fusarium species can directly infect the bracts and pistils of flowers.

In hydroponics, Fusarium can be particularly aggressive [13]. 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 [1]. 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 [14]. 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 [24].

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 [19]

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 [20]

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

Symptoms

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

Left plant is uninoculated. Middle plant inoculated with *F. oxysporum*, right plant inoculated with *F. solani*.
Image adapted from Punja, Z., Scott, C., & Chen, S. (2018). Root and crown rot pathogens causing wilt symptoms on field-grown marijuana ( Cannabis sativa L.) plants.

The crown region of the plants become darkly discolored and sunken. Discoloration of the vasculature can extend up to 15cm from the soil surface.

Image adapted from Punja, Z., Scott, C., & Chen, S. (2018). Root and crown rot pathogens causing wilt symptoms on field-grown marijuana ( Cannabis sativa L.) plants.

Showing discolored pith. Image adapted from Image adapted from Punja et al. (2019). Pathogens and Molds Affecting Production and Quality of Cannabis sativa L.

In hydroponics, roots become discolored

Image adapted from 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.

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.

The Effect Of Fusarium On Marijuana Plants - Grasscity Magazine ... — Image from https://growersnetwork.org/cultivation-resources/growers-networks-disease-profile-fusarium-wilt/

Depending on where the infection occurs, wilting can be evident on some branches/colas but not others.

Image from https://www.lahuertagrowshop.com/blog/como-prevenir-y-eliminar-fusarium-en-plantas-de-marihuana/

Control

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 [4].
Soil treatments aimed at reducing the number of viable fungal propagules in the soil such as anaerobic soil disinfestation (ASD) [5] and solarization [6].
- 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 [7].
  - Soils that are more disease suppressive are sometimes associated with certain microbial groups in the soil microbiome [8]

Resistant Cultivars

Resistant strains undoubdtedly can be bred for. In hemp, SF and CF cultivars appear to be more resistant than the cultivar Iran [3]. 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.

Biocontrol Agents

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) [25].
- 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) [21].
In California, Gliocladium virens, Trichoderma harzianum, and Bacillus amyloliquefaciens strain D747 are approved biofungicides [26].
Other possible biocontrol biocontrol agents include Rhapsody (Bacillus subtilis strain QST 713) and Mycostop (Streptomyces griseoviridis strain K61) [21].

Plant Activators

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 fungicide

Monterey Agri-Fos Disease Control Fungicide – Pint LG3340

Quest Reliant Systemic Fungicide (Agri-Fos/Garden Phos) 1 Gallon

Cultural Methods

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 [21].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 [30]. 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% [31].
- 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.

Foliar/Flower Controls

The most important factor in preventing flower infections from Fusarium is probably humidity [25]. 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) [27].

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% [29]. 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.

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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.1998.88.6.544
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

March 29, 2020 3

Cannabis Viruses, Viroids, and Phytoplasmas

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.

Image from Nagy, P., & Pogany, J. (2011). The dependence of viral RNA replication on co-opted host factors. Nature Reviews. Microbiology, 10, 137–149. https://doi.org/10.1038/nrmicro2692
and uploaded on ResearchGate

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 viruses

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).

Image from https://viralzone.expasy.org/resources/Insect_vectorl.jpg
Representative of circulative and propagative transmission with phloem feeding insects

Semi-persistent viruses

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 [18]. In 2019, Lettuce Chlorosis Virus (LCV) was found in Cannabis grows in Israel [7].

BCTV

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 [17], it can be transmitted by seeds, but this is unknown for Cannabis.

HSV

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 [9]. 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 [3]. 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 [3].

HSV image: taken from https://cannabis.community.forums.ozstoners.com/uploads/gallery/album_1048/gallery_46756_1048_13453.jpg

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:

The leaf in the background shows leaf curling consistent with HSV symptoms, and the leaves in the foreground show streaking symptoms.
Image taken from https://www.growweedeasy.com/wp-content/uploads/2017/02/example-of-drooping-top-leaves-caused-by-broad-mite-damage-marijuana.jpg

HMV

***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 [3].

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:

Image result for hemp mosaic virus — Image taken from: https://scontent-lga3-1.cdninstagram.com/v/t51.2885-15/sh0.08/e35/c0.90.719.719a/s640x640/83170094_132982331526009_6191058825780320376_n.jpg?_nc_ht=scontent-lga3-1.cdninstagram.com&_nc_cat=109&_nc_ohc=sYxJf1-jcoYAX87iiVi&oh=5a216e8a4c195b0fb180d39831356293&oe=5EC01655

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 [16].

