Bud Rot (Botrytis)
Definition: Bud rot is a fungal disease, usually caused by Botrytis cinerea (gray mold), that decays cannabis flowers from the inside out. It thrives in dense colas under cool, humid conditions and often goes unnoticed until a single yellowing, wilting leaf or a patch of gray fuzz reveals rotten brown tissue inside the bud.
How Bud Rot (Botrytis) works
Botrytis spores are everywhere. They only become a problem when moisture lingers inside thick buds long enough for the fungus to take hold. Because it starts near the stem in the center of the cola, the outside can look perfect while the inside is already brown.
Warning signs
- A single sugar leaf poking out of a bud turns yellow or dry while the rest of the plant is green.
- Pistils darken and the bud feels soft or mushy.
- Gray, fuzzy growth appears on the surface as the infection advances.
- Pulling the bud apart reveals brown, crumbly tissue near the stem.
Conditions that invite it
Late flower relative humidity above roughly 60%, temperatures between about 60 and 75°F (16 to 24°C), stagnant air, and big, dense colas. Outdoors, fall rain, heavy dew, and cold nights are the classic setup, which is why growers on the Gulf Coast, in the Northeast, and in the Pacific Northwest watch for it closely.
If you find it
Cut out the infected bud plus an inch or two of healthy-looking material around it. Bag it before carrying it through the grow so spores don’t scatter, and wipe your scissors with alcohol. Moldy flower should be thrown away, not dried or processed.
Prevention
- Hold RH around 40 to 50% in late flower, lower at night.
- Keep air moving through and around the canopy, and use defoliation to open dense spots.
- Outdoors, pick mold-resistant, faster-finishing strains suited to your fall weather.
- Check the biggest colas daily in the final 3 weeks.
Bud rot can also start during drying if the room is too humid.