Hop Latent Viroid (HpLVd): What We Know
Hop Latent Viroid (HpLVd or HLVd) is an insidious, highly-contagious pathogen that sneaks up on crops and can decimate the quality and yields of your harvests.
As its name suggests, HpLVd crossed over into cannabis from hops, the only other member of the cannabaceae plant family. The pathogen is easily-transmitted through cross-contamination—from scissors to gloves to cloners—and, as more and more clones are swapped, traded and sold, the faster it has spread throughout the industry.
“Part of that problem is the ‘hot new strain’ thing,” says Jeremy Warren, Director of Plant Sciences for Dark Heart Nursery.12
“If a nursery misses the hot strain, they need it ASAP. Nurseries often skip the quarantine and testing stage, and ramp up production of clones right away. As a grow, bringing in new genetics is usually the biggest source of a new viroid; unfortunately, we are at the point where most grows already have it.”12
"More than 2/3 of the industry has this problem," adds Zac Ricciardi, National Cannabis Specialist for BioSafe Systems.13
The average HpLVd infection rate in the United States is 30%, with places like California reporting up to 90% of grows testing positive for HpLVd in their facilities.11
This data is the result of over 250,000 tests submitted to Dark Heart from across the country.11 Since 2014, growers and nurseries such as Dark Heart Nursery have been working on researching HpLVd in pursuit of detection and treatment methods for this increasingly rampant issue.1
“We have tested grows in almost every recreational and medical state, as well as Canada,” confirms Richard Philbrook, Molecular Biologist at Dark Heart Nursery. “Every state that has sent in samples has had [HpLVd] to some degree.”12
What is Hop Latent Viroid (HpLVd)?
In short, HpLVd is a small snippet of genetic material (about a tenth of the size of a virus), that does not carry any other genetic material and remains dormant (latent) until bloom, making it easy to spread and hard to detect.
HpLVd is a viroid (not a virus) that originates in hops plants and was confirmed in 2017 to also be present in cannabis.2 Viroids are small, coat protein-free, infectious RNAs, which are considered to be transmissible mainly by mechanical injury of plant tissues.3
The insidious nature of this viroid is that it often remains latent until later in flower, typically affecting inflorescence (flowers) beginning in the second trimester of flower.
While it is possible to see symptoms of HpLVd in vegetative rooms, affected plants are much more easily identifiable after the fourth week of flower. Additionally, HpLVd can persist without symptoms throughout its life, remaining a contagious threat to other plants.4
What can Hop Latent Viroid do to crops?
HpLVd is often referred to as "dudding" due to the stunting effect it has on plants. Symptoms of HpLVd include:1
- Stunted growth
- Reduced vigor
- Brittle stems
- More lateral than apical growth
- Degradation of Inflorescence (aka “Dudding”)
- Lack of bud development
- Minimal resin production
- Rapid maturation of trichomes (premature/accelerated ripening)
- Reduction in concentration of cannabinoids, terpenes, and flavonoids within resin
How is Hop Latent Viroid transmitted?
Viroids like HpLVd are transmitted mechanically through cross-contamination, often through abrasions and cuts.1,5
Most commonly, this spread happens because tools weren't sterilized, or infected and clean plants shared the same reservoir or unfiltered water source. Once introduced into a grow, viroids can be spread in recirculated water, air, and by workers and their tools.5,6 It is also possible that HpLVd can spread from nearby crops, even non-cannabis crops.3
That means that if you have runoff coming from an infected plant that is sharing a saucer or tray with an uninfected plant, there's a chance of contamination. If you take cuts from an infected plant and put them in a shared reservoir or system (aerocloners and hydroponic/aeroponic systems), you are potentially infecting all other genetics sharing that air and water. Similarly, if you are growing outdoors near other crops that are susceptible to or could carry this viroid, you run the risk of introducing it into your grow.
“[Outdoor cross-contamination] is very common with viruses that have broad host ranges,” adds Warren. “Currently, for this specific viroid, these are the only known hosts of the viroid: Cannabis (Cannabis sativa), hop (Humulus lupulus), Japanese hop (H. japonicus) in the family Cannabaceae and stinging nettle (Urtica dioica) in the family Urticaceae14,” though he concedes that this limitation could be a result of a lack of research into other plants that are susceptible.
How do you know if you have Hop Latent Viroid?
Testing the plant is the only way to know for sure.
If possible, the best method is to submit samples to a lab that can run multiple tests including reverse transcription polymerase chain reaction (RT-PCR, not LAMP assays that are commonly used for at-home tests) and molecular hybridization (dot blot) testing.3 If you are a licensed facility or in a market with labs that can service all growers, this will be the preferred and most accurate method for determining if your plants have HpLVd.
