What causes cannabis bud rot
Bud rot is a fungal infection caused by Botrytis cinerea. It also goes by gray mold. The fungus infects dense flower clusters. Spores land on buds. They need free water and 6 or more hours of wetness. Temperatures between 65°F and 75°F (18°C to 24°C) favor infection. Relative humidity above 85% lets spores germinate. Infected buds turn brown, gray, or mushy. The inner stem may show gray fuzz. Bud rot can destroy a crop in days.
Cannabis Bud Rot Causes: Why Gray Mold Appears in Dense Flowers
How to prevent bud rot
1. Keep humidity low in flower
Target 40% to 50% relative humidity during weeks 5 through 8 of flower. Use a dehumidifier. Do not let RH climb above 60%. Run the dehumidifier at night. Night humidity spikes when temperatures drop. Measure RH at canopy level, not at the wall.
2. Move air across the canopy
Install one oscillating fan per 10 square feet of canopy. Aim fans at the walls or ceiling. Do not blow air straight into colas. Run exhaust fans 24 hours a day. Exchange 1 to 2 room volumes of air per minute. Air movement dries leaves and buds after watering.
3. Prune and space plants
Defoliate the lower third of each plant. Remove leaves that block airflow. Keep 12 to 18 inches between main colas. Do not crowd plants. Dense canopies trap moisture. Prune in early flower. Avoid heavy pruning in late flower. Wounds can invite infection.
4. Water at the base
Water the soil or growing medium. Do not wet leaves or buds. Water in the morning. This lets the surface dry before night. Use drip lines or a watering can with a long spout. Stop watering 2 to 3 days before harvest.
5. Inspect plants daily
Check each cola every day during late flower. Pull the bud apart with clean hands. Look for brown spots, gray fuzz, or mushy tissue. Check the stem inside the bud. Rot starts inside and works outward. Early detection limits spread.
6. Remove infected buds
Cut infected buds with sterile pruners. Cut 2 inches below the visible rot. Put the bud in a plastic bag. Seal the bag. Remove it from the grow area. Do not compost infected buds. Do not smoke or use them for CBD oil. The fungus can produce allergens and toxins.
7. Sanitize tools and hands
Wipe pruners with 70% isopropyl alcohol between cuts. Use gloves. Change gloves after touching an infected bud. Wash hands with soap and water. Clean scissors, trays, and stakes. Botrytis spores stick to surfaces.
Late flower checklist
- RH: 40% to 50%
- Temperature: 65°F to 75°F
- Air movement: 24 hours a day
- Inspect each cola: daily
- Water on buds: never
- Pruners: sterile between cuts
- Infected buds: bag and discard
- Nitrogen: reduce in late flower
When to harvest early
If bud rot affects more than 10% of your crop, harvest early. Early harvest saves some CBD and THC. It also stops the fungus from spreading. Check trichomes. Cloudy trichomes mean usable flower. Clear trichomes mean lower potency. An early harvest is better than a lost crop.