What is a plant virus?
A virus is an infectious nucleic acid-based pathogen that is parasitic to the host. Essentially, it is a non-host genome that ‘hijacks’ the replication machinery of the host cell in order to amplify its genome. It also requires host enzymes to translate the viral transcripts into proteins that the virus uses to form a protective coat and move within the host/spread to alternate hosts such as insects. A virion is a viral genome encapsulated in a protein coat. Some viruses will also have a fatty membrane surrounding the protein capsid.
There is some argument as to whether viruses can really be classified as ‘alive’. Though they have a genome, they do not have their own metabolism or replication abilities. They can really be thought of as ‘selfish genes’, meaning that they are a group of genes that evolved together in order to perpetually reproduce. They are truly an insight into the evolution of life: how replication is always selected for, seemingly without purpose in the case of a virus.
Since they are obligate parasites, viruses generally have evolved to not kill their native host before they can reproduce enough. In the case of insect vectored viruses, this may mean interacting with the plant in such a way that the metabolism of the plant becomes more attractive to the insect vector. In the case of seed-transmitted viruses, it may mean inducing early flowering to shorten the period of time that the virus is restricted to the host, one can think of many ways that a virus can affect the phenotype of a plant to favor its spread while harming the agricultural value of the crop.
The very first virus to ever be discovered was actually a plant virus, and one that has had some amount of interest within the Cannabis community, tobacco mosaic virus (TMV). We will discuss TMV further, but for now I will just say there is little evidence of it causing disease symptoms in Cannabis. Different plant viruses are transmitted through different means. Plant viruses are usually spread by an insect/nematode vector, through seed, through pollen, or are mechanically transmitted (usually in the setting of human agriculture).
- Some plant viruses are limited to the plant phloem and cannot infect epidermal/mesophyll cells.. These viruses generally cause ‘yellows’ symptoms, a faily even chlorosis
- Sap-transmissible viruses can infect epidermal/mesophyll cells and generally cause mosaic and mottling symptoms
Generally, the phloem limited viruses are semi persistent (travel to the insect vector’s foregut) or persistent viruses (travel to the insect vector’s haemolymph and salivary glands) that are vectored exclusively by phloem-feeding insects such as aphids, whiteflies, or leafhoppers. Sap-transmissible viruses commonly cause mosaic and mottling symptoms on host tissue, whereas phloem-limited viruses tend to be ‘yellowing’ type viruses with more uniform symptoms. Many viruses are multipartite, meaning they have segmented genomes that are encapsulated separately but must all be present in the host in order to cause disease.
Persistent Circulative plant viruses are able to enter the haemolymph (analagous to blood) and salivary glands of their insect vectors/seconday hosts, but they do not replicate within the host cell. Peristent Propagative viruses are able to actively reproduce within the insect and also infect the insect haemolymph and salivary glands. Circulative viruses remain viable within for long periods of time (often the lifespan of the insect).
Semi-persistant viruses do not enter the haemolymph of the insect vectors, but do travel to the foregut. Some yellows and some mosaic viruses are semi-persistent. They do not take as long to acquire for the insect vector compared to circulative viruses, and the latent period is very short. They remain viable for inoculation for a few days.
Non persistent viruses
Nonpersistent viruses are only carried on the stylets of sucking/probing insects and remain viable for a few hours and must be quickly transmitted to a new host.
What is a plant viroid?
All known viroids infect plants, and most are pathogenic. Viroids are very similar to viruses except that they do not have a protein coat, they are simply self-replicating and transmissible nucleic acid pathogens. Viroids are usually spread through aphids or mechanical transmission.
What is a plant phytoplasma?
Phytoplasmas are very small mollicutes (bacteria that lack a cell wall). They are not much like viruses biologically, but I am including them in this particular article because the symptoms many phytoplasmas cause can be similar to those caused by viruses. In addition they are unculturable and are insect-transmissible (mostly by leafhoppers, planthoppers, and psyllids). Phytoplasmas act very similarly to circulative viruses within the insect vector, they enter the insect haemolymph and colonize the salivary glands.
