How to Treat Powdery Mildew on Cannabis Plants

The short answer

Powdery mildew on cannabis is a surface fungus, not a root or soil problem. It does not need standing water to grow and it will not clear on its own. Treat it with three moves in this order: isolate the plant, remove infected leaves, and dry the room so spores cannot land on new growth. Then spray a contact treatment every 5 to 7 days for two weeks. Once buds form, stop oil-based sprays and manage the plant with airflow, defoliation, and harvest timing instead.

How to Clean Grow Room After Powdery Mildew on Cannabis

Prerequisites

• Nitrile gloves and a change of clothes for each grow room
• Sharp pruning shears, plus a jar of isopropyl alcohol for sanitizing between plants
• Sealed bags for infected plant waste
• A hygrometer and a small oscillating fan
• Potassium bicarbonate or 3 percent hydrogen peroxide, plus a pump sprayer

Confirm what you are looking at

PM shows as white or gray patches with a dusty texture on the top of leaves. The patches start small, often on lower and middle leaves, then spread upward. Rub one patch with a gloved finger. Powdery mildew smears. Hard water spots and dried nutrient residue flake off instead. Check the underside of the leaves too, since some cannabis mildew species colonize both surfaces.

more on this topic

How to treat powdery mildew, step by step

1. Move the affected plant away from healthy plants, or shut the room to stop foot traffic between tents.
2. Put on gloves and a clean shirt. Spores travel on fabric.
3. Cut every leaf with visible patches and any leaf touching it. Cut into the petiole, not through the middle of the leaf.
4. Bag the trimmings inside the room and seal the bag before you leave.
5. Wipe your shears with isopropyl alcohol, then move to the next plant.
6. Drop room humidity to 45 to 50 percent and hold it there. Run the exhaust fan and add an oscillating fan at canopy level, but aim it at the wall so it moves air rather than blasting one plant.
7. Keep daytime temperatures between 68 and 75 F. Wide swings between day and night humidity give the fungus an edge.
8. Mix your spray. For potassium bicarbonate, use 1 tablespoon per gallon of water with a few drops of mild soap as a spreader. For hydrogen peroxide, mix 1 part 3 percent peroxide with 3 parts water.
9. Spray the whole plant, including the undersides of leaves and the stems, until the solution drips. Do this with lights off or at the start of the dark period to avoid leaf burn.
10. Repeat the spray every 5 to 7 days. Alternate products if you need more than three rounds.
11. Remove any new patches between sprays. Do not wait for the next spray day.
12. Sanitize the room after the outbreak clears: wipe walls, trays, and fan blades, and replace intake filters.

If the plant is already flowering

Skip neem, horticultural oil, and any sulfur product once buds are dense. Those leave residue you cannot rinse off and can damage trichomes. Continue with hydrogen peroxide at the weaker mix, keep humidity low, and open the canopy by removing fan leaves that block airflow. Inspect buds daily for gray or brown fuzz, which points to bud rot on top of the mildew. If PM has reached the bud tissue, harvest early and wash the cut branches in a bucket of water with 1 cup of 3 percent peroxide per 5 gallons, then rinse in clean water and dry with strong airflow.

how to treat powdery mildew on cannabis plants

What to avoid

• Do not raise humidity to "drown" the fungus. Cannabis PM spreads in moderate humidity with poor air movement.
• Do not apply sulfur within two weeks of an oil spray.
• Do not use any pesticide that is not registered and labeled for your crop. In the US, that means checking state rules before you spray anything.
• Do not compost infected leaves. Bag and discard them.
• Do not clone from a plant that has shown PM. The cutting carries the infection forward.

Stop it from returning

Space pots so leaves on neighboring plants do not touch. Prune the lower third of each plant in veg so air moves under the canopy. Keep night humidity under 55 percent with a dehumidifier or by running lights-on ventilation longer. Filter incoming air and keep a dedicated set of clothes and shoes for the grow. Inspect the underside of new leaves twice a week, since catching three small patches costs far less than catching thirty.