Mildews are called “powdery mildew” among cannabis growers. This term encompasses all mildews. Mildew grows on foliage, leaves and buds. Most look like foliage was sprinkled with fine flower or talcum powder. It drains the vigor of plants, and can pose a health risk to smokers with lung ailments. Every year powdery mildew destroys a growing percentage of cannabis crops.
The first indication of infection is small spots on the tops of leaves. Next a powdery looking substance (flowering fungus) appears on foliage. At this point the systemic disease has been inside the plant a week or more and the disease can be only arrested by external treatments but not eliminated. Spots progress to a fine, pale, gray-white powdery coating on growing shoots, leaves, and stems.
Damage caused by powdery mildew is subliminal during the first month or two after infection. Powdery mildew is not only on the upper surface of foliage, it is found throughout the plant. It causes growth to slow, leaves to yellow, and plants eventually die as the disease advances. Occasionally fatal indoors, this disease is at its worst when roots dry out and foliage is moist. Plants are infected for weeks before they show the first symptoms.
Mildew is caused by one of the many different species of fungi from the order Erysiphales. Spores are present in the air all the time. Often these spores enter the grow room and greenhouses. They find cool moist grow rooms with poor circulation the perfect place to live.
Preventing powdery mildew is the best policy. Once established, mildews must be eliminated from an empty grow room. They cannot be eliminated from plant tissue without the help of a systemic chemical.Cleanliness! Prevent this mildew by avoiding cool, damp, humid, dim grow room conditions, as well as fluctuating temperatures and humidity. Low light levels and stale air affect this disease. Increase air circulation and ventilation, and make sure light intensity is high. Space containers far enough apart so air freely flows between plants. Allow foliage to dry before turning off lights. Remove and destroy infected foliage. Avoid excess nitrogen. Copper and sulfur-lime sprays are a good prophylactic and potassium bicarbonate is effective as a surface fungicide.
Quarantine new clones brought in from outside the grow room for a week or two. If buying clones at a dispensary, hydroponic store or if acquired from friends, isolate the clones for a week to see if they have powdery mildew and other diseases or pests before introducing them to the main plant population. Dip clones in a prophylactic organic bath before introducing them to the grow room.
Sulfur products such as, Safer’s Defender, prevent powdery mildew on contact with foliage, but the disease is still inside the plant. Applying sulfur make the cannabis taste bad when smoked. It appears to concentrate on resins. Chimera, an outstanding breeder, grower and friend, does not recommend using sulfur on vegetative or flowering plants. He recommends using a sulfur burner in a room to kill all mildew when noplants are in the room.
Apply Serenade® (Bacillus subtilis) or spray with a saturation mix of baking soda and water to kill surface contamination to keep the disease in check. Copper-based sprays may keep this mold in check. A saturation of baking soda spray dries to a fine powder on the leaf; the baking soda changes the surface pH of the leaf to 7, and powdery mildew cannot grow. An organic “systemic” called Aqua-Shield is available from Botanicare that have had very favorable reports. A mixture of 10-20 percent milk and 80-90 percent water is also effective, but it smells bad.
Make sure to destroy damaged plant material so it does not infect other plants.