Hop latent viroid is the pathogen most likely to be in a cannabis garden without anyone knowing. Two studies, from 2024 and 2025, added important detail on how it moves and how to stop it. Some of what they found goes against standard nursery practice. In one controlled study, 84.3% of seedlings from an infected mother carried the viroid, and 58% of seedlings from infected pollen did. In lab inoculations, tomato, cucumber and chrysanthemum became infected without showing symptoms. And two sterilisation methods that small nurseries rely on, ultraviolet light and moderate heat, left the viroid’s RNA intact, while diluted bleach destroyed it.
1. Why this needs its own article
Hop latent viroid already appears on this forum in the tissue culture and micropropagation topic, as one of the reasons to run clean stock. That framing is correct, but there it is only a footnote, and the pathogen deserves more.
Two things make it different from the other diseases covered here. It is often asymptomatic, or shows up only as unexplained underperformance: reduced potency, smaller buds, brittle stems, no clear lesion to photograph. And it is a viroid, a bare circular RNA molecule with no protein coat. That is why measures that reliably kill fungi and bacteria do not necessarily touch it.
The practical consequence: take a hypothetical grower losing 20% of expected potency with no visible disease. They will check feed, light and genetics long before they think of a pathogen with no symptoms. This article is meant to close that diagnostic gap.
2. How it moves: the 2024 transmission data
Source: Viruses 2024, 16(1):30 (see References).
| Route (controlled experiment) | Result |
|---|---|
| Infected mother × healthy pollen | 84.3% of ~300 seedlings positive |
| Healthy mother × infected pollen | 58% of ~300 seedlings positive |
| Crude sap inoculation | 100% of inoculated plants infected |
| Total RNA extract inoculation | 60% |
Conditions: these were controlled hemp crosses. Each cross was replicated three times, the seedlings were tested by RT-PCR, and none of 100 control plants tested positive. The paper’s own abstract gives seed transmission as 58–80%, and the authors did not find the difference between the maternal and pollen routes statistically significant. Their RT-PCR assay detected the viroid in as little as 5 pg of total RNA.
Other work reports different rates. The 2025 Plants study found 60% of seeds from a hemp breeding trial positive, and all 16 seeds tested from one infected cannabis cross. The exact rate varies by study. What the studies agree on is that seed transmission is substantial.
Experimental hosts. In the lab, sap inoculation infected three crops (tomato, cucumber and chrysanthemum) and two laboratory model plants (Nicotiana benthamiana and Arabidopsis thaliana). None showed symptoms except Arabidopsis, which died back.
On this evidence, seed from an infected mother is not a reset. Growers have commonly assumed that if a clone line is infected, you can start again from seed. In this study, most seedlings from an infected mother carried the viroid.
This does not mean seed is a bad route to clean stock. It means seed from a mother you have not tested is not a clean route. Seed from a tested-negative mother, or from a tested breeding programme, is a different proposition. What matters is testing, not seed versus clone.
Pollen matters for breeders. If the pollen donor is infected, a breeding programme spreads the viroid. Pollen donors are usually the least monitored plants in a garden, because nobody is judging their buds. This applies directly to the practices in this forum’s articles on making feminized seed and on phenotype hunting.
The 100% crude-sap result is the reason tool hygiene is not optional. With unsterilised tools, any cut that moves sap from plant to plant should be treated as a likely transmission event. That includes pruning, cloning, defoliation and supercropping.
Neighbouring crops: a hypothesis, not a finding. Tomato and cucumber were shown to be hosts by inoculation in the lab. Natural field infection of tomato or cucumber has not been reported, and the authors of the 2025 Plants study note that nobody has shown these plants act as natural reservoirs. Growers whose cannabis shares structures, tools or workers with tomato or cucumber may want to take the possibility into account, for example by not sharing pruning tools between the crops. It is not an established route.
3. How long it survives, and what destroys it: the 2025 sanitation data
Source: Plants 2025, 14(5):830 (see References). In every test, the endpoint was whether the viroid’s RNA could still be amplified by RT-PCR. That is strong evidence about whether a treatment destroys the viroid. It is not a direct infectivity test, because amplifiable RNA is not automatically infectious.
Survival:
- Sap: still detectable after up to about a week at room temperature
- Dried leaf and root tissue: detectable at 4 weeks
- Greenhouse bench surfaces: about 100 to 10,000 viroid copies recovered
What degraded the RNA: diluted bleach (doses below), and 1,000 ppm hypochlorous acid for 1 minute. At 200 ppm, hypochlorous acid had no effect in the same test.