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 [7]. LCV was not found to be seed transmissible, but can be transmitted by vegetative propagation.

AMV

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).

AMV symptoms — Image taken from https://www.google.com/url?sa=i&url=https%3A%2F%2Fallcropsolutions.com%2Fdisease-testing%2Fhemp-and-hop-diseases%2F&psig=AOvVaw2ErtK3TxRV9yr9hAMiwN2l&ust=1584083662267000&source=images&cd=vfe&ved=0CAIQjRxqFwoTCLDUjtqxlOgCFQAAAAAdAAAAABAD
It is claimed to be AMV on *Cannabis*, though the diagnosis cannot be confirmed

An example of AMV on tobacco is shown below, demonstrating the leaf puckering and chlorotic mosaic:

Image result for alfalfa mosaic virus — Image taken from https://burleytobaccoextension.ca.uky.edu/files/styles/panopoly_image_original/public/tobacco_amv_seebold_img_1630.jpg?itok=sP6CFHK3

The following Cannabis leaf shows some very minor puckering as well as mosaic symptoms:

Image taken from: https://www.rollitup.org/proxy.php?image=https%3A%2F%2Fwww.thcfarmer.com%2Fcommunity%2Fattachments%2Fdsc03378-jpg.275131%2F&hash=64fe3a9a4288140b02e7d5d3efdae5b2

ArMV

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 [3]. 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

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 [15]. 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 [3]. An image of such symptoms on Cannabis is shown below, but I am not sure which virus is causing this:

Image result for cannabis cucumber mosaic virus — Image taken from https://encrypted-tbn0.gstatic.com/images?q=tbn%3AANd9GcSudCZ5olT67BS0jsCOP9_KrdYevd2AYIltNqQdS0woj4utLSnB

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 [3].

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 [5]. P. cannabis has also been shown to vector Pea Mosaic Virus in the lab [6], 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 [3], 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 [4]. Much like HSV, the causal agent of HMV has not been identified and confirmation of insect vectoring has not occurred [7].

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) [10]. 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 [13]. 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 [14].

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) [19]. 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:

Image result for symptoms of cannabis dudding — Image taken from https://i.ytimg.com/vi/W8B9Vp0anKE/maxresdefault.jpg

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.:

Image from https://www.thcfarmer.com/proxy.php?image=https%3A%2F%2Flh5.googleusercontent.com%2FK0ZJMbdQouv0Ky57fu1NZ-iH9VUfBal6bXhLuIyvcHtVi4zFHc0pR49ghjszXmLksiXQyglBEWguMdGfDl4r_cDTYZ4tw1BIpJCV2knnAD2EdiSMGOficnYhy_fgaVGkRSxfrlnf&hash=3015088fed798c1dc89c4b4b077e0260

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:

Image taken from https://www.thcfarmer.com/attachments/duds-in-dried-flowers-side-by-side-jpg.556818/

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 [21].
In 2011, hemp witches’ broom in Iran was confirmed to be caused by a phytoplasma in the stolbur group. [20].
In 2015, Cannabis sativa was identified as a host for the asteris group of phytoplasma in India [22].
In 2019, C. sativa was identified as a host for the trifolii group of phytoplasma in NV, U.S.A. [23].

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 [25].In India, the leafhopper Hishimonas phycitis was found to vector the asteris phytoplasma and is the putative vector in Cannabis [24]. 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.

Image from https://www.cannabisbusinesstimes.com/fileuploads/publications/38/issues/103476/articles/images/IMG_2653-Wiches%27_broom_fmt.png

