The Threat of Viral Cannabis Diseases

Columns - Growing Pains

Growers worldwide are seeing Cannabis disease symptoms with unknown causes among their crops. Here’s what we know about potential causes, detection and what you can do to stop viral spread.

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Distorted leaf margins can be a sign of CDS infection.
Photo by Mojave Richmond

Around the world people are coming to grips with the health issues and economic fallout of COVID-19. Public awareness of viral contagions is reaching unprecedented levels, presenting an opportune moment to address disease problems within the cannabis industry. As we are seeing with the novel coronavirus pandemic, harmful viruses emerge and adapt, and this is not exclusive to humans. Cannabis growers are increasingly experiencing the negative economic outcomes of decreased vigor, lower flower yields and reduced production of primary target compounds including both cannabinoids and terpenoids. What we are calling “Cannabis disease syndrome (CDS)” exhibits a suite of consistent symptoms, but with no readily apparent single cause. These symptoms, which do not appear to be caused by nutrient deficiencies or other pathogens, are often collectively referred to as “dudding” or “dudders.” (The term originated when growers would think a plant with decreased vigor or stunted growth was “just a dud.”)

Steadily declining vigor in commercial Cannabis clones is not a new phenomenon. As vegetative reproduction by rooting cuttings became popular in the 1980s, growers would occasionally see a clone that became weaker and less productive each time cuttings were flowered. Apart from lowered yield, there were few other symptoms of infection. We tentatively called this a “photocopy effect” based on our analogy that copying a copy of a copy of a copy, results in a faded image that eventually becomes a mere ghost of the original.

We knew that because lost vigor was appearing in asexually multiplied serial cuttings the problem could not be explained by “genetic drift,” which is a shift in the frequency of genes within a small sexually reproducing population. Growers wondered what the causes might be, and even addressed the possibility that simply making serial cuttings might result in diminished vigor.

Soon we realized the symptoms were caused by transmission of an infectious disease that became more and more prevalent through successive rounds of multiplication. (More on this later.) We destroyed clones exhibiting symptoms, carefully sterilized benches, pots and tools, and began to use fresh blades when taking cuttings from each mother plant. There were no known causes, just obvious adverse effects. Yet we found practical solutions, and soon the problem nearly disappeared.

Some similarities exist between the CDS we are experiencing today and COVID-19. Much like the human coronavirus, CDS is difficult to detect at first, as there is a wide range of symptoms. Through our and other growers’ observations of affected plants during the past few years, we have learned that vegetative plants can transmit CDS, while flowering plants are more likely to suffer the consequences. Because symptoms are not readily visible and are easily confused with other diseases, they both lie hidden within populations, and can very quickly become economically impactful. Another similarity between CDS and COVID-19 is that asymptomatic plants can infect the otherwise healthy, with more serious outcomes for some than others. Molecular testing is required to identify potential infections, and there are few laboratories that can effectively identify the causal organisms. Other than practicing social distancing and establishing quarantines, there are as yet no solutions to stopping their spread.

Cannabis disease syndrome cannot be attributed to a single pathogen, although there is a primary candidate for its cause (more on this later). In symptomatic plants, several infectious organisms may be involved, making accurate diagnosis and effective control even more difficult. If CDS killed more of its hosts rather than simply making them sick, then it would have been noticed much earlier, and should not have already spread so widely. As we also have observed while studying affected plants, the cannabis disease syndrome spreads most quickly by taking cuttings from infected plants, using them as mother plants, and thereby multiplying the disease through future generations.

 

What causes CDS?

We are unsure which pathogens cause CDS, but likely candidates include viral infection accompanied by opportunistic fungal and/or bacterial infections. Viral diseases are common in crop plants including Cannabis. Cannabis cryptic virus (CanCV) was recently identified in several European industrial hemp cultivars, and infected plants exhibit symptoms that may previously have been confused with tobacco viruses. Alfalfa mosaic virus (AMV), cucumber mosaic virus (CMV) and Arabis mosaic virus (ArMV), all originating in other plants, have also been identified in European industrial hemp cultivars. Recently, lettuce chlorosis virus (LCV) was found in Israeli glasshouse medicinal cannabis crops, and beet curly top virus (BCTV) was reported from plants grown outdoors in Colorado. All could pose economically significant problems in the future.

 

In the Spring of 2019, two laboratories independently identified hop latent viroid (HpLVd) within a number of California sinsemilla clones, and this pathogen is the prime suspect in our search for the causal pathogen of the CDS we experience today. Originally discovered in Cannabis’s closest relative Humulus lupulus, the source of hops cones used in brewing beer, it is unclear how it spread to Cannabis. Unlike viruses and other diseases with more readily apparent symptoms, HpLVd is symptomless in most hop cultivars, affecting only the most susceptible. And, co-infection is often first indicated by the symptoms of opportunistic fungal pathogens that infect hop plants weakened by HpLVd, or plants weakened by HpLVd may become more susceptible to fungal or bacterial diseases, similar to the situation we face in diagnosing its infection of Cannabis.

