Bud rot in cannabis is a fungal disease caused by Botrytis cinerea. It thrives in cool, damp conditions and can destroy flower buds. The best prevention is to maintain low humidity, provide strong air circulation, and remove infected plant material at once. Below you will find the main causes of bud rot and steps to prevent it.
What Causes Cannabis Bud Rot?
Bud rot develops when fungal spores land on cannabis buds and the environment stays wet for several hours. Several factors increase the risk:
- High humidity: Relative humidity above 55% during flowering creates ideal conditions for Botrytis.
- Poor air circulation: Stagnant air lets moisture sit on buds and leaves.
- Cool temperatures: Botrytis grows best between 60°F and 75°F (15°C to 24°C).
- Dense canopy: Overcrowded plants block airflow and keep inner buds wet.
- Overwatering: Wet soil and foliage encourage fungal growth.
- Contaminated tools: Pruning tools can spread spores from plant to plant.
Outdoor growers face additional risks from rain, fog, and morning dew. Indoor growers may see bud rot when ventilation fails or when plants are packed too close.
Cannabis Bud Rot Prevention for Outdoor: A Grower's Guide
How to Prevent Bud Rot
Prevention relies on three pillars: environment, plant care, and early detection.
Cannabis Bud Rot Prevention Guide for Indoor Grow
Control the Environment
- Keep relative humidity below 50% during the flowering stage. Use a dehumidifier if needed.
- Maintain temperatures between 68°F and 77°F (20°C to 25°C). Avoid cool nights that promote condensation.
- Install oscillating fans and an exhaust system to move air through the canopy.
- Water plants at the base, not overhead. Water early in the day so foliage dries before night.
Manage Plant Canopy
- Prune lower branches and large fan leaves that block airflow.
- Space plants so each one receives good air movement. In a grow room, use trellis nets to spread branches.
- Remove dead or dying leaves at once. They are prime spots for Botrytis to start.
- Avoid high-nitrogen fertilizers late in flower, which can create soft, dense growth that traps moisture.
Inspect Plants Often
- Check buds each day during the last weeks of flowering. Look for brown, gray, or fuzzy patches. Infected buds may feel mushy and fall apart when touched.
- Use a flashlight to inspect dense colas from the side and bottom.
- If you find bud rot, isolate the plant. Cut away infected buds with sterilized scissors, cutting at least one inch below the rot. Seal the waste in a plastic bag and throw it away. Do not compost it.
- Sterilize tools with alcohol between cuts and between plants.
What to Do If Bud Rot Appears
Act fast. Botrytis can spread to healthy buds within days. Remove all infected material and improve airflow and humidity right away. If rot has reached the main stem, the plant may not recover. In that case, harvest what you can and discard the rest. Do not smoke or consume moldy buds. Mold can cause respiratory problems and other health issues.
For outdoor plants, cover them during rain and shake excess water off branches in the morning. For indoor plants, check that your exhaust fan is sized correctly for your grow space. A small hygrometer placed near the buds helps you track humidity.
By keeping your grow space dry and well-ventilated, and by checking plants often, you can prevent bud rot from ruining your harvest.