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SYMPTOMS: The most obvious sign of powdery mildew is the presence of a whitish, powdery or felt-like growth on the leaves, first along the margins and lower surfaces, and eventually over the entire leaf surface. Infected leaves are distorted, becoming narrow, cupped towards the middle, and brittle. Fruit on heavily infected trees may become severely russeted.
DISEASE CYCLE: The powdery mildew fungus survives the winter within buds which became diseased the previous summer. When these buds open in the spring, the fungus resumes growth in the newly expanding leaves and produces the white, powdery spores which give the disease its name. These spores are then blown by the wind to emerging tissue and produces secondary infections until tree growth stops or temperatures become limiting.
Disease development, although rapid between 60-80 0F, is slow both between 40-50 0F, and at temperatures above 90 0F. High relative humidity favors disease development; however, rain and dew are not required for spore movement or for infection to take place.
SCOUTING: Powdery mildew is most likely to be a problem on highly susceptible varieties, including Jonathan, Rome, Cortland, Baldwin, and Idared. Disease pressure is also likely to be higher in seasons following a mild winter where fungal survival is greatest.
Record the number of terminal shoots showing powdery mildew infections out of 100 observed on each tree (20 per limb x 5 limbs). Begin monitoring at shoot emergence, and continue until terminal growth has ceased. If powdery mildew becomes chronic, control measures may be needed in future years.
Apple IPM web site created by Kerry Kirk - 2001 - Send questions or comments to Patty Lucas