CDS causal organisms travel through the plant’s vascular system, spreading from lower parts of stems (on the left) upward toward younger tissues (on the right).
Photo by Mojave Richmond

How can we know if our plants have CDS?

Infected plants vary from the healthy norm. CDS symptoms are subtle and difficult to perceive and are therefore most noticeable in clones with which a grower has intimate familiarity. The foremost outcome of a CDS infection is lowered productivity resulting from a loss of vigor. Plants grow more slowly, flowers are smaller, and resin gland development is slowed. Lower branches appear to grow away from the central stalk more than usual and sag, while the main stem grows erect. Other common symptoms include brittle stems that easily break when bent, distorted leaf growth, variegated and chlorotic leaves and overall stunted growth, resulting in drastically lowered expression of terpenes and cannabinoids.

Although the presence of HpLVd may initially be asymptomatic, as it progresses, additional symptoms begin to appear such as stunted growth, general yellowing of the foliage, discolored mosaic blotches or streaks, interveinal yellowing, deformed leaf margins of younger leaves and discoloration within the stems, all characteristic symptoms of possible opportunistic coinfection by fungal or bacterial pathogens.

Adding more confusion, HpLVd-infected plants may actually exhibit symptoms that can be perceived as favorable, such as darker green foliage and increased branching, which complicates detection and control while increasing its spread through the propagation of seemingly healthy cuttings. Molecular testing by experienced laboratories is the only way to verify the presence of HpLVd.

Viral infections spread internally from infected to clean parts of the same plant, making sampling difficult. Only part of a plant might be infected, maybe just one or a few limbs. Since HpLVd may hide in a minute portion of the vascular system, the only way to know for certain that a plant is healthy is to test tissues from several parts of the plant before destroying it. Sampling each clone for pathogens, and verifying that the cutting remains clean, can be a costly process.

How can we control CDS?

Long before scientists discovered that a human immunodeficiency virus (HIV) is the primary cause of acquired immune deficiency syndrome (or AIDS), health care organizations were already advising people of preventative measures: use condoms, do not reuse needles, sterilize razors, etc. They understood the severity of AIDS and how to curtail its spread long before they knew its cause. As with human viruses, awareness, mitigation and suppression are the key elements in the control of plant pathogens. Once awareness is raised throughout our community, testing must begin to learn the causes of the disease and the extent to which it has spread. Infections must be traced back to their sources, and diseased plants either quarantined or destroyed. If the spread of CDS is stopped, it may eventually die out, or at least reach levels that do not cause widespread economic losses. There are several steps that growers should take to prevent the spread of pathogenic diseases in general, and specifically CDS.

1. Control pests.

Viral vectors include common Cannabis pests such as aphids, whiteflies, thrips and mites that transmit and widely spread viruses and other pathogens by first feeding on an infected plant, and then feeding on an uninfected plant. Effective pest control should be a part of every grower’s strategy.

HpLVd infection also attenuates flower and resin development. These plants of the same cultivar are the same age and grown under identical conditions.
Top Photo by Ali Bektas
2. Keep it clean.

Each time you take a cutting, the plant’s vascular system is susceptible to infection. When establishing new mother plants, use fresh, sterile blades directly from the package or use sterilized scissors for every cut to assure you are not transmitting a disease. Viral and fungal infections can also spread via contaminated containers, benches and growing media. Household bleach diluted with five parts water is an inexpensive and effective sterilizer. Spray clean tools, soak scissors, drench containers and wipe down benches. Use sterile media for rooting cuttings and growing flowers. Wash and sterilize your hands or change gloves before moving on to the next mother plant. When possible, buy clean stock from a trusted nursery.

3. Quarantine.

Set up a sterile quarantine area for any new arrivals to your facility, and keep new additions isolated within until you become confident that they are not infected. (Again, the only way to be entirely sure a plant is healthy is testing.) Flower a few cuttings in isolation and watch for disease symptoms before multiplying your mother plants. Better yet, do not accept any foreign plants. As we have observed time and again, most grows become infected by taking in cuttings from others.

4. Limit visits.

An infected plant can touch an uninfected neighbor and spread diseases, and humans who come into contact with infected plants can also spread diseases. Prevent the spread of CDS by keeping people who may have been in contact with potentially infected plants, especially other growers, out of your quarantine and propagation areas.

