How to treat powdery mildew on cannabis plants
Remove infected leaves. Hold relative humidity at 40% to 50%. Keep air moving across the canopy. Spray a fungicide on a 7 day schedule until two rounds pass with no new colonies. Powdery mildew does not need standing water, so wet leaves are not the trigger.
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What powdery mildew is
Powdery mildew on cannabis is a fungal infection from Golovinomyces species. The fungus is an obligate biotroph. It needs live plant tissue and it feeds through haustoria pushed into leaf cells. The white patches on the leaves are mycelium and spores sitting on the surface.
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Spores land on a leaf, germinate in 2 to 3 days, and form a visible colony in 5 to 7 days. One colony releases thousands of spores into the air.
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Conditions that drive an outbreak
- Relative humidity above 50% with no air movement
- Leaf temperature between 68 F and 77 F
- Dense canopy that blocks light and traps moisture
- Nitrogen heavy feed that pushes soft new growth
- Clones taken from an infected mother plant
Leaf wetness from rain or overhead water slows spore germination. That is why outdoor plants often show mildew on the top of the canopy after a dry stretch, not after a storm. Do not use overhead water during flower because of bud rot risk.
Step 1: Strip infected tissue
Cut every leaf with a visible colony. Bag the material and remove it from the room. Do not compost it. Wipe pruners and gloves with 70% isopropyl alcohol between plants. Work on the least infected plants first and the worst plants last.
Step 2: Fix the room
- Set exhaust and intake for one full air change per minute
- Point an oscillating fan at the canopy, not at one fixed spot
- Hold 68 F to 77 F in veg and flower
- Hold 40% to 50% relative humidity in flower
- Remove the bottom third of leaves at day 21 of flower
Step 3: Pick a spray
Potassium bicarbonate
Mix 1 tablespoon per gallon of water with 1/2 teaspoon of liquid soap. The spray raises leaf surface pH and breaks spore cell walls. Apply every 7 days. The EPA lists potassium bicarbonate as exempt from residue tolerance. It leaves a white film on buds.
Sulfur
Wet sulfur and micronized sulfur both work. Do not spray above 85 F, because sulfur vapor can burn leaf edges. Wait 14 days between sulfur and any oil spray. Sulfur leaves residue and a taste on flower, so stop by week 2 of flower.
Hydrogen peroxide
Mix 1 cup of 3% hydrogen peroxide per gallon of water. This is a contact kill with no residual. Use it in veg and the first 3 weeks of flower. It also works as a harvest bud wash: 1 cup of 3% peroxide per 5 gallons, dip branches for 30 seconds, then rinse in plain water.
Biofungicides
Products built on Bacillus amyloliquefaciens strain D747, Bacillus subtilis QST 713, and Trichoderma harzianum colonize the leaf surface and block spore attachment. Many carry an OMRI listing and a 0 day preharvest interval. Rotate strains so the fungus does not adapt.
Oils
Neem oil and jojoba oil coat spores. Do not use them after week 3 of flower. Oil holds on stigmas, traps resin, and can seal in moisture.
Milk
Mix 1 part milk to 9 parts water. Apply in sunlight. The proteins in milk react with UV light and suppress colonies. This is a home option with limited residual action.
Spray schedule
- Day 1: strip infected leaves, then spray the full plant, including leaf undersides
- Day 7: spray again
- Day 14: spray again if new colonies appear
- Rotate modes of action at each round
Spray at lights off or at dusk. Slow evaporation lowers the risk of leaf burn.
Check your state rules
The EPA has not set pesticide tolerances for cannabis because the crop is federally illegal. State departments of agriculture publish their own allowed product lists for cannabis. Check the list for your state before you spray. Some products with a federal label are banned in cannabis programs.
Prevention
- Quarantine new clones for 14 days before they enter the room
- Keep mothers in a separate area from flowering plants
- Hold 40% to 50% relative humidity in flower
- Run fans during lights on and lights off
- Keep nitrogen moderate in weeks 1 to 3 of flower