Hop latent viroid (HLVd) can live in a cannabis line for years without visible symptoms. The only signs may be lower potency, smaller buds and brittle stems. Only a laboratory test can confirm it. This part explains which tissue to send, what an RT-PCR result means and how to read it.
This part also looks at a 2025 study that tried to clear the viroid from infected lines with heat and tissue culture. The success rate ran from 100% to 0% depending on the cultivar. Finally, it lists the claims about HLVd that have no published support and the questions growers in this region can help answer.
This is Part 2. Part 1: Hop Latent Viroid: 84% Seed Transmission, Asymptomatic Tomato and Cucumber Hosts, and the Disinfectants That Fail
Key points from Part 1, in brief. A viroid is a bare ring of RNA (genetic material) with no protein coat. In a 2024 study, 84.3% of seedlings from an infected mother carried the viroid. Sap from an infected plant infected 100% of the plants it was put into. Diluted bleach at about 0.8% sodium hypochlorite for 2 minutes destroyed the viroid on surfaces, while UV light, 70% alcohol and moderate heat did not.
1. Testing for the viroid: which tissue to send and how to read an RT-PCR result
What the test detects
RT-PCR stands for reverse transcription polymerase chain reaction. In the first step, the lab turns the viroid’s RNA into a matching DNA copy. In the second step, it makes many copies of one specific piece of that DNA so its instruments can detect it.
The test is very sensitive. In a 2024 study in the journal Viruses (reference 1), it found the viroid in as little as 5 pg of total RNA. One picogram (pg) is a millionth of a millionth of a gram. RT-qPCR is a version of the test that also estimates how much viroid RNA is in the sample.
Which tissue to send
Send roots. In a 2025 study in the journal Plants (reference 2), roots tested positive 2–3 weeks after infection. Leaves tested positive only after 4–6 weeks. In a propagation programme, that gap can decide whether you catch an infection or spread it in cuttings. Include leaves as well if the lab asks for them.
- Test every mother plant before you take cuttings from her.
- Test every pollen donor before you use its pollen.
- Retest any line that has been through a clean-up, and any new line brought in.
How to read the result
- Positive: the lab found viroid RNA in that sample. Treat the plant as infected. Stop taking cuttings, pollen or seed from it. Clean the tools and bench it touched with bleach.
- Negative: the lab found no viroid RNA in that sample at that time. A recent infection may not have reached the tissue sampled yet, especially the leaves. That is why roots are the better sample and why a second test later is useful. In the clean-up study in section 2, plants were retested about 10 weeks after the first test before a line was called clean.
- RT-qPCR results: ask the lab how it reports the amount of viroid and where it draws the line between positive and negative.
- A positive swab from a bench or tool: viroid RNA is present on that surface. RNA that can be detected is not automatically able to infect a plant. Still, treat the surface as contaminated and clean it with bleach.
Getting a sample to a lab
Look for a laboratory that offers an RT-PCR or RT-qPCR test designed for HLVd. Before you take the sample, ask the lab three things: how much tissue it needs, how to pack and ship it, and how long results take. University plant pathology departments and agricultural diagnostic laboratories are reasonable places to ask. No verified provider in Thailand or Southeast Asia was found while preparing this article, so none is named here.
Check the rules before you ship. Cannabis plant material is regulated in Thailand and elsewhere. This matters most when a sample would cross a border.
Translator’s note: in Thailand, cannabis flower is a controlled herb under the Notification of the Ministry of Public Health Re: Controlled Herbs (Cannabis), B.E. 2568. That notification has been in force since 26 June 2025. Send only roots and leaves, never flower. Before you send anything, ask the lab what paperwork it needs. At 30–35 °C, a sample can spoil in transit, so ask the lab whether to keep it chilled on the way.
What to do: before your next round of cuttings, contact one lab and ask the three questions above. Then send root samples from your mother plants.
2. Cleaning an infected line: meristem culture plus heat treatment
What the method is
The method combines two techniques. Thermotherapy means keeping plants at a raised temperature for several weeks. Meristem culture means cutting out the meristem, the tiny growing point at the tip of a shoot, and growing a new plant from it in a sterile lab container.
The data come from a study in the journal Plant Cell, Tissue and Organ Culture in 2025 (reference 3). The published paper reports that the success rate depended on the cultivar, across thirteen cultivars. The figures for each cultivar below come from the 2024 preprint of the same work. A preprint is an early version posted online before it has been reviewed by other scientists.
The steps, as described in the preprint
- Tissue-culture plantlets were kept at 28–30 °C for 4–5 weeks.
- Meristems were cut out, and new shoots were started over 2–3 weeks.