5. Destroy affected plants.

The easiest, cheapest and most effective strategy is to destroy any mother plants you suspect are infected. Always label and number cuttings so you will know which mother they came from, then later each cutting can be traced back to its source. Should cuttings prove to be infected you should destroy the entire clone—the mother plants and all of their cuttings.

Stunted growth, as seen on the right, can be observed in plants where HpLVd has taken hold. These plants of the same cultivar are the same age and grown under identical conditions.

Can I get rid of CDS?

Once CDS is suspected and/or confirmed in a crop there are several possible solutions for its control.

1. Outgrow it.

Viral and fungal infections translocate through the vascular system of the plant and may be left behind in older tissues as plants develop, so it may be possible to simply outgrow the problem. Under vegetative daylength (more than 16 hours of light each day), transplant small mother cuttings into fresh sterile media, feed them well, and grow them rapidly under strong artificial lighting, or better yet, under full natural sunlight. Take small cuttings from the uppermost meristems or growing tips of the fastest growing plants and root them in sterile media or an air rooter with sterile water. The more cuttings a grower roots, the higher the chances of selecting a clean one.

Unfortunately, many of society’s most popular clones, some of which have become industry standards, have the highest chance of being infected. By now they have been through countless vegetative propagation cycles, and each cycle exposed them to possible infection. Whenever feasible, replace potentially infected cuttings with an earlier version of the same clone. Smaller growers who focus on fewer varieties may have started with and still maintain an uninfected clone. Quarantine any new arrival and be sure it performs better before replacing your old favorite.

There are other possible solutions that should be outsourced unless a grower has the resources to try them on their own. Micropropagation of meristems in vitro has been touted as a way to clean an infected clone. Many confuse micropropagation with in vitro culture of callus or undifferentiated cells, the plant version of stem cells. Meristems are the last place pathogens reach as they spread through a plant, and the smaller the cutting, the less chance it will be infected. Micropropagation protocols root tiny meristems in sterile conditions, much smaller cuttings than can be rooted in a typical commercial nursery. Once a clean clone is maintained in sterile culture it remains clean, and when multiplied under sterile conditions its offspring will also be clean.

Micropropagation alone does not remove pathogens. However, nurseries are developing protocols to reduce and possibly eliminate pathogen loads by passing meristems through a series of conditions that kill disease organisms without killing the plant. Although theoretically possible, no company has shown that this process actually works, and results in a disease-free plant that also retains its favorable characteristics. One can only know if cleanup has been effective by first testing to be sure a pathogen such as HpLVd is present before treatment, and then cannot be detected following treatment. A clone that has been “cleaned” should also exhibit the favorable traits it had before it became infected. Before spending money, one must decide whether their clone is worth attempting to recover, or whether it would be more efficient to restart with clean nursery stock or seeds.

The localized chlorotic yellowing of leaves can be caused by HpLVd infection as well as other opportunistic fungal and bacterial pathogens associated with CDS.
PHoto by Robert C. Clarke
2. Sow your seeds.

Unsanitary cuttings are the main root of Cannabis diseases, and rather than trying to clean an infected clone, growers can simply sow seeds. Seed and pollen transmissions of viruses and viroids have been shown for several crops but are unconfirmed in Cannabis. Traditional seed propagation interrupts a pathogen’s life cycle, allowing a fresh start each year. Seeds provide the best chance of procuring clean plants and establishing a new clone. Who knows? You might even discover the next OG Kush!

Conclusion

CDS rarely kills individual plants, which is one reason for its silent spread, but it may well be lethal to our industry. We have no measure of how fast it is spreading. We are only beginning to examine the problem, and as our awareness increases and growers begin to experiment, more effective solutions will certainly appear. As cannabis is increasingly commercialized, disease control will become the responsibility of licensed and certified nurseries that will preserve and distribute clean stock, like all clonally based commercial agriculture from potatoes to berries.

It is high time for our industry to be proactive. CDS as well as other diseases have become part of our industry and are likely here to stay. As with viruses within our human population, we will learn how to live with them and manage them efficiently.

Robert C. Clarke is a freelance writer, photographer, ethnobotanist, plant breeder, textile collector and co-founder of BioAgronomics Group Consultants, specializing in smoothing the transition to a wholly legal and normalized cannabis market. info@bioagronomics.com

Mojave Richmond is the developer of many award-winning varieties such as S.A.G.E., which served as a springboard for creating many notable cultivars. Richmond is a founding member of the international consulting company BioAgronomics Group. info@bioagronomics.com

Ali Bektas is an Istanbul native who received his Ph.D. from University of California, Berkeley where he worked on field-based DNA detection methods for indigenous farmers in Mexico to monitor local corn landraces against transgenic contamination. He was staff scientist at Phylos Bioscience from 2017 to 2019.