What viruses have been reported in Cannabis?
In my opinion, viruses are significantly underdiagnosed in Cannabis and are the least understood of all pathogen classes in the species. There appears to be more research into Cannabis diseases recently, but up until the recent past, only 5 viruses were reported as commonly and naturally causing problems in commercial Cannabis production. These viruses are: Hemp streak virus (HSV), alfalfa mosaic virus (AMV), cucumber mosaic virus (CMV), arabis mosaic virus (ArMV), and hemp mosaic virus (HMV). As far as I am aware, HSV has only been reported on fiber cultivars in Europe. HMV, on the other hand, has been reported in some drug cultivars in Pakistan. AMV, CMV, and ArMV have been reported on European hemp. There is next to no information in regards to infection of modern North American drug cultivars by these viruses
In 1971, Hartowicz et al. screened 22 common plant viruses and found that over half of them were able to infect Cannabis, but only 8 actually caused symptoms. Keep in mind that these were all manual innoculations and may not actually be vectored to Cannabis in nature. Of the manually inoculated viruses, 5 of them caused severe mosaic symptoms and plant stunting: Tobacco Ringspot Virus (TRSV), Tomato Ringspot Virus (TomRSV), Tobacco Streak Virus (TSV), and Cucumber Mosaic Virus (CMV). Two of the viruses caused mosaic symptoms but did not cause heavy stunting: Alfalfa Mosaic Virus (AMV) and Eunoymous Rinspot Virus (ERSV). Elm Mosaic Virus (EMV) also caused some symptoms of necrotic flecking.
Most recently (2020), Beet Curly Top Virus (BCTV) was found to be a common and naturally occurring infectious agent on plants in Colorado . In 2019, Lettuce Chlorosis Virus (LCV) was found in Cannabis grows in Israel .
BCTV symptoms were first noticed in a field in CO in 2015. Leaves would begin yellowing in a mosaic pattern from the petiole to the tips of leaves. Plants with advanced symptoms displayed stunted growth, malformation of new leaves, leaf chlorosis, and necrotic flecking. BCTV was confirmed to be the causal agent by utilizing next generation total active RNA sequencing and using control plants to find the viral genome present. It was confirmed through PCR as well. This has been the method of discovering viruses in high value crops such as grapevine stock, and it appears to be utilized more for discovering diseases of Cannabis. BCTV is a Curtovirus within the Geminiviridae family. It is fairly unique because its genome is ssDNA instead of RNA. This means that it needs to utilize the host plant’s transcription and translation mechanisms. It has a monopartite genome that is encapsulated in a dual icosahedral capsule. It is the only virus in Cannabis that is transmitted by a leafhopper, Neoaliturus tenellus, native to the western USA. BCTV is a circulative nonpropagative virus, has a very broad host range, and can have significant impacts on yield. In some species , it can be transmitted by seeds, but this is unknown for Cannabis.
HSV is reportedly one of the most common viral diseases, at least in commercial hemp in Europe. However, researchers have been unable to identify any causal agent for the symptoms associated with Hemp Streak Virus. It has long been assumed that HSV is viral and transmissible, but more recent molecular studies suggest that an abiotic factor may be at play because no pathogenic viruses were found in symptomatic Cannabis plants from screening with targeted PCR reactions and RNA sequencing . It has been suggested that if this is indeed a viral disease, that it may also be responsible for certain leaf curling symptoms found on Hemp in Hungary in the late 1990s . It is also reportedly vectored by aphids and seed, but this has not been demostrated experimentally. Insect vectoring would certainly point towards HSV being a vectored pathogen .
