Hop Latent Viroid (HLVd, or sometimes HLV) is the single most economically significant pathogen affecting commercial cannabis today.
Industry estimates suggest that HLVd causes upward of $4 billion in annual losses across the cannabis industry. A 2021 survey by Dark Heart Nursery found that approximately 90% of cannabis-growing facilities in California were infected.
Despite this, many operations still do not test for it routinely. This guide covers what HLVd is, how it spreads, what it does to your plants, and what you can do about it.
What HLVd actually is
HLVd is not a virus, not a bacterium, and not a fungus. It is a viroid: a tiny, circular strand of RNA with no protein coat. At just 256 nucleotides, it is one of the smallest known plant pathogens. It was originally identified in hop plants in the 1980s and first reported in cannabis in California in 2019.
Unlike viruses, viroids do not encode proteins. They cause disease by interfering directly with the host plant’s gene expression. HLVd replicates using the plant’s own cellular machinery, which is part of why it is so difficult to eliminate once established.
How HLVd spreads
HLVd spreads primarily through mechanical transmission. Contaminated tools, hands, gloves, and other surfaces can transfer the viroid from an infected plant to a healthy one through contact with plant sap during routine cultivation activities such as pruning, cloning, defoliation, and harvesting. HLVd can also move through shared irrigation or recirculating water systems if infected plant material contaminates the water, allowing the viroid to spread rapidly throughout a crop.
Propagation is the highest-risk vector. Every clone taken from an infected mother plant potentially carries the viroid. A single infected mother plant can introduce HLVd throughout a cultivation facility if infected cuttings are propagated and distributed. Because the viroid is unevenly distributed within infected plants, not every cutting will necessarily carry HLVd. However, once infected clones are introduced into production, the viroid can spread through mechanical transmission during routine handling and, in facilities with shared irrigation or recirculating water systems, potentially spread to additional plants beyond the original cultivar.
Tools and hands are the most common day-to-day vectors. Cutting tools used for defoliating and pruning that contact sap from an infected plant can transmit the viroid to the next plant they touch. Gloves, scissors, scalpels, and pruning shears all require disinfection between plants.
Water can also serve as a transmission vector, particularly in recirculating hydroponic systems where runoff from infected plants reaches healthy ones.
Seed transmission has been documented in hops at a rate of approximately 8%, and preliminary studies and observations suggest HLVd occurs in cannabis seed as well (Zamir Punja and Laura Campanella, personal communication), although rates vary by strain and level of infection in the parent plants.
What HLVd does to your crop
The term “dudding disease” has been around long before HLVd was identified (Allison Justice, personal communication) and describes the most visible impact: plants that look dull, underperform, and produce flower that is noticeably inferior to expectations for familiar cultivars.
Yield loss. Published research reports reductions of 12–42% in plant height, fresh biomass, and inflorescence stem length in infected plants compared to uninfected controls (Punja et al., 2023). Some commercial operations have reported more severe losses depending on environmental stress and cultivar susceptibility.
Cannabinoid reduction. HLVd suppresses secondary metabolite production. Research indicates that infected plants can have significantly reduced THC and terpene content, dropping flower from premium pricing tiers to mid or extraction grade.
Trichome density. Infected plants produce fewer and smaller trichomes, which directly impacts the visual quality, aroma, and potency that determine market value.
The silent damage
Perhaps the most economically devastating aspect of HLVd is that approximately 70% of infected plants show no visible symptoms (Punja et al., 2023). These asymptomatic plants continue to be propagated, spreading the viroid through the facility while the grower attributes the declining performance to genetics, environment, or nutrients.
Why visual inspection is not enough
A cultivation director can walk the rooms every day, inspect every plant, and still miss an HLVd infection. The viroid does not produce dramatic symptoms in most cases. Instead, it causes a gradual decline in performance that registers as a disappointing harvest, lower test results, or a batch that went to extraction instead of premium flower.
The connection to HLVd is rarely made because the symptoms are nonspecific: they look like bad genetics, an off cycle, or suboptimal conditions. Without molecular testing, distinguishing between a cultivar that is underperforming and a cultivar that is infected is challenging at best.
How to detect HLVd
The only reliable method of detecting HLVd is molecular testing, specifically RT-qPCR (Reverse Transcription quantitative Polymerase Chain Reaction). This technology detects the viroid’s RNA directly, even when the plant shows no symptoms.
RT-qPCR testing can identify HLVd in various plant tissues, including leaves, petioles, and roots. Research has shown that the viroid is most reliably detected early on in root tissue, while leaf tissue becomes suitable for detection as pathogen load increases in the leaf as the plant ages.
When to test
- Every mother plant, on a quarterly schedule at minimum
- Every new genetic introduction, before integration into the facility and again 4-6 weeks later
- Any plant showing unexplained decline in vigor, yield, or potency
- Periodically across production rooms, even when no symptoms are present
How to manage HLVd
There is no cure for HLVd. Once a plant is infected, the viroid cannot be eliminated. Integrated Pest Management (IPM) protocols should focus on prevention, early detection, and containment. To reduce the risk of transmission, tools should be disinfected between plants.
Prevention
- Remove any visible plant debris or sap from tools, as organic material can reduce the effectiveness of disinfectants
- Disinfect all cutting tools between plants (10% bleach solution prepared fresh daily, 70% isopropyl alcohol, or commercial horticultural disinfectants labeled for use on tools)
- Apply the disinfectant according to the manufacturer’s instructions, ensuring the recommended contact time is met before using the tool on another plant
- Use clean gloves and change them between plants during pruning and propagation
- Implement dedicated tool sets for the mother room
- Source genetics from tested, verified-clean stock
- Quarantine all new genetic introductions and test before integration
If HLVd is detected
- Remove and destroy confirmed positive plants immediately
- Do not take cuttings from any plant that has not been tested
- Increase testing frequency across the facility
- Trace back to the source: which mother plant did the infected clones come from?
- Consider tissue culture as a clean-start propagation method for high-value cultivars
The economics of testing vs. not testing
The cost of testing a mother plant is a fraction of the cost of one compromised harvest. A single infected mother that goes undetected can distribute HLVd to hundreds or thousands of clones over multiple cycles, compounding losses across rooms, batches, and quarters.
Operations that implement routine molecular screening are not reacting to problems. They are preventing them.
Is HLVd harmful to people?
No. Hop Latent Viroid is a plant pathogen. It infects plant cells and replicates using plant cellular machinery. HLVd is not known to infect humans, animals, or any organism outside the plant kingdom. There are no documented cases of human illness caused by HLVd exposure.
The impact of HLVd is entirely economic and agricultural: it reduces plant vigor, yield, and cannabinoid production. Consuming cannabis from an HLVd-infected plant is not a known health risk to the consumer, but the product will likely be lower in potency and quality.
For a deeper look at which cannabis pathogens affect human health, read our full guide: Cannabis Pathogens and Human Health.
Screen your mothers before they clone the problem
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