What did not:
- UV-C for 3–5 minutes. The circular form of the RNA survived.
- Heat at 70–90 °C for 15–30 minutes. Amplifiable RNA remained.
- 70% ethanol.
- Virkon and Zerotol on sap, at the concentrations tested.
Detection timing: roots tested positive 2–3 weeks after inoculation; foliage at 4–6 weeks. The viroid spread faster under a 12:12 photoperiod than under 24-hour light.
4. A sanitation and testing protocol that follows from the data
Bleach for tools and benches. Not UV, not alcohol, not heat alone. UV boxes are a common convenience method for blades because they are dry and need no consumables. On this evidence, a 3–5 minute UV-C exposure does not destroy the viroid. A nursery that relies only on a UV box has no effective tool hygiene against this pathogen.
On flaming: the study tested 70–90 °C for up to 30 minutes. It did not test flaming, and it did not test autoclaving as a decontamination step. It did happen to detect viroid RNA on reused plastic pestles that had been autoclaved. A flame is hundreds of degrees hotter than 90 °C, so the heat result cannot be extended to flaming in either direction. It does not show that flaming fails, and nothing here shows that it works. Do not rely on heat alone. Bleach is the method with published support.
Bleach dose, as active ingredient: about 0.4–0.8% sodium hypochlorite, aiming for the upper end. The study used bleach containing 8.25% sodium hypochlorite. A 10% dilution (≈0.8%) applied for 2 minutes fully degraded viroid RNA in sap. The same dilution for 1 minute degraded it only partly. The authors put the minimum effective level at roughly 0.4%. Check the strength on your bottle’s label, because household bleach strengths vary:
| Stock strength (NaOCl) | Mix | Approx. working strength |
|---|---|---|
| 8.25% | 1 part bleach + 9 parts water | 0.8% |
| 6% | 1 part bleach + 6.5 parts water | 0.8% |
| 4–5% | 1 part bleach + 4 parts water | 0.8–1.0% |
- Wipe off visible sap and plant debris first.
- Keep surfaces wet for at least 2 minutes.
- Mix fresh each day. Diluted bleach loses strength.
- Afterwards, rinse blades in clean water, dry them and oil them lightly, because hypochlorite corrodes steel. Rinse benches before plants go back on them.
- Bleach treats surfaces, not plants. In the study, bleach did not destroy viroid inside intact roots or leaves.
Benches are a transmission route too. Up to 10,000 copies on greenhouse benches means bench sanitation belongs in the protocol alongside tool sanitation. Neither UV nor heat is practical for a bench. Treat any surface that held an infected plant within the past week as contaminated, even though the plant has gone.
Treat pollen donors as monitored plants, and do not treat seed from an untested mother as a clean restart.
How to get tested
- Sample roots. In the 2025 study, roots tested positive two to three weeks earlier than foliage. In a propagation programme, that difference decides whether you catch an infection or distribute it. Include leaves if the laboratory asks for them.
- Send the sample to a laboratory that offers an HLVd-specific RT-PCR (or RT-qPCR) test. Before sampling, ask the lab how much tissue it needs, how to package and ship it, and how long results take. University plant pathology departments and agricultural diagnostic laboratories are reasonable places to ask. We have not verified a provider in Thailand or Southeast Asia, so we do not name one.
- Check the rules before you ship. Cannabis plant material is regulated in Thailand and elsewhere, and that matters most if a sample would cross a border.
- Test the mother before propagating from her. Retest any line that has been cleaned or newly brought in.
5. Clearing an infected line: meristem culture + thermotherapy
Source: Plant Cell, Tissue and Organ Culture 2025, 161:66 (see References). The published paper reports cultivar-dependent clearance across thirteen cultivars. The per-cultivar figures below come from the 2024 preprint of the same work.
Protocol as described in the preprint: tissue-culture plantlets were held at 28–30 °C for 4–5 weeks. Meristems were then isolated and shoots initiated over 2–3 weeks, followed by about 8 weeks of growth. Plants were tested, then retested about 10 weeks later to confirm negative status. By our arithmetic, that is roughly 14–16 weeks to the first test and about six months to a confirmed clean line.
| Cultivar | Cleared (initial test) |
|---|---|
| Valarie | 100% |
| Athena | 94% |
| FRB1.4 | 69% |
| AnnaLee | 62% |
| Wife | 50% |
| Hybrid 9 | 26% |
| Hybrid 5 | 14% |
| EarlyPearly | 14% |
| Abigail | 3% |
| S1-4 | 2% |
| River Rock | 0% |
| Pure CBG | 0% |
Survival: the preprint reports overall survival above 70% after heat treatment. The version we could read does not give survival for each cultivar.
