My first experience with a virus- or viroid-infected cannabis plant was in 1996. I was employed as a consultant by one of the largest cannabis caregivers in the U.S. to confirm whether his genetics were infected with a suspected virus or viroid. He was experiencing a dramatic decline in yield, a diminished aroma and flavor, as well as a reduction in potency. At that time there were no lab testing services to identify a virus or viroid in cannabis crops.
Editor’s note: While some use the terms virus and viroid interchangeably, they are not the same. Although both are infectious and can cause diseases to plants, viroids consist of low molecular weight RNA that lacks a protein coat and can only infect plants, whereas viruses can infect a variety of living organisms, including humans, and are composed of a nucleic acid and enclosed in a protein coat.
I was tasked with growing his genetics in a different environment and growing system to identify quality and yield differences. At the end of the experiment, we, along with other cannabis experts, surmised that the plants were probably infected with a virus or viroid. In retrospect, it could have been hop latent viroid disease (HLVd) or hemp streak virus.
After this experience, I became vigilant for virus and viroid symptoms whenever I consulted in grows across multiple states. I began to notice a pattern of symptoms: fingers of leaves curling to the left or right, new shoots with lackluster or stunted growth, as well as yellow streaking in large and medium leaves. At the time, all I could do was speculate the cause.
And so, it wasn’t until about 2016, when I mentioned to a colleague—a government entomologist—that I suspected a virus was infecting cannabis genetics on a multistate level, and that each time a new market opened, cultivators typically sourced genetics from an existing market that was potentially infected with the virus, thus spreading the virus unknowingly throughout both legal and illicit cultivation facilities. My colleague eventually mentioned my theory to a fellow entomologist at Cal Poly Tech, who grew concerned that if I was correct and an unchecked virus was unknowingly being spread throughout the industry, it could mean billions of dollars in devastating crop losses.
Today, many labs have confirmed at least one widespread infection. A study from Dark Heart Nursery found that up to 90% of California’s cannabis genetics are infected with HLVd, which is said to have “jumped” from hops to cannabis.
Increased Proliferation
Cannabis growers have battled a plethora of pests and diseases in the industry’s long-dated history. But legalization has allowed large-scale cultivation to flourish. Thirty years ago, before the 1996 passage of Proposition 215 (legalizing medical cannabis in California), and long before California voters passed Proposition 64 (legalizing adult use), one very rarely saw powdery mildew in an indoor garden. As cannabis clubs and home grows became popular, clone points of sale multiplied.
This represented a seismic shift in how cannabis was cultivated. Historically, a large portion of cannabis was grown outdoors where cold winters killed most molds. If anyone was growing indoors, they typically kept a smaller number of plants to minimize the legal consequences if caught.
But as the demand for clones grew in tandem with the number of indoor farms, people made businesses out of being cannabis nurseries selling clones. Without the benefit of cold winters killing mold spores, the mass trading of clones seemed to rapidly spread powdery mildew spores. Sales of neem oil skyrocketed, and soon many cultivators started utilizing a myclobutanil fungicide to eradicate the mildew. The use of myclobutanil on a crop destined for combustion poses serious health risks for end consumers—many of them patients living with severe health conditions—as it turns into hydrogen cyanide when set ablaze. Unsurprisingly, most regulated markets ban its use on cannabis crops.
As the industry matured, growers learned to better control their environments, not to take for granted that new genetics are not infected, and to properly quarantine before introducing new clones. Despite these efforts, powdery mildew remains a serious threat to crops today.
A Growing Problem
The cannabis industry’s production increase exacerbated plant pests and diseases, in turn creating conditions that are conducive to the spread of new pathogens. For example, aphids can transmit potato virus to cannabis.
While growers are generally aware of potential fungal and bacterial diseases, as well as many pests, less attention has been directed to viral infections. Researchers will soon face challenges in identifying and managing these previously unidentified viruses, viroids, and diseases in cannabis.