I wonder if HSV symptoms may have been misdiagnosed mite damage, such as broad mites or russet mites which produce symptoms very similar to those described of HSV. Some mites may even be vectored by insects by hitching a ride The following image is taken from a plant with broad mites, and displays symptoms very similar to those described from HSV:
***Please be aware that there are many online sources that believe that Sunn-Hemp Mosaic Virus is the same as HMV and claim that Sunn-Hemp Mosaic can infect Cannabis. There is absolutely no evidence of HMV being caused by SHMV, and this misidentification does not even align with reports of HMV being only an insect-vectored disease. Sunn-hemp is a legume and is a completely different plant than hemp, but the name seems to be confusing for some. Reputable websites including Dinafem’s website seem to equate Sunn-Hemp virus to Hemp Mosaic Virus without justification.***
The causal agent of HMV is also unidentified, but has been suggested to be a Cucumovirus or a Nepovirus. It is reportedly vectored by aphids, but I have seen one report that it was vectored by onion thrips, which doesn’t make sense as no Cucumoviruses or Nepoviruses are vectored by thrips. In one experiment, an Argentine sunflower virus was inoculated onto hemp and the plant contracted HMV-like symptoms and could be transmitted by aphids .
HMV causes chlorotic leaf lesions that expand, become necrotic, and can kill leaves. HMV may cause leaf enation, leaf curl, bunchy top, and reduced leaf size. Severe symptoms may look something like this:
This is simply a plant with symptoms consistent of those with HMV, it is not a confirmed diagnosis.
Cannabis cryptic virus
Another virus known as Cannabis cryptic virus has been identified to be ubiquitously present in Cannabis without causing any disease symptoms [8, 9]. Virions were visualized and sequences were obtained, but viral presence does not seem to be correlated to symptoms. The cryptic virus appears to be a Partitivirus which is likely seed transmissible .
Lettuce Chlorosis Virus
Recently (2019), a new virus was reported in Cannabis. LCV is a Crinivirus within the Closteroviridae. Lettuce Chlorosis Virus (LCV) was reported as causing interveinal chlorosis, leaf brittleness, and necrotic spots in licensed Cannabis grows in Israel. The virus was shown to be transmissible by the whilefly species Bemisia tabici . LCV was not found to be seed transmissible, but can be transmitted by vegetative propagation.
AMV is an Alfamovirus within the Bromoviridae. AMV was first identified on hemp in Germany through sap transmission tests. It is most commonly spread through aphid and seed/clone transmission, but can also be spread through dodder or root grafts. It is a ssRNA virus and the genome is split between 4 virions.
Symptoms include leaf chlorosis in a mosaic or mottle pattern, sometimes presenting as a gray mosaic. Young leaves may have strange morphology (puckering).
An example of AMV on tobacco is shown below, demonstrating the leaf puckering and chlorotic mosaic:
The following Cannabis leaf shows some very minor puckering as well as mosaic symptoms:
ArMV is a Nepovirus within the Secoviridae family. Both CMV and ArMV are tripartite viruses that cause similar symptoms. ArMV has been described as causing chlorotic spots and stripes, and has also been described as displaying symptoms of chlorotic ‘check mark’ shapes . It appears to have a negative impact on the growth of the plant as well. ArMV has a broad host range. It infects many vegetables, and also infects hops. It is primarily transmitted by nematodes, but may also be seed transmissible in many species and can be transmitted by vegetative propagation (cloning).
CMV is a tripartite Cucumovirus. It is within the Bromoviridae like AMV. It has a very broad host range and has been found on dicots and monocots including various vegetable, ornamental, and grass crops . It is vectored by aphids and is also seed transmissible in many species. It will also be transmitted by cloning. This will be difficult to distinguish from other mosaic viruses, but has been described as having light green ‘check mark’ chlorosis, similar to that described for ArMV . An image of such symptoms on Cannabis is shown below, but I am not sure which virus is causing this:
Can TMV Infect Cannabis?
In Hartowicz et al., they also mention that TMV is indeed able to infect Cannabis, but it did not cause any noticeable symptoms [2, 3]; rather, Cannabis appears to act as a carrier for the virus. They confirmed the presence of TMV in nonsymptomatic Cannabis by back-inoculating to indicator plants. It seems that in almost every Cannabis forum site or popular media site, it is commonly reported that TMV can cause symptoms in Cannabis. However, these claims are usually uncited and unsubstantiated. In fact, in the cases of people actually posting serological test results of plants with mosaic symptoms, I have never seen a positive result for TMV. Of course, different cultivars of Cannabis may react differently to infection by TMV and some may actually produce noticeable symptoms that were not seen in experiments conducted on hemp cultivars. While it may be possible for TMV to cause symptoms in Cannabis, based on published information, it is more likely that TMV is symptomless in Cannabis and that mosaic symptoms are caused by viruses that have been reported to affect the health of Cannabis plants.