Recently, many companies have begun releasing at-home LAMP tests for HpLVd. While these are a great resource and tool, the accuracy of a LAMP test isn’t as reliable as a lab test. At-home tests are significantly impacted by calibration, which, when off by small amounts, can impact results (triggering false results).7
When lab testing is not an option, a combination of at-home testing and symptom identification will often be enough to identify potential infections, but thorough lab tests are the only definitive way to confirm with certainty.
According to Dark Heart, the only way to ensure your garden is free from this invasive viroid is to sample and test regularly.1
“Once you have no infection, rigorously test any clone you bring in from outside source,” says Philbrook. “That’s where people are messing up; quarantine and test [new clones] before you start bulking them out in your production space.”
Best Ways to Prevent a Hop Latent Viroid Infestation
"The best thing for a grower to do is prevent HpLVd from ever finding its way into the grow," reiterates Zac Ricciardi.
Eventually, it will become standard practice in the industry for clones and cuts to come with certificates of authenticity (COAs) verifying that they are free of pathogens, including HpLVd. But, until such a time, Ricciardi advises that you only take in cuts from vetted sources, and, ideally, start from seed. "Starting from seed is the best way to avoid it. Seed is definitely much cleaner than taking in clones."
If your plants are already growing, preventing HpLVd starts with cleanliness to prevent cross-contamination.
It's important to make sure that growers and grow employees are not reusing equipment (scalpels, containers, feed reservoirs) in between plants. If you are growing multiple plants in shared trays or tables, be aware of runoff, insects, and even touching plant material that can spread viroids from plant to plant.
Additionally, while aerocloners are great for single strains, they can be a mixed bag when more than one plant or genetic is being cloned. As a result, Ricciardi says that it is no longer advisable to mix plants and cuts within the same cloner.
All it takes is one cutting with the viroid in a cloner or hydro system to potentially contaminate the rest of the plants.
According to Philbrook and Warren, commercial cleaners like Virkon work well to clean tools in between cuttings, further cautioning that isopropyl alcohol/isopropanol does not work. “Yes, [isopropanol] kills viruses by removing the coat protein, but HpLVd does not have a coat protein. Iso precipitates RNA, so it actually makes transmission worse.” This holds true for any alcohol-based cleaner (ethanol or isopropanol).
Can you get Hop Latent Viroid from seed?
To date, there have been no published studies specific to cannabis that demonstrate that HpLVd is seed-transmissible in cannabis.
In 1999, the first study of its kind on the transmission of HpLVd claimed that the viroid is potentially transmitted in one specific chemotype of hops at a frequency of 8%, but study data is not available.8 Prior to this study published, there was no information on the transmissibility of HpLVd through the generative (seed-making) or seed phases.3 Successive studies reiterate that "low transmission efficiency has been reported through pollen transfer or by seed." These studies have since disproved the claims of seed-transmissibility.3,8,9
In one follow-up study, 100% infected parent plants were used to produce F1 hybrids, which showed no infection when grown under greenhouse conditions.3
The researchers found that, once these F1's were reintroduced into an outdoor garden, HpLVd reemerged gradually over a period of ten years; the study suggests that is a result of contamination from an uncontrolled environment. Throughout the entirety of the study, only two "weakly-positive" samples were found amongst the F1 hybrids, having 0.5% of the minimum viroid level to be considered "infected."9
Their results suggest that HLVd is not readily transmissible through seed and that the re-infection appears either due to some threshold viroid content in some plants and/or as a result of viroid transmission from other infected materials."3
Eight years later, those same researchers authored another study in 2008 confirming that "Hop latent viroid (HLVd) is not transmissible through hop generative tissues and seeds," continuing further that, while HLVd can propagate in hop pollen, it is eliminated during the first stages of mitosis, and "no viroid was detectable in in vitro germinating pollen, suggesting complete degradation of circular and linear HLVd forms."9
While we can infer from these results that, like hops, HpLVd is also not transmissible through cannabis seeds, there has yet to be any published data confirming transmissibility specific to cannabis.
It appears that properly taken tissue culture and seeds seem to be an effective way to avoid transmission of HpLVd.
Can you treat Hop Latent Viroid? Is there a cure?
Through the efforts of nurseries like Dark Heart and studies such as those cited here, new methods for treatment of HpLVd are being discovered.1,10
Among the various studies on HpLVd in hops plants, regional and anatomical differences in titer suggest further methods for reducing and eliminating traces of HpLVd among varieties. Specifically, HpLVd concentrations have been found to be lower in warmer months and climates, and to have higher concentrations in roots and lower parts of the plant.10 This suggests that new growth in warm conditions could be a source of genetic material that is more likely to have little-to-no viroid traces.
Dark Heart previously explained their patented HpLVd treatment process in a 2019 podcast, discussing how a plant goes through pretreatment, tissue culture, and re-testing before being propagated and grown into a new mom from which to source "clean" cuts.1
At the nursery, infected cuts are identified through lab testing. Once identified, infected plants are put through an undisclosed pretreatment involving "extreme temperatures" before tissue culture is taken from the tip of the meristem (the newest growth of the plant, only a few millimeters in size). This culture is then propagated until it can be re-tested for the viroid. Once the viroid is undetectable, the culture is grown out as a source of new, clean cuts. This technique is based on protocols used by Foundation Services at UC Davis to clean grape cultivars of viruses.