Phytoplasma Control

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 [26], 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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Valdez, R. B. (1972). Transmission of raspberry ringspot virus by Longidorus caespiticola, L. leptocephalus and Xiphinema diversicaudatum and of arabis mosaic virus by L. Caespiticola and X. diversicaudatum*. Annals of Applied Biology, 71(3), 229–234. https://doi.org/10.1111/j.1744-7348.1972.tb05086.x
Cucumber mosaic virus – cucumber mosaic. (n.d.). Retrieved March 12, 2020, from http://www.extento.hawaii.edu/Kbase/Crop/Type/cucvir.html
Ziegler, A., Matoušek, J., Steger, G., & Schubert, J. (2012). Complete sequence of a cryptic virus from hemp (Cannabis sativa). Archives of Virology, 157(2), 383–385. https://doi.org/10.1007/s00705-011-1168-8
Anabestani, A., Behjatnia, S. A. A., Izadpanah, K., Tabein, S., & Accotto, G. P. (2017). Seed Transmission of Beet Curly Top Virus and Beet Curly Top Iran Virus in a Local Cultivar of Petunia in Iran. Viruses, 9(10), 299. https://doi.org/10.3390/v9100299
Giladi, Y., Hadad, L., Luria, N., Cranshaw, W., Lachman, O., & Dombrovsky, A. (2019). First Report of Beet Curly Top Virus Infecting Cannabis sativa in Western Colorado. Plant Disease, 104(3), 999. https://doi.org/10.1094/PDIS-08-19-1656-PDN
Warren, J. G., Mercado, J., & Grace, D. (2019). Occurrence of Hop Latent Viroid Causing Disease in Cannabis sativa in California. Plant Disease, 103(10), 2699. https://doi.org/10.1094/PDIS-03-19-0530-PDN
Fereshteh Vali Sichani, Masoud Bahar and Leila Zirak, 2011. Characterization of Stolbur (16SrXII) Group Phytoplasmas Associated with Cannabis sativa Witches’-broom Disease in Iran. Plant Pathology Journal, 10: 161-167.
Zhao, Y., Sun, Q., Davis, R. E., Lee, I.-M., & Liu, Q. (2007). First Report of Witches’-Broom Disease in a Cannabis spp. in China and Its Association with a Phytoplasma of Elm Yellows Group (16SrV). Plant Disease, 91(2), 227. https://doi.org/10.1094/PDIS-91-2-0227C
un nabi, S., Priya, M., dubey, D., & Rao, G. (2015). Identif ication of Cannabis sativa L. ssp. sativa as putative alternate host of sesame phyllody phytoplasma belongs to 16Sr I group in India. Medicinal Plants, 7, 68–70. https://doi.org/10.5958/0975-6892.2015.00010.6
Feng, X., Kyotani, M., Dubrovsky, S., & Fabritius, A.-L. (2019). First Report of ‘Candidatus Phytoplasma trifolii’ Associated with a Witches’ Broom Disease in Cannabis sativa in Nevada, U.S.A. Plant Disease, 103(7), 1763. https://doi.org/10.1094/PDIS-01-19-0098-PDN
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March 15, 2020 1

Disease Profile: Powdery Mildew of Cannabis

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.

White powdery mold growing on cannabi leaves like spots of flour — Photo taken from *White Powdery Mildew on Cannabis Plants – Identification & Solution!* (n.d.). Retrieved February 12, 2020, from https://www.growweedeasy.com/cannabis-plant-problems/white-powdery-mildew

Photo taken from *Is Powdery Mildew Systemic? | Medicinal Genomics*. (n.d.). Retrieved February 12, 2020, from https://www.medicinalgenomics.com/powdery-mildew-systemic/

Conditions

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 [1]. 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 [2].

Golovinomyces spadiceus grows best in warm, low humidity climates. It is commonly found on wild plant species such as wild sunflower [4] 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 [7]. The tomato-infecting PM species, which has also been reported as infecting Cannabis, is signifiantly reduced at low humidity levels (20-40% RH) [8], 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 [9].

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

*Powdery Mildew of Grape | Ohioline*. (n.d.). Retrieved February 15, 2020, from https://ohioline.osu.edu/factsheet/plpath-fru-37

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.

Control Strategies

Climate

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 [10]. 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.

Genetic Resistance

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
GDP
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.

Cultural

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.

Chemical

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.

Bicarbonates

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.

Recipe:

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)

Citric Acid

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 [13] (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:

14.0%……….Thyme Oil
10.0%……….Clove Oil
9.0%…………Garlic Oil
4.0%…………Peppermint Oil
3.0%…………Corn Oil
3.0%…………Geraniol
2.0%…………Citric Acid
2.0%…………Rosemary Oil

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

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
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February 17, 2020 4

How Do I Germinate Cannabis Seeds and Transplant Clones?

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.

Light

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.

Temperature

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.

Method 2:

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.

Light

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.

Temperatures

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.

Humidity

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?

