Mold on Your Cannabis Plant? Here's What to Do

Two kinds of mold ruin cannabis plants: powdery mildew (white dust on the leaves) and botrytis, better known as bud rot (gray/brown rot inside the bud). Both are fungi, both thrive in high humidity with poor airflow, and both can wipe out an entire harvest in a week if you don't act. But each one calls for a different response.

Powdery mildew: the white dust

It shows up as a flour-like white powder on the leaves — small patches at first, then it coats the whole leaf. Catch it early and the plant survives.

• Quarantine the affected plant immediately so it doesn't infect the rest.
• Drop ambient humidity to 40-50%. Improve ventilation.
• Spray with baking soda (1 teaspoon per liter of water) or milk diluted 1:9 with water. Hit the affected leaves every 3 days for 2 weeks.
• Don't use chemical fungicides in late flower: they end up in your buds.
• Remove the worst leaves with sterilized scissors. Burn them or dump them far away from the grow.

Botrytis: gray mold in the bud

This one's more dangerous. It grows from the inside of the bud outward, so by the time you see it on the surface, the damage runs deep. Look for gray/brown fuzz, buds that turn soft and mushy, and the sugar leaves around them drying out overnight. The smell is sour, almost vinegary.

• Cut out the affected buds with a 5 cm (2 in) margin into healthy tissue.
• Burn or dispose of those buds far from the grow. Don't toss them in your regular bin.
• Drop humidity to 45% or lower. Add an extra fan to boost circulation.
• Inspect the entire plant every 12 hours for the next 7 days.
• If more than 30% of the plant is hit, consider harvesting the rest early before you lose it all.

Prevention (the only thing that scales)

While you're treating one outbreak, you're already losing plants. The real defense is your environment:

• Relative humidity: 65-70% in veg, 40-50% in flower.
• Night temperature: never more than a 5°C (9°F) swing from daytime (condensation is the enemy).
• Selective defoliation: pulling leaves that shade the inner colas improves airflow.
• Ventilation: keep air moving at all times, especially between the buds. One fan isn't enough — run two on crossing planes.
• Water discipline: never mist the buds in late flower.

If you grow outdoors in a humid region, look into botrytis-resistant strains (genetics with looser, airier buds and better internal airflow). At our club we keep a curated list of genetics that hold up in Argentina's trickiest climates.