On leaves of new shoot growth symptoms of powdery mildew are feltlike, white patches on the margins and lower surfaces. Infected leaves curl upward and soon become covered with a powdery coating of spores. Photo by K. Peter.
Powdery mildew interferes with the proper functioning of leaves, reduces shoot growth, reduces fruit set, and produces a netlike russet on the fruit of some cultivars. It is often a serious problem in apple nurseries.
The first sign of powdery mildew in spring is a 3- to 4-day delay in the opening of infected buds. Leaves and blossoms of these buds soon become covered with a white to light gray powder, the spores of the powdery mildew fungus. Flowers do not develop normally, are likely to be greenish white, and produce no fruit. On leaves of new shoot growth symptoms of powdery mildew are feltlike, white patches on the margins and lower surfaces. Infected leaves curl upward and soon become covered with a powdery coating of spores. New infections of succulent leaves and growing shoots reduce the size of the entire shoot. By midsummer, leaves and shoots may turn brown.
The fungus overwinters as mycelium (fungus threads) inside infected buds. As these buds open in spring, all of their parts become covered with a powdery coating of spores. The spores, easily windblown, infect new leaves, fruit, and shoots. Fruit infection takes place during and shortly after the blossom period. Leaf and shoot infection may continue as long as shoot growth continues. Buds can become infected as they begin to form until they are matured for overwintering.
Infections occur at temperatures of 65 to 80°F when relative humidity is high, such as at night. No moisture is required for spore germination to occur. Consequently powdery mildew is often called the "dry weather disease."
Mildew sprays should begin at the tight cluster bud stage, where the disease is severe. Fungicides need to be continued until new shoots stop growing or about the fourth cover spray. Sterol inhibitor fungicides (FRAC Group 3 fungicides) applied to control apple scab are very effective in controlling powdery mildew. Specific chemical recommendations for home gardeners are in Fruit Production for the Home Gardener, and recommendations for commercial growers are in the Penn State Tree Fruit Production Guide.