What is Botrytis cinerea and why is it the most dangerous fungus in flower?
Botrytis cinerea, known as gray mold or bud rot, is a necrotrophic fungus that infects both live and dead plant tissue. In cannabis it becomes critical in late flower because dense buds create humid microclimates where spores germinate within hours. Once the mycelium pushes into the interior of a bud, the infection is essentially irreversible: tissue rots, turns brown, and releases millions of spores that contaminate the rest of the grow. Unlike most pests, Botrytis can destroy an entire bud in 48–72 hours if environmental conditions don't change.
What environmental conditions trigger infection?
Botrytis needs two concurrent conditions to establish and spread (Punja, 2023, DOI: 10.1139/cjb-2022-0139):
- Sustained relative humidity above 70% — above 80% RH, spore germination is nearly immediate.
- Temperature between 59–77°F (15–25°C) — the fungus's optimal growth range lines up exactly with cannabis flowering temps.
- Free water on buds — condensation droplets or foliar sprays are direct infection vectors.
- Stagnant air — without circulation, the boundary layer of air sitting over the buds accumulates vapor and crosses the condensation threshold well before the rest of the room does.
The highest-risk window is lights-off. As room temperature drops, relative humidity climbs automatically and can exceed 70% in under an hour if there's no dehumidification or active airflow running.
How much does airflow help and how do you implement it?
Active air circulation is the environmental intervention with the strongest evidence for reducing Botrytis. Under controlled conditions it cuts incidence by 66% to 92% (Punja, 2023, DOI: 10.1139/cjb-2022-0139). The mechanism is twofold: it breaks up the stagnant boundary layer sitting over the buds and speeds up evaporation of surface moisture before it reaches the dew point.
- Use oscillating fans at canopy level aimed at the sides of the plant, not blowing directly into the buds (direct wind causes mechanical stress).
- Target at least one complete air exchange of the room volume every 1–3 minutes.
- Do not turn off circulation fans during lights-off — that is the highest-risk period.
- In tents, check for dead-air zones in the lower corners.
What RH and temperature ranges should you hold in late flower?
In late flower the environmental targets shift compared to earlier stages, specifically to reduce Botrytis risk (Punja, 2023):
- Relative humidity: 40–50% (never exceed 55% in late flower with dense buds).
- Temperature lights-on: 68–77°F (20–25°C).
- Temperature lights-off: 61–68°F (16–20°C) — keeping the day/night swing under 8°C limits the nighttime RH spike.
- Target leaf VPD: 1.2–1.5 kPa — a VPD in that range means RH is staying below the Botrytis risk threshold.
If your room can't get below 55% RH in the final two weeks of flower, airflow and strategic defoliation become even more critical.
What does defoliation do and when should you do it?
Defoliation doesn't kill the fungus, but it dramatically reduces the sites where it can establish. Removing diseased, yellowing, or water-pooling leaves lowers the inoculum load in the room and improves airflow between buds.
- At the flip into flower (weeks 1–2): moderate defoliation to open the canopy and improve light and air penetration.
- Mid-flower (weeks 4–6): remove leaves blocking bud sites and any that are already showing yellowing.
- Never defoliate in the last 2–3 weeks before harvest: late mechanical stress can affect ripening.
- If you find a bud with mold: remove it immediately with clean scissors into a sealed bag, without shaking it (spores release with movement). Check the adjacent buds right away.
What does a week-by-week prevention checklist look like?
Botrytis isn't fought once it shows up — it's prevented with a sustained environmental protocol. These are the control points by week through flower:
- Check nighttime RH minimum twice a week — the critical data point is the overnight peak, not the daily average.
- Confirm circulation fans are running 24 hours a day (including lights-off).
- Visually inspect buds from week 5 onward, with a loupe if possible.
- Check that no leaves have pooled water or necrotic tissue sitting inside the bud structure.
- In the final 2 weeks: reduce irrigation to lower transpiration and the total moisture load in the room.
