Powdery mildew, caused by the fungus Podosphaera leucotricha, attacks buds, blossoms, leaves, new shoots, and fruit of wild and cultivated apples and crabapples. It 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.

Symptoms

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 spores are windblown and infect new leaves, fruit, and shoots. Fruit infection takes place during and shortly after the blossom period. Infected fruit of some cultivars have a russet appearance. 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 degrees F when relative humidity is high.

Disease Management

Protective mildew sprays should begin at the tight cluster bud stage. Where the disease is severe, mildew fungicides need to be continued until new shoots stop growing or about the fourth cover spray. Choose cultivars that are resistant to powdery mildew. Some scab-resistant apple cultivars are resistant to this disease. Mildew-susceptible cultivars include Jonathan, Rome Beauty, Cortland, Baldwin, Monroe, Ida Red, Granny Smith and Stayman.