One protocol, one laboratory, and results from 100% to 0%. Four of the twelve cultivars with a reported rate cleared at 3% or less. Most of these are North American lines. We found no published clearance rates for the varieties grown in this region, so you cannot look your cultivar up.
Clean or replace? Given those odds, the first question is not “how do we clean this line?” but “should we clean it or replace it?” For a valuable line with no substitute, six months for an uncertain attempt may be worth it. Otherwise, replacing it with tested-negative seed, or with a tested-negative source of the same genetics, is often faster. A laboratory offering clean-up should price it as an attempt with an uncertain outcome, not as a guaranteed result. Cleared plantlets are free of the viroid but not resistant to it, so only the sanitation in section 4 keeps them clean.
A tropical note. The 28–30 °C heat step is close to ambient temperature in much of Thailand, so the thermotherapy itself is not the hard part here. The hard part is keeping a tissue-culture lab sterile in a humid climate, where contamination is a constant risk. The tissue culture and micropropagation topic covers sterile technique.
6. What is claimed about this pathogen that we could not verify
“90% of California operations have hop latent viroid.” This figure is widely repeated. One trade article cites 200,000-plus tissue tests and a Canadian diagnostics figure of “about a quarter of samples”, but it gives no formal citation, and the underlying data belong to the laboratories. The biology of this viroid is well published. The headline prevalence figure is not. Cite the studies, not the press releases.
Prevalence in Thailand and Southeast Asia. We found no published survey, testing programme or incidence figure for this region. That absence of data should not be read as absence of the pathogen.
Behaviour at tropical temperatures. We found no study of how high ambient temperature affects the viroid’s spread within the plant. The 12:12-versus-24-hour result is interesting, and the study did not explain it.
7. What it would take to know whether this is a problem here
- Has anyone in Thailand or the region tested for it? A handful of results from farms here, positive or negative, would be among the first local data points.
- Is anyone growing cannabis next to tomato or cucumber and sharing tools or workers between them? The 2024 host data makes that worth knowing, even though field spread has not been shown.
- Is anyone using a UV box as their main tool sterilisation? On the 2025 data, that protocol needs replacing, and it is a cheap fix.
- Has anyone attempted a meristem clean-up locally? A single result, success or failure, with the cultivar named, would be the first such data point for this region.
If a line has quietly got worse over several generations, with lower potency, smaller buds, brittle stems and nothing you can photograph, test for this before you blame the feed or the genetics. If you do test, please post the result, positive or negative. We found no published incidence data for Southeast Asia, so a dozen test results from this forum would be among the best information available on whether this pathogen is here.
Related reading: the tissue culture and micropropagation topic; the articles on mother plants and cloning, on making feminized seeds, on storing seeds and pollen, and on phenotype hunting; the article on root rot, Pythium and Fusarium.
References
- Atallah OO, Yassin SM, Verchot J. New insights into hop latent viroid detection, infectivity, host range, and transmission. Viruses 2024; 16(1):30. https://doi.org/10.3390/v16010030
- Punja ZK, Scott C, Tso HH, Munz J, Buirs L. Transmission, spread, longevity and management of hop latent viroid, a widespread and destructive pathogen affecting cannabis (Cannabis sativa L.) plants in North America. Plants 2025; 14(5):830. https://doi.org/10.3390/plants14050830
- Torres A, et al. Differential gene expression analysis of Cannabis sativa following hop latent viroid (HLVd) eradication therapy in micropropagation tissue culture. Plant Cell, Tissue and Organ Culture 2025; 161:66. Client Challenge. Preprint with the per-cultivar figures: bioRxiv 2024, Differential Expression Analysis of Cannabis sativa response to thermotherapy of Hops Latent Viroid (HLVd) infection and clearance of the viroid in Tissue Culture Micropropagation | bioRxiv
- MMJ Daily. Hop latent viroid: a global threat to cannabis cultivation. Trade article; its prevalence figures are unverified. Hop latent viroid: A global threat to cannabis cultivation
This article is technical horticultural information for readers operating legally in their jurisdiction. Cannabis cultivation is regulated differently in every country; in Thailand, cannabis inflorescence is a controlled herb under the applicable Ministry of Public Health rules and cultivation and sale require licensing. Nothing here is legal advice, and nothing here is a diagnostic service — laboratory testing is the only way to confirm this pathogen.