HLVd is just the tip of the iceberg regarding cannabis/hemp and its susceptibility to viruses and viroids. For instance, lettuce chlorosis virus (LCV) has become a threat to cannabis cultivation as it can spread to clones from infected mother plants, and pests, with some studies claiming transmission through water.
The field of cannabis virology is fairly new, but I suspect it will become common for all commercial cultivators to engage with and employ labs that can perform both RNA and DNA testing, as well as PCR pathogen detection assays, for genetic fingerprinting for cultivar verification, and for detecting a multitude of potential viruses and viroids capable of infecting cannabis.
There is a very long list of other viruses/viroids that are capable of infecting cannabis plants or possibly even “jumping” to cannabis plants as the HLVd is suspected of doing. This is a inexhaustive list of potential threats (with most identified by researchers from the University of Milan and the University of Bologna):
Grapevine line pattern virus (GLPV)
Tobacco streak virus (TSV)
Tobacco mosaic virus (TMV)
Potato virus Y (PVY)
Potato virus X (PVX)
Cucumber mosaic virus (CMV)
Sunn-hemp mosaic virus (SHMV, also known as hemp streak virus)
Alfalfa mosaic virus (AMV)
Arabis mosaic virus (ArMV)
Raspberry ringspot virus (RRSV)
Broad bean wilt virus (BBWV)
Tomato spotted wilt virus (TSWV)
Cannabis cryptic virus (CanCV)
Lettuce chlorosis virus (LCV)
Beet curly top virus (BCTV)
Opuntia umbra-like virus (OULV)
Citrus yellow vein-associated virus (CYVaV)
Hop latent viroid/hop stunt viroid (HLVd)
Hemp leaf roll
Knowing that one infection (e.g., HLVd) increases the risk of another, the propensity for an outbreak of other viruses and viroids is very high.
Having witnessed powdery mildew, HLVd, and various mite outbreaks over the decades, I must ask when one of the aforementioned viruses or viroids listed above will become the outbreak of tomorrow. This seems especially likely considering some of the largest commercial vegetable greenhouses in the U.S. also host separate bays for cannabis cultivation. Others utilize converted greenhouses that previously housed flowers and are surrounded by agricultural crops that are susceptible to a multitude of pests and diseases.
The vegetable and flower industries are well-versed in the threats to their crops. But cannabis is just now learning exactly what its crop is susceptible to and discovering how to prevent and treat an outbreak.
Treatment and Prevention Protocols to Consider
Especially given the narrow range of pest and disease preventatives at the disposal of a cannabis cultivator, it is crucial to establish strong pest and disease prevention protocols. Many commercial-scale cannabis cultivation facilities currently utilize tissue culture to start with clean mother plants. In addition to starting mothers through tissue culture, many growers routinely send samples to a viral identification laboratory to pinpoint any infections as soon as they arise and ensure they can confidently take clones from the guaranteed clean plant. If the lab identifies an infection, these growers can generate new clean genetics in the form of new tissue cultures.
Commercial cultivation facilities that don’t grow from tissue cultures must carefully and methodically scrutinize all new genetics they plan to grow. Both clones and seeds can be transmission vectors for viruses and viroids. Growers must always lab test new genetics to confirm they are pest- and disease-free prior to introducing them to their facility. Seeds should be germinated in a quarantine environment and grown until they have enough biomass to send samples to a lab for viral verification.
Mother plants should be routinely screened to ensure they are always virus- and viroid-free to avoid taking contaminated clones and spreading the infection. Growers can observe plants for any and all anomalies, symptoms, or changes such as diminished yields, diminished aroma and flavor (i.e., terpene production), as well as noticeable potency reduction.
Growers must study and understand the signs and symptoms of virus and viroid infections, their damaging effects, methods of transmission, prevention, and methods to eliminate the virus. As the saying goes: An ounce of prevention is worth a pound of cure.
H/T: www.cannabisbusinesstimes.com