February 10, 2020 1

How Do I Select Which Cultivar (Strain) to Grow?

A cultivar is a cultivated crop variety that has been produced from selective breeding. In the Cannabis world, cultivars have long been called ‘strains’, similar to wine grape cultivars being referred to as ‘varietals’. The term strain generally refers to unique genotypes of microorganisms, but has been somewhat co-opted by the Cannabis industry.

While there are some Cannabis breeders that truly pursue the production of stabilized genetic lines with unique phenotypic properties, in my opinion, most of the Cannabis strains on the market are produced quickly and have not undergone a long enough selective breeding process to stabilize the lines. Many strains on the market are a result of breeders simply crossing the best sellers of a given season in order to cash in on the hype of particular genetics. This results in most of the products on the market being unstable polyhybrids that have not undergone rigorous breeding on a large enough scale to select phenotypes for traits such as disease resistance that are commonly overlooked.

This is my list for the most important aspects in selecting genetics for your grow:

The most important aspect to consider in microgroweries is whether or not you enjoy the effects of the strain you are growing.
1. For example, some of my favorite strains of all time have Cherry Pie genetics including GDP and Purple Punch. I tend to get anxiety with sativa-dominant (cerebral) strains and prefer strains with more sedating effects. I also really enjoy strains with high CBD and moderate THC.
Secondly, it is extremely important to choose strains that have been released by reputable breeders that place importance on stable genetics (particularly when growing from seed). If you are purchasing clones, it is important to know that the genotype does not have a strong propensity for hermaphroditism and has phenotypic traits that are favorable.
In an indoor grow, some of the most important phenotypic traits to look for besides the effect of the strain are:
1. Flowering time (In general, strains that are closer to ‘indica’ on the cannabis spectrum are ready to harvest quickly as compared to plants that are considered more ‘sativa’). I tend to select strains that can finish in approximately 8-10 weeks after the start of flowering. If you are buying seed, one should consider the possibility of growing an ‘autoflowering’ strain. I will go into a bit more detail on autoflowering Cannabis a bit later on.
2. Yield (Plants that produce large and dense colas are generally preferable when growing on a small scale).
3. Stature (Height, bushiness, the degree that the plant stretches in flower, and density of branching are all important factors to consider.)
4. Cannabinoid profile and potency (This may be a very important factor to some. While cannabinoid profile is important to me, I tend to not select my strains on the basis of THC levels simply because most of the strains on the market are too strong for my personal preference, and I do not care much about maximizing THC).
5. Other metabolite profiles of your plant: Terpenes and flavonoids are important to consider. Terpenes are mainly responsible for the odor of the strain you are growing, but many are psychoactive and influence the effects of the strain you are growing.
6. Resistance to disease and pests: Different strains have different levels of resistance to various diseases and insect pests. For example, if you have had trouble with powdery mildew in the past, it would be useful to avoid strains that are particularly known for powdery mildew susceptibility.
  1. In my last grow, I had three strains in one grow tent (Dream Queen, Purple Punch, and Venom OG). I had a whitefly infestation occur, and I noticed an obvious difference in whitefly populations on the different strains. Dream Queen had very few whiteflies, while Venom OG seemed to have the most.
  2. For Powdery Mildew, it appears that the most resistant strains have Afghani heritage, please see my post on PM under pest and disease profiles for strain recommendations.
  3. For bud rot, it appears that the most resistant strains are equitorial sativas. Please see my post on bud rot under pest and disease profiles for strain recommendations.

As you can tell, indoor growing really lends itself to strains that have classically been considered ‘indica heavy’ and have a short, bushy stature, quick finishing time. Although plants with a more ‘sativa’ phenotype can be grown indoors, they tend to be more difficult because they are more lanky, they tend to have a longer internodal spacing, they stretch a lot during flower, buds tend to be less dense, and the time to finish can be quite long. However, there are techniques to help keep these plants more manageable such as training. Do not let yourself be deterred if you are set on growing such a strain.

If you are new to Cannabis and do not have a firm grasp on your strain preference, I would do the following to select a strain to grow:

Look for local vendors selling cannabis clones and look through their offerings. Select at least a few of the strains (some indica dominant strains, some sativa dominant strains, and some hybrid strains).
See if there are any dispensaries in your region selling flower of your selected strains and purchase a small amount of each of the selected strains.
Sample the flower on different days and select the one with the effects you enjoy the most for purchasing clones. Often times, the dispensaries in the region may not have the flower of the strain you want to sample. In that case, it might be worth doing some searching for the offerings of local dispensaries and only selecting strains that you can purchase clones for.

