Plant Health & IPM

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