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Bud Rot (Botrytis): Catching It Before It Takes the Harvest

By the Cultivar grow team · published 2026-08-04 · updated 2026-08-04 · how we source these numbers

Bud rot is the problem that costs the most and gives the least warning. Botrytis cinerea — grey mould — establishes inside dense flower, where it is dark, still and humid. It works outward. By the time anything is visible on the surface, the core of that bud is already gone.

There is no treatment that saves affected flower. Everything useful here is either early detection or prevention.

What to look for

The single most reliable early sign is subtle: a leaf emerging from within a bud that has gone yellow or brown, and pulls out with no resistance.

A healthy sugar leaf is anchored. A leaf whose base is sitting in rotting tissue slides free with a light tug. If that happens, open the bud.

Inside an affected bud you will find:

• Grey, brown, or white-grey fuzzy growth at the core.
• Dry, crumbling, almost dusty material where dense flower should be.
• A musty smell, distinct from the plant's aroma.

Inspect the biggest, densest colas first. They trap the most moisture and rot first. In late flower, check every few days — and check the day after any humidity spike.

Do not confuse it with powdery mildew, which grows on leaf and bud surfaces as a white powdery coating and is visible from the outside. Botrytis hides.

The lights-off spike

This is the mechanism most growers miss.

Warm air holds more moisture than cool air. Your tent might sit at a comfortable 26°C and 55% RH with lights on. When the lights go off, temperature falls — and because the amount of water in the air has not changed, relative humidity climbs sharply. A tent that reads 55% during the day can be well above 65% an hour into the dark period.

That is when botrytis gets its opportunity, and it is invisible to a grower who only checks the hygrometer while the lights are on.

Get a hygrometer with min/max memory, or log overnight. The number that matters is the peak in the dark period, not the one you happen to see.

Prevention

Humidity control in flower. Bring relative humidity down as flower progresses; late flower is where dense buds and high humidity combine most dangerously. A dehumidifier on a humidistat is the direct answer. Consistent with our VPD targets, aim for the flower band rather than a single fixed RH.

Airflow through the canopy, not just above it. Botrytis needs still air. Fans should move air within the leaf canopy, and dense plants need help getting air into the middle.

Defoliation and structure. Selective removal of leaves crowding the interior of large colas improves air movement where it matters. This is a judgement call — leaves are photosynthetic machinery — but interior leaves buried inside a cola contribute little and hold a lot of moisture.

Avoid wetting flowers. No foliar spraying once buds form. This is one of the main reasons to be careful with pest treatments in flower — see spider mites.

Watch the temperature swing. A large drop at lights-off produces a large RH spike. Reducing the swing reduces the spike.

Outdoor growers: rain sitting in dense colas is the classic trigger. Shaking plants gently after rain, and considering an early harvest ahead of a wet spell, are both reasonable.

If you find it

Act immediately — spores spread, and disturbing infected material releases them.

1. Remove the affected bud and a generous margin of apparently healthy flower around it. The rot extends further than it looks.
2. Handle it gently. Do not shake or squeeze. Bag it where it sits if you can.
3. Dispose of it away from the grow space, not in a bin inside the room.
4. Wash your hands and consider changing clothes before handling other plants.
5. Fix the conditions — humidity and airflow — for the whole space, not just that plant.
6. Re-inspect everything else, especially the densest colas.

Do not attempt to salvage affected material. Mouldy cannabis is not made safe by drying, trimming, or curing, and inhaling mould is a genuine health risk. This is not a place to be economical.

Deciding whether to harvest early

If you find rot in late flower and conditions are hard to fix quickly, harvesting early is often the right call. You lose some ripeness; you keep the crop.

Weigh it honestly:

• Rot spreading across multiple sites and conditions you cannot correct within a day or two — harvest.
• A single site, caught early, with humidity control available — remove it, fix the environment, and continue with daily inspection.

Whatever you decide, the drying environment matters just as much: buds that survived the tent can still be lost in a badly-managed dry. Our drying and curing guide covers the targets.

The honest summary

Bud rot is not really a disease you treat. It is an environment you either controlled or did not. The growers who rarely see it are not luckier — they are the ones who know what their tent does in the first hour after lights-off.

FAQ

Keep reading

• Powdery Mildew on Cannabis: White Dust That Wipes Off
• Heat Stress in Cannabis: Taco Leaves, Foxtails and What to Fix First
• Brown or Rust Spots on Cannabis Leaves: Calcium, pH or Pests?
• Drooping Cannabis Leaves: Overwatered or Underwatered?
• Fungus Gnats: Small Flies in Your Grow and the Larvae That Matter
• Hermies and Nanners: Why Plants Turn and What to Do About Seeds

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