Plant Health & IPM

Some problems shout: brown bud rot spots, yellow leaves, mite webbing. HLVd whispers. Your plant seems to grow normally, and yet the harvest comes up short: less flower, weaker aroma, thinner trichome cover. That is not bad luck. Often it is a viroid. When an experienced grower suddenly gets worse results without changing anything in the setup, it is worth looking for an invisible lodger: Hop Latent Viroid, HLVd for short. HLVd in cannabis often goes unnoticed. It is not a virus in the classic sense but a viroid, a tiny circular RNA molecule of just 256 nucleotides (RNA building blocks) with no protein coat at all (Adkar-Purushothama et al., 2023). Small, but expensive. What HLVd does to cannabis plants: “dudding” disease Among growers the phenomenon is called “duds” or “dudding disease”. The nasty part is that you cannot see it: in many cultivars the infection stays symptomless for a long time and only shows late. When symptoms do appear, they come as shorter internodes, smaller leaves, stunted growth and brittle stems, and above all in flower as smaller, looser buds, a weaker smell and fewer trichomes (Adkar-Purushothama et al., 2023). What it costs you: reported losses of up to 50% in cannabinoids and terpenes. That widely quoted figure comes from industry sources, though, not from a peer-reviewed cannabis study. The solid, quantitative effect data come from closely related hops, where HLVd has been studied well since the 1980s and measurably lowers yield and bitter acids. The direction is clear; the exact size of the effect in cannabis is less so. How widespread the problem is shows in a frequently cited industry survey (not peer-reviewed): around 90% of Californian facilities tested positive, roughly 30% of plants were symptomatic, and the viroid has long since reached Canada as well. And HLVd is not the only pathogen in play: Miotti et al. (2023) catalog a whole cannabis “virome” (including beet curly top virus and lettuce chlorosis virus). HLVd is considered the most important one economically, but when dudding-like symptoms are unclear, another pathogen can be behind them. And not every flower problem is a viroid: what looks like a dud can also be a climate problem, see Bud rot and mold prevention. Europe does not yet have a harmonized detection protocol. Testing is dominated by private providers. How HLVd spreads, and why that is good news HLVd travels mechanically: through cuttings and clones, through grafting and especially through contaminated tools (the scissors that go from plant to plant without being cleaned). No insect transmission has been documented (Adkar-Purushothama et al., 2023). That is the good news: every main route of spread is under your own control. The less good news came in 2024. For a long time seed transmission was considered “negligible”. Atallah et al. (2024), however, showed experimentally 58 to 80% transmission through hemp seed. That clearly contradicts the old consensus, and for you it means: your own seedlings and traded seed are not automatically a clean route either. The same study also found new host plants (tomato, cucumber and others). HLVd is more adaptable than assumed. Testing for HLVd: affordable and doable today Because you cannot see HLVd, there is no way around testing, and testing has become affordable. In a large-scale HLVd detection study, Martí et al. (2023) showed that leaf tissue is the best sample material (100% detection in leaves) and that thousands of plants per week can be screened at low cost. Useful in practice: the viroid titer rises with plant age, so older plants are easier to test reliably; the detection limit of RT-PCR is a tiny 5 picograms of RNA (Atallah et al., 2024). Commercial kit or RT-qPCR lab: according to Martí et al., a study by the test’s developers, both are comparably reliable. Choosing a provider still deserves a second look, because the protocols of private labs are not public. And if it’s too late? Cleanup is expensive If a valuable mother plant is infected, there is a way to rescue it, but not a convenient one. Torres et al. (2025) combined meristem tip culture with thermotherapy and eliminated HLVd in 5 of 13 cultivars: cultivar-dependent, reproducible, stable over months. In plain terms: laborious, expensive, a lab procedure, and not successful for every cultivar. The review by Adkar-Purushothama et al. (2023) is clear on this: a cleaned plant is viroid-free, but not resistant. Without strict hygiene it gets reinfected right away. Cleanup only pays off for irreplaceable genetics. For everything else, prevention is cheaper. What we don’t know for sure yet The dramatic cannabis numbers (50% loss, 90% prevalence) come from gray literature; the hard effect data come from hops. Whether the high seed transmission (measured in hemp) applies equally to THC genetics is unresolved. And there is no HLVd-resistant cannabis cultivar so far. In short: we know enough to act, but not enough to rely on a miracle fix. Takeaways for your grow Treat HLVd as what it is: a silent, costly, unwelcome long-term guest that you only keep out with discipline. Test new clones and mother plants with a leaf sample before they enter your grow. Keep your scissors clean. Tools are a main transmission route, and one you control yourself. Stay alert with seeds too. 58 to 80% transmission in hemp is not a fringe effect. Quarantine new arrivals until the test comes back negative. Test first, then tweak. If your yields drop for no obvious reason, test for HLVd before you adjust your nutrients or your lighting. What a realistic intake check with a quarantine routine looks like will be covered in the practical follow-up “Clean clones: how to keep HLVd and pests out” (coming soon). Sources Adkar-Purushothama, C. R., Sano, T. & Perreault, J.-P. (2023). Hop Latent Viroid: A Hidden Threat to the Cannabis Industry. Viruses, 15(3), 681. doi:10.3390/v15030681 Atallah, O. O., Yassin, S. M. & Verchot, J. (2024). New Insights into Hop Latent Viroid Detection, Infectivity, Host Range, and Transmission. Viruses, 16(1), 30. doi:10.3390/v16010030 Martí, Á. et