A farmer once reported that he infected a Cannabis plant with a tobacco virus that caused stunting and mosaic symptoms, but it is speculated that virus transferred was Tomato ringspot virus, Tobacco rinspot virus, or Tobacco streak virus as these can all infect Cannabis in cases of mechanical inoculation and are sap transmissible .
What Insects Vector Viruses in Cannabis?
According to Ceapoiu (1958), the worst vectors of Cannabis viruses are bhang aphids (Phorodon cannabis), greenhouse whiteflies (Trialeudodes vaporariorum), onion thrips (Thrips tabaci) and green peach aphids (Myzus persicae). P. cannabis has been shown to vector at least 2 viruses to Cannabis: CMV and AMV . P. cannabis has also been shown to vector Pea Mosaic Virus in the lab , but I am unaware of any cases of natural infection of PMV in Cannabis. In 1955, P. cannabis was also reported to be the vector for HSV , though as mentioned earlier, it is still unknown if HSV is actually a viral disease. P. cannabis has also been reported to vector Hemp Mosaic Virus (HMV), though molecular evidence of this is lacking . Much like HSV, the causal agent of HMV has not been identified and confirmation of insect vectoring has not occurred .
Despite T. vaporariorum being reported as a vector for Cannabis viruses, there is no information in the literature as to which Cannabis diseases it may vector. The only viruses shown to be vectored by T. vaporariorum in plants are within the Crinivirus genus. The only confirmed crinivirus in Cannabis is the recently reported Lettuce Chlorosis Virus, which was reported B. tabaci as a likely vector, but T. vaporariorum may be able to vector this virus as well.
In regards to thrips, the two main species affecting Cannabis are onion thrips (Thrips tabaci) and western flower thrips (Frankliniella
occidentalis) . Again, there is no evidence here of whether or not any viruses are actually vectored by thrips in Cannabis. The only viruses known to be vectored by thrips are within the Tospovirus genus, none of which have been reported in Cannabis. Of course, this does not mean that tospoviruses don’t affect Cannabis or that thrips do not transmit viruses, just that there is not yet evidence of this.
Green peach aphids (Myzus persicae) have also been reported as vectors in Cannabis. They are at least capable of transmitting AMV, CMV [11, 12], but it is unknown what other viruses they may vector.
In regards to ArMV, it appears that it is most readily vectored by dagger nematodes within the genus Xiphinema . However, no Xiphenema species have every been reported on Cannabis. Despite an uncited claim on Wikipedia, ArMV has not been demonstrated to be vectored by insects such as aphids or whiteflies, but may be able to be transmitted by other genera of nematodes such as needle nematodes withing the Longidorus genus .
BCTV is the only Cannabis virus to be vectored by a leafhopper (specifically Neoaliturus tenellus, at least that is the only reported vector of BCTV on the wide range of hosts it can infect.
What to do if you suspect your plant(s) have a virus
First off, there is no easy answer to this. There is no ‘cure’ to viral diseases; there is no spray that will eliminate the problem for you. Growers have to be on top of insect control, proper sanitation, and culling of infected plants in order to prevent infection in the first place. Of course, sourcing virus-free growing stock (seeds or clones) is of upmost importance.
Second, it is important to have an IPM program in place to control insects that may be vectors for viruses. I would say that aphids and whiteflies are the two biggest threats as insect vectors.