I believe that for first time growers, it is best to grow from clone as opposed to seed if you have access to them. The reasons for this include: genetic uniformity between plants and phenotypes that have already been selcted for favorable traits. When you buy seeds, as I mentioned earlier, many strains are not stable and can produce a variety of phenotypes, some of which may be unfavorable. It is best to have a bit of experience in finding trusted breeders before buying seeds. However, with a bit of web searching, you can find some well-reviewed genetics from trusted breeders. If you are buying seeds, I believe autoflowering seeds tend to be the easiest beginner plants to grow; you will not need to change the light cycle to induce flowering, they finish quite quickly, and you do not need to fuss about physical manipulation techniques such as ‘topping’, ‘FIMing’, trellising, or other training techniques. Though historically considered to be of lesser quality, autoflowering Cannabis has come a long way and there are many strains out there that produce high quality flower.

What to Look for in a Clone Nursery

Due to my location, I have only ever bought my clones from one source: Dark Heart Nursery (with the exception of when I was running Ken’s Cut GDP for a while). Dark Heart is a very reputable nursery that puts a lot of care into their strain selection, and most importantly for me, they produce disease-free clones. They have a dedicated Plant Health Department that ensures operating procedures are kept as sterile as possible, and they go through the necessary steps to ensure that their clones are free from known viruses and viroids. The causal agent of cannabis ‘dudding’ disease was discovered and described as the Hop Latent Virus by their plant health department and necessary steps were taken to ensure the viroid was eliminated from all plants [1].

It is important to select your clones from a nursery that takes care to ensure they are providing healthy, disease free clones.

How Do I Select a Reputable Breeder?

In my opinion, if you are not going to test out seeds or cuts from various different breeders, the best way to find reputable breeders is to read forums discussing the topic.

One of my favorite strains of all time that I kept in production for multiple years was Ken Estes’ Granddaddy Purple. I personally do not have a ton of experience with trying a large variety of breeders, but I have also had success with Greenhouse Seeds (I did a few runs of their Bubba Kush).

Purple Punch is a favorite of mine, and though uncertain, breeders at Supernova Gardens claim to have produced this legendary strain. Often times, the breeder of well-known strains are debated and the actual crosses that occurred are unknown as well. In such cases, you generally will grow the strain because it has a reputation of being high quality regardless of the breeder. One such strain I have grown is Dream Queen, though the genetics I grew came from the highly reputable Humboldt Seed Co.

I enjoyed growing Venom OG from Rare Dankness, but as with many ‘OG’ strains, it was a bit lanky and the buds were a bit loose for my liking. However, I enjoyed the diesel odors, rich colors, and effects of this strain. One of the most reliably consistent seed-grown strains I have ever run is Skunk #1 from Sensi Seeds. Though the strength of this strain is mild to moderate, I enjoy the lower THC content, the consistency of the plants, the hard to come by skunky odor, and the yield. Sensi Seeds has a lot of very stable strains that I enjoy, and they don’t seem to play the THC-hunting game or the name game so prevelant in today’s Cannabis culture. Their Big Bud is also a nice strain to grow. It produces well, is not overly powerful, and is very forgiving to the new grower.

I am also a big fan of Barney’s Farm. Though I cannot speak to all of their products, I grew their LSD one time. It yielded well, had no issues to speak of, and I enjoyed the moderately strong but relaxing effects. I would grow this strain again.

In terms of autoflowering plants, I have ever only tried Blueberry Auto by Dutch Passion Seeds. By introducing ruderalis genetics to DJ Short’s legendary Blueberry genetics, they have managed to produce a plant that yields very well, finishes very quickly under long day conditions, and is fairly potent with a nice nose, which can be difficult to achieve with autoflowers.

Aside from the breeders/seed companies mentioned above, popular seed companies in forums include DNA Genetics, Bomb Seeds, Sin City Seeds, Buddha Seeds, Anesia Seeds, and Royal Queen Seeds. These are all companies I see popping up in forums (recently I have seen a lot of praise for Sin CIty, Buddha, and Anesia in particular). I have not tried anything from them.

What is the Takeaway?

