Cannabis Bud Rot: Early Signs and Treatment

What bud rot is

Bud rot is gray mold. The pathogen is Botrytis cinerea. It infects more than 200 plant species. On cannabis it eats flower tissue from the inside out. The first damage sits at the stem where a bud meets a branch. The outside of the bud can look normal for 3 to 5 days after infection.

One sugar leaf at a bud site turns yellow, then brown, then gray. It stays dry and crisp.
The leaf pulls out of the bud with no resistance.
Opening the bud at the stem shows brown or gray tissue.
A gray fuzz sits on the tissue. That fuzz is mycelium and spores.
The spot smells musty or like ammonia.
Stem tissue inside the bud feels soft or hollow.

Rot spreads outward from the stem. A brown patch on the outside of a cola means the infection is late.

Relative humidity above 55 percent in the canopy.
Temperatures from 59 to 77 degrees Fahrenheit.
Still air. Airflow below 1 foot per second at the canopy.
Dense colas and heavy defoliation that blocks light and air.
Rain or overhead irrigation during weeks 5 through 9 of flower.
Spores on tools, hands, and clothing.

Botrytis spores travel on air. One infected plant in a room can seed the next plant within days.

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How to check a plant

Look at the sugar leaves at each bud site. Check the top of the canopy first.
Tug the leaf. A healthy sugar leaf holds. A rotted one slides out.
Open the bud at the stem with clean hands.
Shine a light into the gap. Gray or brown tissue means rot.
Tag the plant. Check it each day.

Treatment

There is no cure for infected flower tissue. Remove it.

Turn off fans and set humidity control before you cut.
Cut 1 to 2 inches below the visible rot with sterilized scissors.
Put the waste in a sealed bag. Do not compost it. Do not leave it in the room.
Wipe the blades with 70 percent isopropyl alcohol between cuts.
Change gloves before you touch a clean plant.

After removal:

Drop room humidity to 40 to 50 percent.
Keep canopy temperature at 68 to 75 degrees Fahrenheit.
Add oscillating fans. Aim air across the canopy, not into the buds.
Remove fan leaves that block airflow around colas.
Stop overhead watering. Water the medium, not the plant.
If rot covers more than a third of a plant, harvest the plant and inspect each bud.

Fungicides

Few fungicides are registered for cannabis flower in the United States. Labels for greenhouse ornamentals do not transfer to cannabis. Check state rules before any spray. Residues stay in flower tissue and show up in lab tests.

Drying and curing

Botrytis keeps growing after harvest. Dry at 60 degrees Fahrenheit and 60 percent relative humidity. Keep buds from touching. Check each stem each day. Rot in the drying room shows as brown spots and gray fuzz. Remove the whole bud, not the spot.

Safety

Moldy flower is not smokeable. Heat does not destroy mycotoxins or remove spore allergens. Inhaled spores can cause allergic response and lung infection. People with weak immune systems face the highest risk. Discard any bud with visible mold.