Powdery mildew on cannabis is a fungal infection that shows as white or gray patches on leaves, stems, and buds. Potassium bicarbonate, sulfur, and Bacillus amyloliquefaciens products kill it on contact. No spray cures tissue that is already infected. You must remove infected leaves and change room conditions at the same time.
What powdery mildew is
Cannabis powdery mildew comes from several fungi, including Golovinomyces ambrosiae and Podosphaera macularis. The fungus grows on the leaf surface and sends feeding structures into the outer cell layer. It does not need standing water to start. Spores move on air currents and land on new tissue.
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It is not bud rot. Bud rot (Botrytis) turns buds brown and slimy. Powdery mildew stays white to gray and wipes off at first. Later it spreads into bud tissue and cannot be washed out.
cannabis powdery mildew remedies
Conditions that cause it
Powdery mildew favors 60 F to 80 F and 40 to 60 percent relative humidity. That range sits inside the normal range for most grow rooms. The trigger is a swing between humid nights and dry days, plus still air around the canopy. Dense plants with little airflow get it first.
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Spores germinate in 24 to 48 hours. A full patch can form in 5 to 7 days.
Remedies that work
Potassium bicarbonate
Mix 1 tablespoon per gallon of water. Add a surfactant. Spray the top and bottom of every leaf. It raises leaf pH and breaks the cell walls of the fungus. Reapply every 7 days. It leaves a white residue. Rinse buds after harvest if you used it in flower.
Sulfur
Sulfur works on contact and as a vapor. It is one of the oldest treatments for powdery mildew. Do not use sulfur within 2 weeks of any oil spray. The mix can burn leaves. Do not use sulfur after week 3 of flower. It leaves a taste and smell.
Bacillus amyloliquefaciens
Strain D747 and similar strains are sold as wettable powders. The bacteria colonize the leaf surface and block the fungus. They work as prevention and as an early treatment. Most states set no residue limit for cannabis. Check your state rules.
Hydrogen peroxide
A 3 percent solution mixed at 1 part peroxide to 4 parts water kills spores on contact. It leaves no residue. It does not stop the fungus from coming back. Use it for cleanup between cycles and for washing harvested buds.
Neem oil and other oils
Oils coat the leaf and smother spores. They work, but they leave residue and can damage buds. Do not use oils in flower. Do not use oils within 2 weeks of sulfur.
Milk spray
Mix 1 part milk to 9 parts water. Spray every 7 days. Lab tests show a small effect. It is cheap and safe but weaker than the options above.
What to do in flower
Options drop in flower. Potassium bicarbonate and hydrogen peroxide are the two most used. Remove every infected leaf you can reach. Cut airflow up. Hold humidity below 50 percent. If patches reach the buds, harvest early or destroy the plant. Do not sell or process infected flower.
Prevention
- Keep relative humidity below 50 percent in late flower.
- Run oscillating fans below and above the canopy.
- Space plants so leaves do not touch.
- Defoliate the lower third at day 21 of flower.
- Keep night and day temperatures within 10 F.
- Filter intake air with a HEPA filter.
- Clean tools and hands between rooms.
- Quarantine new clones for 14 days.
Post-harvest wash
Mix 1 cup of 3 percent hydrogen peroxide in 5 gallons of water. Submerge branches for 30 seconds. Rinse in plain water. Hang to dry with strong airflow. This removes surface spores and dust. It does not remove mildew that has grown inside bud tissue.
Why this matters for CBD products
CBD oil, tincture, and vape cartridges come from flower. Mildew on the flower can raise mold spore counts and change the smell of the extract. A CO2 or ethanol extract carries some of that through. Lab tests for microbials and mycotoxins screen for this. Buy CBD from sellers who post a batch COA with microbial results. Ask for the test date and the lab name.
Safety
Sulfur and oil sprays irritate skin and eyes. Wear gloves, eye protection, and a respirator. Do not mix sulfur with oil. Do not spray anything in the last 2 weeks before harvest unless the label says it is safe. Check state pesticide rules. Most states publish a list of allowed products for cannabis.