There are so many strains out there it can be overwhelming to choose. The bottom line is you should choose a strain that you enjoy consuming, but other factors (mainly yield and finishing time) should come into play in choosing between strains you like. Buy your seeds from a reputable breeder/seed company and buy your clones from nurseries that ensure you are receiving a healthy plant.

If you don’t know where to even begin and don’t want to sample a bunch of cannabis, simply choose any of the companies I have mentioned above and buy a product with positive reviews, go on Leafly and determine if the described effects are desirable to you, and just do it. When it really comes down to it, it’s hard to go wrong.

Warren, J. G., Mercado, J., & Grace, D. (2019). Occurrence of Hop Latent Viroid Causing Disease in Cannabis sativa in California. Plant Disease, 103(10), 2699. https://doi.org/10.1094/PDIS-03-19-0530-PDN

February 8, 2020 1

Cannabis Pest Profile: Whiteflies

Pictures from top left: The first three pictures show leaves infested with whitefly nymphs on the tops of the leaves. The third picture shows what appears to be a fourth instar nymph taken under a microscope. The fourth picture shows a mature adult whitefly on the top of a leaf. The final picture is again displaying nymphs on a leaf.
NOTE: There is some leaf clawing and tip burn in the 4th photo. This is a sign of nitrogen toxicity and generally an overabundance of nutrients. This soil was particularly hot on this run.

What is a Whitefly?

The most common cannabis pests are those with the physiological capability to bypass plant trichomes with piercing-sucking mouthparts. Such insects include aphids, leafhoppers, and whiteflies [1]. Whiteflies are annoying little pests that are related to aphids (in the order Hemiptera). They feed on the sap of plants, which is a sugar-rich solution transported in the plant phloem.

What Species of Whitefly affects Cannabis?

The most common whiteflies found in indoor grows in California have a wide host range and are mostly classified into a few species: Bemisia tabaci (sweetpotato whitefly), aleurodes vaporariorum (greenhouse whitefly), and silverleaf whitefly (Bemisia argentifolii) [1]. There are many more species of whitefly that may potentially affect Cannabis. However, control methods are largely the same between species.

What is the life cycle of a Whitefly?

Whitefly life cycle

Illustration adapted from M. L. Flint. July 1995. Whiteflies in California: a Resource for Coop. Ext. UC IPM Publ. 19.

The length of time for a whitefly to mature differs between species (32 days from egg to adult for greenhouse whiteflies, 39 days for silverleaf whiteflies [2]). After hatching, whiteflies must progress through four nymph stages and a pupal stage before becoming adult whiteflies. An adult whitefly can live between one and two months, though different species have different lifespans. Whiteflies tend to lay eggs and spend most of their early development underneath leaves. However, nymphs and adult whiteflies can often be seen in high concentrations on top of Cannabis leaves as well. Whiteflies reproduce most prolifically in warm weather, though I have also had a whitefly outbreak occur in a grow tent that was in the garage during winter. It got quite cold during the nights (50 Farenheit), which for the record, is not ideal for plants either.

What Danger do Whiteflies Pose to my Plants?

Whiteflies can cause direct mechanical damage to your plants. They feed by piercing plant tissue and in high populations, they can cause yellowing (chlorosis) of leaves and even leaf death [3]. Generally damage caused by feeding alone is mild unless populations are very high.

They also cause indirect damage. As whiteflies feed, they exrete excess sugar-rich liquid onto plant leaves known as honeydew. Honeydew can attract other insects such as ants, and can be a substrate for sooty black mold to grow on leaves. Sooty mold and honedew can negatively impact photosynthesis. Furthermore, whiteflies are known to transmit many plant viruses. Though viral Cannabis diseases have not been heavily researched, a 1971 study found that a majority of screened common plant viruses can infect Cannabis [4]. Greenhouse whiteflies have been reported as some of the worst vectors for viruses that can infect Cannabis [5]. Please see my post on Cannabis viruses under pest and disease profiles to learn about what viruses may be transmitted by whiteflies.

What Can I do to Prevent and Treat Whitefly Infestations?

