Powdery mildew needs three things: spores, humidity above 60 percent, and still air. Break one link and it stops. Cannabis grow tents create all three. Sealed walls block airflow. Transpiration from dense leaves pushes humidity up. Spores arrive on clones, on clothing, or through an unfiltered intake. Prevention is a set of numbers and habits, not a spray.
how to treat powdery mildew on cannabis leaves
Keep humidity in the 40 to 60 percent range
Powdery mildew conidia germinate between 60 and 90 percent relative humidity. Free water is not required. Optimum temperature is 68 to 77 degrees Fahrenheit. Above 86 degrees, germination falls.
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Run a dehumidifier sized for the tent volume. A 4 by 4 foot tent holds about 64 cubic feet. A 30 pint unit handles it. Measure with a calibrated hygrometer at canopy height, not at the tent wall. During flower, drop to 40 to 50 percent. Water at the base of the pot. Do not wet leaves. If you hand water, do it early in the light cycle so any splash dries.
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Move the air
An exhaust fan and carbon filter should exchange the tent volume once or twice per minute. A 4 by 4 by 6 foot tent is 96 cubic feet. Use a 100 to 200 CFM inline fan. Add one oscillating fan per 3 square feet of canopy. Aim it at a wall or above the canopy, not at leaves. Direct wind causes wind burn and raises transpiration.
Open the canopy
Powdery mildew starts in shade and low airflow. Prune the bottom third of each plant at week 3 of flower. Remove leaves that point inward. Space plants so leaves from one do not touch another. A common target is 1 plant per 2 square feet in a screen of green. More plants means more transpiration and higher humidity at the leaf surface.
Filter the intake
Spores are 5 to 15 microns. A HEPA filter on the intake removes them. A MERV 13 filter removes most. Keep the tent zipped. Change clothes or wear a clean smock before entering if you have been in another garden.
Clean between runs
Spores survive on fabric, corners, and fan blades for months. Wipe all surfaces with 10 percent bleach or 3 percent hydrogen peroxide. Remove and wash the tent liner. Replace the carbon filter pre-filter. Wash pots in hot water. Do not compost infected leaf material. Bag it and put it in the trash.
Quarantine new plants
Hold clones and seedlings in a separate room for 14 days. Inspect the underside of leaves with a 10x loupe every 3 days. Look for white, circular patches 1 to 2 mm across. Destroy any plant that shows signs.
Check weekly
Inspect the lower and middle canopy every 7 days. Powdery mildew appears first on the upper leaf surface as small white spots, then spreads to stems and buds. Remove infected leaves with clean scissors. Bag them without shaking. Do not spray buds in late flower.
Preventive options include Bacillus subtilis, potassium bicarbonate, and sulfur. Sulfur burns leaves above 85 degrees Fahrenheit and cannot be used within 2 weeks of an oil spray. Efficacy data for cannabis is limited. Most products are labeled for other crops.