- The shoots grew for about 8 weeks.
- The plants were tested.
- The plants were retested about 10 weeks later to confirm they were still negative.
Adding up these steps gives roughly 14–16 weeks to the first test. It takes about six months to reach a line confirmed clean.
| Cultivar | Cleared (first 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 that more than 70% of plantlets survived the heat treatment overall. The version available did not give survival figures for each cultivar.
What the results mean
With one method in one laboratory, the clearance rate ranged from 100% to 0%. Of the twelve cultivars with a reported rate, four cleared at 3% or less. Most of these cultivars are North American lines. No published clearance rates were found for cultivars grown in this region, so you cannot look up your own cultivar’s chance.
Clean the line or replace it?
With these odds, the first question is whether to clean the line or replace it. For a valuable line with no substitute, six months for an uncertain attempt may be worth it. Otherwise, it is often faster to replace it with tested-negative seed, or with a tested-negative source of the same genetics.
A laboratory offering a 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. Only the cleaning routine in Part 1 keeps them clean.
Tropical conditions. The 28–30 °C heat step is close to the normal air temperature in much of Thailand, so the heat itself is not the hard part here. The hard part is keeping a tissue-culture lab sterile in a humid climate, where contamination by mould and bacteria is a constant risk. The tissue culture and micropropagation topic covers sterile technique.
Translator’s note: for a grower, this method offers a chance to keep a rare line that would otherwise have to be thrown away. No laboratory in Thailand or Southeast Asia offering HLVd clean-up of cannabis lines was found while preparing this article. If you cannot find one, the realistic option today is to replace the line with tested-negative stock. Meanwhile, keep the infected mother separate and do not propagate from her.
Translator’s note: if you or a lab try the method, plan for power cuts. A cut in the hot season can push a closed room above the 28–30 °C range within hours. It also stops the clean-air bench used for sterile work. Log the room temperature, and put the temperature controller and clean-air bench on a UPS (uninterruptible power supply, a battery that takes over when the power fails). In the rainy season, expect more contamination and start extra plantlets to allow for losses.
What to do: list your lines by value. For each infected line, decide whether to replace it or attempt a clean-up. If you attempt one, start with a small batch from one line, test before and after, and retest about 10 weeks later.
3. Claims about HLVd that could not be verified
“90% of California operations have hop latent viroid.” This figure is widely repeated. One trade article cites more than 200,000 tissue tests, plus a Canadian diagnostics figure of “about a quarter of samples” (reference 4). It gives no formal source, and the underlying data belong to the testing laboratories. The biology of the viroid is well published, but the headline prevalence figure is not.
Prevalence in Thailand and Southeast Asia. No published survey, testing programme or infection rate was found for this region. A lack of data does not mean the pathogen is absent.
Behaviour in tropical heat. No study was found on how high air temperatures affect the viroid’s spread inside the plant. The study that found faster spread under 12 hours of light than under 24 hours did not establish the reason.
What to do: when you see a prevalence figure for HLVd, ask which study it comes from. Quote the studies in the reference list, not press releases. Do not assume your farm is free of HLVd just because there is no local data. Test it.
4. What growers here can find out
Four questions would show whether HLVd is a problem in Thailand and Southeast Asia:
- Has anyone in the region tested for it? A handful of results from local farms, positive or negative, would be among the first local data.
- Does anyone grow cannabis next to tomato or cucumber and share tools or workers between them? The 2024 lab results on other host plants make this worth knowing, although spread in the field has not been shown.
- Does anyone use a UV box as their main way to sterilise tools? On the 2025 data, that routine needs replacing, and bleach is a cheap fix.
- Has anyone tried a meristem clean-up locally? A single result, success or failure, with the cultivar named, would be the first such data for this region.
Has a line slowly got worse over several generations, with lower potency, smaller buds, brittle stems and nothing visible to photograph? Test it for HLVd before you blame the feed or the genetics.
Post your test result below this topic, positive or negative, with the type of tissue sent. No published data on HLVd infection in Southeast Asia was found. A dozen results from this forum would be among the best information available on whether the pathogen is here.
Related reading on this forum: Mother Plants & Cloning; Storing Seeds and Pollen; Making Feminized Seeds; Phenotype Hunting; Root Rot, Pythium & 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 figures for each cultivar: 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 information on growing plants, for readers who operate legally where they live. Cannabis cultivation is regulated differently in every country. In Thailand, cannabis flower is a controlled herb under the Ministry of Public Health rules, and selling or processing it requires a licence. Nothing here is legal advice or a diagnosis. Only a laboratory test can confirm this pathogen.