Environmental Conditions

As mentioned, whiteflies do best in warm, humid weather. An indoor grow tent provides perfect conditions for whiteflies to reproduce. Keeping your grow area cool is one way to help, but Cannabis grows best at warm temperatures as well, so you will be stressing your plants and inhibiting growth by doing this. My rule of thumb is to keep your grow area in the low 70s (Farenheit). Humidity should be controlled not only to inhibit the proliferation of insect pests, but also microbial pathogens. My personal preference is to keep the relative humidity of my growing space around 40%, certainly no higher than 50% (for mature plants only, seedlings may require high humidity).

Prevention

Cultural methods: Keep your grow area clean. Do not bring plants into your grow area that have been kept outside. Sterilize all tools used in the grow area with 70% isopropanol or ethanol. Be sure to clean the inside of your grow tent before introducing new plants, remove all plant debris from the area.

Physical control methods: I like to use reflective mulches in all my grows. If you are doing indoor container gardening, a simple method is to make a carboard cutout that sits inside of your pot, on top of the soil. Make sure to cut a hole in the center for the plant stem, and cut a small strip from the outside of the collar to the center so that you can slide it around the plant stem. Finally, wrap the collar in aluminum foil and slide it around your plant stem. Reflective mulches can repel invading insects and provide more light to the underside of plant leaves which is also beneficial to plant photosynthesis. Furthermore, it helps control other insect pests that utilize the soil to complete their life cycle, and helps maintain soil moisture and temperatures. Collars should be removed for watering and feeding of plants.

Chemical prevention methods: Neem oil is the hydrophobic extract of the seeds of the neem tree [8]. It is typically bought as an extract of 70% neem oil. I advocate using neem oil sprays even before encountering any insect or fungal issues with your plant. I recommend spraying your plants once every two weeks up until flowering is visible. I do not use neem oil after flowering begins because it can affect the taste and the smell of your buds if it gets on flowers. Furthermore, neem oil can cause allergies in some, and has been shown to be a potential toxin to humans when consumed [6]. I would recommend using neem oil only on healthy vegetative plants.

To spray plants, you will need a one-hand pressure sprayer. I like to use 2 Tbsp of neem oil per gallon of water and I add a couple of drops of dish soap to help emulsify and spread the neem oil. Always test one leaf of a new cultivar/strain you are growing before spraying the whole plant by soaking a single leaf in your prepared neem oil solution. Check in a 24-48 hours for any adverse reactions. Spray your plants at night time only, and be sure that there are fans on your foliage to ensure that the solution dries before your lights switch back on. It is important that the solution dries because lights can easily burn your leaves if they are still wet, especially with a neem oil in it. I also like to rinse my leaves 3 days after the last spray. First off, it helps remove extra fats and oils that are just sitting on your leaves, and secondly, it can help rinse off any potential insect eggs. It is a good idea to use a spray bottle that you only use water in to do this. Do not rinse your leaves frequently as this increases risks of foliar diseases developing. Only do this every other week, three days after your neem oil application.

Another good leaf rinse besides water is a citric-acid based rinse such as Nuke Em by Flying Skull. Doing a weekly or biweekly leaf rinse with such a product can help prevent insect outbreaks as well as powdery mildew outbreaks. I cannot recommend this enough. You could even use this rinse instead of the water rinse after your neem application.

Because indoor grow tents tend to be isolated, natural predators of pests are not present to help control pest populations. Releasing lacewing larvae and/or ladybugs in your grow tent can be a good idea soon after planting to help reduce any potential pest populations before they get out of control. You must be aware that you may need to alter your spray program if you are introducing beneficials as many pesticides will also kill your beneficial insects.

Treatment

The most important thing after prevention is monitoring. If you have taken proper preventative measures and you still notice a growing population of whiteflies in your grow area, it is time to be more aggressive. At first sight of whiteflies, prune and remove infested leaves from your growing area.

Chemical Control Methods

Insecticidal Soap [7]

At this point, it is a good idea to spray down your plants with insecticidal soap. However, if your plant is in flower, do not spray your buds with insecticidal soap. Though safe, the soaps may affect the taste of your buds. Insecticidal soaps will also kill beneficial insects, so it is not recommended to use insecticidal soaps if you are utilizing beneficials. Always test the sprays on a small portion of the plant before spraying the entire plant.

