Bud Rot and Mold in Cannabis
Gray fuzz inside colas, wet spots, hay smell — catch botrytis early in late flower before it spreads through the canopy.
What bud rot (botrytis) looks like
Botrytis cinerea — bud rot — starts inside dense colas where airflow can't reach. Outer bracts look fine until you pull a bud apart and find gray, fuzzy mold or a wet brown core.
It spreads fast in late flower when buds are largest and RH is high. One infected cola can release spores across the tent within days.
- Gray or brown fuzzy mold inside the bud — not on leaf surface only
- Single leaves dying randomly on otherwise green colas
- Musty, wet hay smell when you brush past infected sites
- Dark, slimy stem when you peel back bracts
Pro tip
Inspect the center of fat colas weekly in weeks 6–8 — that's where rot starts.
Powdery mildew vs bud rot
Powdery mildew (PM) shows as white powder on leaf surfaces — often upper leaves first. It's a different fungus but thrives in similar stale, humid air.
PM is easier to catch early; bud rot hides until damage is done. Both mean your environment or airflow failed before your genetics did.
- PM: white dust on leaves, wipes off initially, returns
- Bud rot: inside flower tissue, often not visible externally
- Both: improve airflow and lower RH — fungicides in flower are a last resort
Why it happens indoors
Dense indica colas + 55% RH + stagnant air under the canopy = rot waiting to happen. Exhaust fans move tent air but don't always move air through the bud mass itself.
Big temperature drops when lights go off push RH up overnight — the dew point hits your colas while you sleep.
Humidity and VPD — late flower targetsImmediate response
Remove infected material immediately — cut 2–3 inches below visible mold, bag it, and take it out of the house. Don't compost inside the tent.
Increase airflow: add under-canopy fan, defoliate sparingly for tunnel airflow through colas. Lower RH to 40–45% with a dehumidifier if needed.
Inspect every cola on the plant and neighboring plants. Assume spores spread — one plant often means check the whole tent.
Prevention checklist
Prevention beats salvage in late flower:
- RH 40–45% from week 5–6 of flower onward
- Under-canopy airflow — clip fans pointed up through lower branches
- Avoid foliar sprays and free water on buds in flower
- Defoliate strategically in week 1–3 of flower — not a stripped skeleton
- Don't pack tent wall-to-wall — air needs paths
- Check colas after any power outage or AC failure
Disclaimer: This article is for educational purposes only. Cannabis laws vary by jurisdiction. Always comply with local regulations.