It is the week before harvest. The flowers are dense, the trichomes almost ripe, and then, on your walk through the room, you find the first brown, rotten spot in the middle of a cola. Bud rot. When a member asks tomorrow whether the batch is still safe, you need an answer that does not rest on gut feeling. Four studies give you one. For a cultivation association, mold is not a fringe topic but the point where yield, quality and safe distribution meet. A moldy batch is not just lost work, it is a health issue for your members and a problem you have to document and prevent. The good news: the pathogens that do the most damage are well studied, and the most effective levers sit in cultivation and harvest management, not in expensive technology. The Main Enemy: Botrytis, the Bud Rot Fungus Botrytis cinerea is the pathogen behind the infamous bud rot. It attacks more than a thousand plant species, but on cannabis it eats its way out from inside the dense flower. Mahmoud et al. (2023) summarize its development dynamics: the fungus destroys flowers especially fast at relative humidity above 70 % and moderate temperatures between 17 and 24 °C; full-blown epidemics need humidity above 90 %, mild temperatures and, above all, leaf wetness with poor air movement. The tricky part: this exact microclimate often forms inside a dense, mature cola on its own, even when your room climate looks fine on paper. The flower creates its own humid microclimate, and the most vulnerable stage is peak bloom itself. Two details from the full text explain your climate control. First, the dew point: at over 90 % humidity and 30 °C, a temperature drop of just 1.8 °C is enough to form condensation on the flowers, and condensation is the launch pad for spore germination. That is exactly what happens at night when the temperature falls. Second, VPD (vapour pressure deficit): Mahmoud names a concrete threshold. Below 0.4 kPa, the air is near saturation, a film of moisture stays on the leaf, and that directly promotes fungal growth. So Botrytis is not a pure “too warm/too cold” problem, but a humidity, dew point and air movement problem. It does not start at the thermostat, it starts in the canopy: where the air stands still. And an uncomfortable point: the fungus can sit symptomless inside the plant as an endophyte and only turn pathogenic during flowering. Meaning: by the time you see the first brown spot, the pathogen may have been there for a long time. That is why clean starting material works earlier than any measure taken during bloom. In passing, the study clears up a myth: varieties that seem “less susceptible” are usually not genetically resistant, they just have looser, smaller flowers and therefore a less fungus-friendly microclimate. True Botrytis resistance has not been demonstrated in cannabis so far. The Bigger Picture: It Is Not Only Botrytis Punja et al. (2019) systematically recorded which fungi occur on cannabis in a three-year field study across several commercial sites in Canada, from the clone stage to the dried bud. The result is a map of risks, and every risk hangs on a specific growing condition: Botrytis (bud rot): at high humidity and poorly ventilated canopies. Fusarium at the root and stem base: with overwatering and poor root-zone hygiene. An irrigation and substrate issue. Powdery mildew (Golovinomyces): with condensation and on over-fertilized, “soft” plants. Penicillium, Aspergillus, Cladosporium: as contaminants after harvest, during drying and storage. One finding from this work is especially relevant for fertilizing practice: over-fertilized plants, above all with too much nitrogen, build soft tissue and are more susceptible to fungal attack. That makes restraint in feeding not only a yield question but a plant-protection question. If you believe “more helps more”, you grow your own fungal risk right along with the plant. An important reality check from the same study: not every fungus on the plant is an enemy. Many of the species found are endophytes that live alongside the plant without symptoms. So it is not about sterility at any price, but about controlling the few genuine pathogens. The Harvest Phase Decides the Microbial Count For a club that has to deliver distribution-grade quality, the study by Punja et al. (2023) on total yeast and mold (TYM) is the most practically valuable. It worked out which factors significantly lower the microbial load of the dried flower, and almost all of them are things within your own control: Varieties with less leaf mass in the flower: less trapped moist material. Active air movement from fans during flower ripening. Harvesting in the cooler months (in the study: November to April). Hang-drying whole flower stalks instead of drying them shredded. Drying down to 12 to 14 % residual moisture (water activity 0.65 to 0.70) or lower: the value correlated inversely with the microbial count, drier means fewer microbes. Under these conditions, most of the dried samples came in below roughly 1,000 to 5,000 CFU/g (colony-forming units per gram). One important caveat the authors make themselves: the usual plate-count methods do not distinguish between harmless and potentially harmful species. Of the 21 fungal and yeast species found, only a few are a real health risk. A high TYM count is therefore a warning sign, but not an automatic verdict. For communication inside the club that means: take it seriously, but do not panic. The Common Thread: Integrated Management, Not a Single Measure Buirs & Punja (2024) bring the individual findings into a system: integrated pathogen management (ICM). The core message: there is no silver bullet. Effective protection comes from combining several layers across all growth stages, from the mother plant through the cuttings to the flower: Clean starting material: pathogen- and viroid-free mother plants and cuttings. What you bring into the room clean, you do not have to fight later. Especially relevant for the Hop Latent Viroid (HLVd), the agent of “cannabis stunt disease”: it spreads mainly