For all pathogens covered in this article including viruses, viroids, and phytoplasmas, one of the simplest things you can do to prevent spread is to practice sanitation such as sterilizing your tools with 70% alcohol between each cut or plant, to sterilize your indoor facility after each grow, and to practice cleanliness in your grow areas and what you wear in your grow areas. It is never a bad idea to shower before going into your grow area, wear scrubs that you wash frequently, and have dedicated boots that your sterilize the soles of frequently.
What Viroids Infect Cannabis?
As of now, the only viroid that has been reported in Cannabis is the Hop Latent Viroid (HpLVd) . It is a 256 bp circular RNA within the Cocadviroid family. It was identified by Dr. Jeremy Warren at Dark Heart Nursery through total RNA sequencing of symptomatic plants. It was confirmed as the disease-causing agent through development of infectious RNA constructs and RT-PCR of infected plants. In Cannabis, the disease is known Cannabis Dudding. Plants with Dudding may have reduced vigor, smaller size, more stretch and weaker branching, and small leaves in vegetative growth:
In flower, buds lack trichome development, terpenes are reduced, bud is more leafy and more airy (larfy), buds can be irregularly shaped, chlorosis and leaf death may occur, and buds appear behind schedule. In the following picture, the cola on the left is from a healthy plant, and the cola on the right is from a dudded plant.:
The infected bud does look frosty in this picture, but the lack of density and high amount of vegetative growth is evident. The following picture shows dried buds from the same plants:
Dark Heart nursery has been able to provide clean stock by doing tissue culture to produce mother plants. This is a process where cells from the very tip of the apical meristem are cultured and generated into a new plant, and many viruses and viroids have not yet been able to infect the newest growth of plants. HpLVd does not appear to be insect-vectored, but is transmitted through mechanical means, usually by using non-sterile shears and tools on plants. It may be transmissible by seed at low rates as well.
To prevent HpLVd, it is important to begin with clean stock and use proper cultural controls in your grow including spraying tools with 70% ethanol in between each plant.
What Phytoplasmas Infect Cannabis?
Cannabis is susceptible to a wide range of phytoplasmas.
- In 2007, hemp witches’ broom in China was identified as a phytoplasma in the Elms Yellows (EY) group .
- In 2011, hemp witches’ broom in Iran was confirmed to be caused by a phytoplasma in the stolbur group. .
- In 2015, Cannabis sativa was identified as a host for the asteris group of phytoplasma in India .
- In 2019, C. sativa was identified as a host for the trifolii group of phytoplasma in NV, U.S.A. .
All of these phytoplasmas were identified through nested PCR using conserved phytoplasma primers followed by sequencing.
Phytoplasma symptoms in Cannabis can include:
- A high amount of branch proliferation from a branch node
- Shortened internode spacing
- Small leaves or leaf dieback
- Phyllody (abnormal development of flowers as leafy structures.
What vectors phytoplasmas in Cannabis?
The primary vectors of phytoplasmas are leafhoppers, planthoppers, and psyllids .In India, the leafhopper Hishimonas phycitis was found to vector the asteris phytoplasma and is the putative vector in Cannabis . Elms yellows diseases are usually transmitted via leafhoppers. Stolbur group phytoplasmas are frequently transmitted by planthopper species. Trifolii group phytoplasmas are frequently transmitted by leafhopper species.
Generally, Phytoplasma diseases are controlled by preventing infection through insect control. Some phytoplasmas can be seed transmissible, but this has not been demonstrated in Cannabis. Phytoplasma diseases may be able to be controlled through application of antibiotics such as tetracycline and rifampicin , but these are not approved for commercial production anywhere that I am aware of. A plant genotype can be recovered through meristematic tissue culture, but once infected, a plant will not produce well or have marketable bud.
In short, viruses, phytoplasmas, and viroids are mostly controlled through prevention and include virus free seed, tissue cultured clones, and insect control. One a plant has one of these problems, it will not be able to be treated and will not be worth producing with. In fact, it is important to cull any plants you find with these symptoms unless you want to preserve the plant’s genetics, in which it may be worth having the plant go through tissue culture.
I hope this article shed light on the confusion surrounding these types of diseases in Cannabis and addressed some of the misinformation commonly spread on forums and message boards.
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