Bud Rot (Botrytis) in Cannabis: Signs and Fixes

A leaf is browning and pulling out of a cola, and you are worried about bud rot. Botrytis starts inside dense flowers where you cannot see it. Catching it early and acting fast can save the rest of your harvest.

Cannie showing early bud rot (botrytis) — Early

Cannie showing advanced bud rot (botrytis) — Advanced

“A leaf just slipped right out of my cola. Peek inside before it spreads!”

Where it shows

Inside dense flower, hidden. Not on the surface.

What it looks like

A cluster goes gray and mushy at its core while everything outside still looks healthy. Leaves right at the site brown and pull away easily.

How to confirm it

Check inside dense flowers, not just the surface.
A single browning leaf that pulls out with no resistance is the early sign.
The core turns gray and mushy while the outside still looks healthy.
Leaves at the site brown and dry out.
Look closely at the biggest, densest colas first.

Why it happens

Moisture trapped inside dense flower. The densest, best-looking colas are the most vulnerable, which is why it is so often found late.

Moisture trapped inside dense flowers.
High humidity in flower, especially at night.
Poor airflow through the canopy.
Big, tight colas, which are the most vulnerable.

How to fix it

Cannie's quick fix, in order:

Check INSIDE dense flower regularly. By the time it shows on the outside it has been spreading for days.
A single browning leaf pulling out of a cola with no resistance is the early sign. Open it and look.
Remove affected material generously and immediately. Never smoke or cure it.
Drop humidity below 50% and increase airflow through the canopy.

Check inside dense flowers regularly. By the time it shows outside, it has been spreading for days.
Open any cola where a browning leaf pulls out easily, and look.
Remove affected material generously and immediately. Cut well past the visible rot.
Bag removed material and wash hands and tools to avoid spreading spores.
Drop humidity below 50 percent and increase airflow through the canopy.
Never smoke or cure affected material.

Organic options

Dehumidifier

The main tool. Hold humidity below 50 percent in flower, especially during lights off.

Airflow

Oscillating fans moving air through, not just over, the canopy keep moisture from settling.

Defoliation

Remove crowded fan leaves to open up the canopy. Do it gradually to avoid stressing the plant.

Biofungicides in veg

Some Bacillus based products help as a preventive in veg. Never spray buds in mid to late flower.

Read and follow every product label. Organic friendly does not mean certified organic; check the listing if that matters to you.

Veg or flower?

Bud rot is a flower problem, worst in the final weeks. Never spray buds; focus on humidity, airflow, and removing infected material quickly.

Prevent it next time

Keep flower humidity below 50 percent.
Space plants and thin dense growth.
Avoid watering late in the light cycle.
Inspect colas often in the final weeks.

Questions growers ask

Can I save a plant with bud rot?

Often, yes. Remove infected material generously and right away, then lower humidity and raise airflow. Keep checking every day.

Is it safe to smoke bud with rot?

No. Never smoke or cure affected material. Remove and discard it.

What humidity prevents bud rot?

Keeping it below 50 percent in flower helps a lot, along with strong airflow through the canopy.

Where these figures come from

These points match published grow books, page by page. They are grower practice, not research: none of these books cites a study, so where science and the books differ, this page follows the science.

Drop humidity below 50% and increase airflow, especially lights off. Danko (2018), e-book p. 133; e-book p. 132 to 133. Paull and Zetta (2020), e-book p. 147 to 149; e-book p. 71 to 73.

Books: Danko, Cannabis: A Beginner's Guide to Growing Marijuana (Hampton Roads, 2018); Green, The Cannabis Grow Bible (4th ed., 2001); Paull and Zetta, Foolproof Guide to Growing Cannabis (2020) and The Cannabis Grow Bible (2020).