Cannabis bud rot is caused by the fungus Botrytis cinerea, a gray mold that germinates when spores settle on dense flower clusters and stay wet in still, humid air. It is rarely the result of one mistake; it builds from humidity, weak airflow, temperature swings, and overcrowded canopies stacking up during the final weeks of flower. Understanding the causes is the fastest way to keep a harvest clean and your flowers dry, firm, and free of mold.
Key Causes at a Glance
- High relative humidity: sustained levels above roughly 60% in flower give spores the moisture they need to germinate.
- Poor airflow: still air pockets inside a canopy trap moisture directly against buds.
- Dense, tight flowers: thick colas dry slowly and hide the first signs of infection.
- Cool nights with warm days: condensation forms on bud surfaces when temperatures swing sharply.
- Overwatering and wet foliage: soggy media and late-day watering push humidity up around the plant.
- Contaminated tools, soil, and debris: spores overwinter in plant litter and travel on hands, scissors, and fans.
- Overcrowding: leaves touching leaves block light and stop evaporation.
- Late-season rain or irrigation overspray: outdoor plants are most exposed in the last three weeks before harvest.
Conditions and Specs to Watch
Bud rot causes all come down to moisture balance. In late flower, aim for 40-50% relative humidity with daytime temperatures in the 68-77F range and gentle, constant air movement across the canopy. Space plants so leaves never overlap for long, defoliate lower growth to open the interior, and water early in the day so the root zone has time to dry. Inspect the densest buds every day, since Botrytis starts as a soft brown patch or a gray-brown leaf at the base of a cola and spreads outward quickly.
Can bud rot be reversed once it starts?
No. Botrytis is not curable on an infected bud. Cut affected flowers out with sterile scissors, bag them without shaking, and remove them from the grow area so spores do not spread to healthy colas.
How to Prevent Cannabis Bud Rot: A Comprehensive Guide
How do I prevent bud rot during late flower?
Hold humidity below 50%, keep fans running around the clock, avoid wetting buds, and harvest promptly when trichomes are ready. Clean tools and hands between plants so spores are not carried from one cola to the next.
Cannabis Bud Rot Prevention Methods
Is bud rot the same as powdery mildew?
No, they are different pathogens, but both are favored by the same conditions: high humidity, stagnant air, and dense growth. Fixing airflow and moisture usually addresses both.
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