Neem Products [8]

If an infestation is in progress, I personally like to swap out neem oil for an azadirachtin product such as Azamax. If you continue to use neem oil, begin applying on a weekly basis as opposed to a biweekly basis. Azadirachtin is the most active chemical found in neem oil, and I find it to be more effective than just neem oil. Azadirachtin is OMRI certified. It can become systemic in the plant and has low toxicity. However, there have been negative effects reported from neem products in some people, and so I would recommend not to use any neem products on your plants during flower, and certainly not during the last 4 weeks before harvest. If you are growing cannabis to sell it or give to others, I also do not recommend using neem products as some people may have allergies. I have found this pesticide to be very reliable in getting rid of whitefly infestations (as well as certain spider mites). This product will also kill most whitefly predators, with the possible exception of ladybugs, so it is not recommended to use this product along with beneficials.

Citric Acid Products [9]

My chemical of choice would have to be a citric acid based pesticide such as Nuke Em by Flying Skull. Citric acid pesticides are OMRI certified and very effective at killing whiteflies. They are nontoxic, and can be sprayed directly on buds without worry of altering the flavor or smoke of your bud. In my personal experience, this should be your go-to chemical product if you are worried about using neem products at all. I would recommend reapplication every 5 days until the infestation clears up. It would be useful to have a one-hand pressure sprayer for application.

Pyrethrin Insecticides [10]

Pyrethrin insecticides can be quite effective, but I would not recommend using them at any point during the flowering cycle of your plant as they are toxic to humans as well as insects. I would recommend to stick to neem and citric acid products.

Spinosad Insecticides [11]

Spinosad is composed of two bacterial-derived chemicals that are toxic to a wide range of insects. It can be quite toxic to beneficial insects as well as bees, and so I recommend spraying at night and only utilizing it if necessary (especially if you are venting to outside). It should not negatively affect beneficials if they are released after the spinosad completely dries. However, it is nontoxic to humans, the label claims that it can be used safely up to the day before harvest (though I would not spray this on flowering plants as according to the UC IPM website for whiteflies [3]) and can be useful in a microgrowery IPM program if you are not using beneficials. Spinosad can be quite effective at killing whiteflies.

Beneficial Insects

Beneficial insects can be very effective at controlling whiteflies. Often, whitefly populations only get large due to the lack of natural predators present in greenhouse and grow room operations. If you are growing inside and do not want a lot of insects flying around, this may not be the best option. However, if you want to avoid using chemical control methods, this can be an effective replacement. If you are using chemical control, beneficial insects can be used in conjunction with certain sprays to maximize your pest suppression.

Green Lacewings

Green lacewings (Chrysoperla rufilabris) are particularly effective at controlling populations of soft-bodied insect pests. Adults are not predatory, but their larvae will feed on whitefly eggs and nymphs [12]. If you are not using sprays in your IPM program, it is a good idea to release lacewing adults early in the vegetative stages. You must provide the adults with flowers so that they can feed on nectar and pollen (spring-flowering plants for vegetative growth, fall-flowering plants for your flowering cycle) [13]. They will lay eggs and newly hatched larvae will help control pest populations. Alternatively, if there are already pests present in your grow room, you can release green lacewing larvae directly to address the infestation. If you want to maintain your lacewings, you must add flowers for them to feed on when they mature.

Whitefly Parasitoids

Encarsia formosa (for greenhouse whitefly) and Eretmocerus eremicus are two commonly used parasitoids of whiteflies that lay their eggs in whitefly nymphs, eventually causing the death of their hosts [14]. They are selective and highly effective at addressing large populations of whiteflies and should be released in the case of a heavy infestation.

Combining Chemical and Insect Control Methods

The only chemicals I would recommend combining with insect releases are neem oil and citric acid-based sprays. At the first sign of an infestation, begin with a wash of insecticidal soap. Following this, apply a neem or azadirachtin spray to your plants. 3 days following this, rinse your plants with water followed by a citric acid-based pesticide. After your plants dry completely, it is safe to release your beneficial insects. Do not apply any other sprays for at least 2 weeks after releasing your beneficials. If you are still experiencing an infestation (which is highly unlikely), I would recommend trying a spinosad insecticide.

McPartland, J. M. (1996). Cannabis pests. Journal of the International Hemp Association, 3(2), 52–55.
Whiteflies in the Greenhouse. (n.d.). Retrieved February 5, 2020, from http://www.ladybug.uconn.edu/FactSheets/whiteflies-in-the-greenhouse.php
Whiteflies Management Guidelines–UC IPM. (n.d.). Retrieved February 5, 2020, from http://ipm.ucanr.edu/PMG/PESTNOTES/pn7401.html
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