This method of treatment further suggests that temperature exposure combined with tissue culture derived from the newest growth of the plant offers a method for "cleaning" cuts affected by HpLVd.
Admittedly, this process can take months, costing growers time and a significant amount of financial investment to clean affected genetics. But, without, the viroid has shown to be systemic. As of this writing, no other method has been documented for removing the viroid from affected material.
If you do have plants that you suspect (or have confirmed) to be infected with HpLVd, there are methods to help you get to the finish line by reducing expression of the viroid, but no treatment that will cure your plant(s) mid-cycle.
Some growers have found products such as CalOx FT that help to amplify the calcium uptake of affected plants, delivering nutrients and helping to prevent the full expression of the viroid. This won't cure the plant, but it will treat the symptoms. "It's not the end of the world if you have [HpLVd]," adds Ricciardi, "you can combat the symptoms, but the plant still has the disease, and can still transmit it to other plants in the garden if not attended to."
Sources:
1. Nursery, D. H. (2019). Dark Heart Nursery Identifies “Dudding” Pathogen - Hop Latent Viroid [Video]. On YouTube. https://www.youtube.com/watch?v=W8B9Vp0anKE
2. J. G. Warren, J. Mercado, and D. Grace. (2019). Occurrence of Hop Latent Viroid Causing Disease in Cannabis sativa in California. APS Publications. https://apsjournals.apsnet.org/doi/full/10.1094/PDIS-03-19-0530-PDN
3. Matoušek, J., and Patzak, J. (2000). A low transmissibility of hop latent viroid (HLVd) through a generative phase of hop (Humulus lupulus L.). Biol. Plant 43:145-148. https://bp.ueb.cas.cz/pdfs/bpl/2000/01/33.pdf
4. A. Bektaş, K. M. Hardwick, K. Waterman, and J. Kristof. (2019). Occurrence of Hop Latent Viroid in Cannabis sativa with Symptoms of Cannabis Stunting Disease in California. APS Journals. https://apsjournals.apsnet.org/doi/10.1094/PDIS-03-19-0459-PDN
5. Adams, A.N., Barbara, D.J., Morton, A. and Darby, P. (1996). The experimental transmission of hop latent viroid and its elimination by low temperature treatment and meristem culture. Annals of Applied Biology, 128: 37-44. https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1744-7348.1996.tb07087.x
6. Punja, Z. K. (2021). Emerging diseases of Cannabis sativa and sustainable management. Pest Management Science, 77(9), 3857–3870. https://onlinelibrary.wiley.com/doi/full/10.1002/ps.6307
7. Hospodsky, D., Yamamoto, N., & Peccia, J. (2010). Accuracy, Precision, and Method Detection Limits of Quantitative PCR for Airborne Bacteria and Fungi. Applied and Environmental Microbiology, 76(21). https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2976253/
8. Darby, P. (1999). New selection criteria for hop breeding. Pages 3-6 in: Proc. Sc. Comm. Int. Hop Grow. Convn. Pulawy.
9. Matousek J, Orctová L, Skopek J, Pesina K, Steger G. (01 February 2021). Elimination of hop latent viroid upon developmental activation of pollen nucleases. Biol Chem. 2008 Jul;389(7):905-18. doi: 10.1515/BC.2008.096. PMID: 18627315. https://pubmed.ncbi.nlm.nih.gov/18627315/
10. Sarah J. Pethybridge, Frank S. Hay, Dez J. Barbara, Kenneth C. Eastwell, Calum R. Wilson. (n.d.). Viruses and Viroids Infecting Hop: Significance, Epidemiology, and Management. https://apsjournals.apsnet.org/doi/epdf/10.1094/PDIS-92-3-0324
11. Sandy, E. (2021). Dark Heart Nursery Research Finds 90% of California Facilities Test Positive for Hop Latent Viroid. Cannabis Business Times. https://www.cannabisbusinesstimes.com/article/cannabis-hop-latent-viroid-infections-dark-heart-nursery-crop-loss/
12. Interview with Jeremy Warren, Director of Plant Sciences, and Richard Philbrook, Molecular Biologist at Dark Heart Nursery, California. Interviewed on 6/1/22.
13. Interview with Zac Ricciardi, National Cannabis Specialist at BioSafe Systems, Colorado. Interviewed on 5/25/22.14. Scheck, H. S. (2020). California Pest Rating Proposal for Hop Latent Viroid. CDFA. https://blogs.cdfa.ca.gov/Section3162/wp-content/uploads/2020/06/HopLatentViroid_PRP_ADA-1.pdf