How to Treat Powdery Mildew on Cannabis Plants

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Treat powdery mildew on cannabis by removing infected leaves, lowering humidity, increasing airflow, and applying a labeled potassium bicarbonate or horticultural oil product. Repeat at the interval on the label. Discard plants with mildew on buds near harvest.

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Identify the problem

Powdery mildew starts as white or gray patches on leaf tops. Patches look like flour. Leaves may yellow, curl, or dry. Check leaf undersides and stems. Mildew can spread to buds. Bud mildew is a harvest problem.

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Conditions that drive powdery mildew

Powdery mildew grows in moderate temperatures and high humidity. It can grow in low light and poor airflow. Crowded leaves raise risk. Spores travel on air, tools, hands, and clones.

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• Temperature range: about 60 F to 80 F.
• Relative humidity above 60%.
• Dense canopy with blocked air.
• Low light on lower leaves.
• Clones or tools with spores.

Treatment steps

1. Isolate the plant. Move it away from other plants. Use gloves. Do not touch healthy plants after infected plants.
2. Remove infected leaves. Cut leaves with visible patches. Put waste in a sealed bag. Do not compost it.
3. Lower humidity. Set humidity below 50% if possible. Use a dehumidifier. Run exhaust fans.
4. Increase airflow. Add a fan that moves air through the canopy. Space pots. Defoliate dense lower growth.
5. Apply a labeled product. Use potassium bicarbonate or horticultural oil. Mix at the label rate. Test on one leaf. Wait 24 hours. Spray leaf tops and undersides. Do not spray buds within 2 weeks of harvest unless the label allows it.
6. Repeat. Follow the label interval. Rotate products if the label lists a rotation. Stop use if leaf burn appears.

Product limits

Potassium bicarbonate changes leaf surface pH. It can stop spore growth. Horticultural oil coats spores and insects. Neem oil is an option in veg. Check state rules. Cannabis pesticide rules differ by state. Some products are not legal for cannabis in all states. Follow the label. Do not use sulfur near oil. Do not use oil in high heat or direct light.

Bacillus subtilis and Trichoderma products can help prevent mildew. They do not cure visible infections.

Prevention

• Keep humidity at 40% to 50% in veg and flower.
• Keep temperature stable.
• Space plants. Prune lower leaves.
• Clean tools with 70% isopropyl alcohol.
• Quarantine clones for 7 days to 14 days.
• Avoid overhead watering. Water at the base.
• Clean grow room between runs. Use a labeled disinfectant.

Flower-stage decision

Do not spray buds with oil or bicarbonate late in flower. Residue can remain. Mildew on buds can raise mold risk. If mildew covers buds, discard the plant. If patches are small and on fan leaves, remove leaves and check buds with a loupe. Do not sell or share infected plant material.