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 appear as felt-like, 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 can turn brown.
The fungus overwinters as mycelium (the body of the fungus) inside infected buds. As these buds open in spring, all of their parts become covered with a powdery coating of spores. The spores are easily windblown and infect new leaves, fruit, and shoots. Fruit infection takes place during and shortly after the blossom period. Leaf and shoot infection can 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. Mildew-susceptible varieties include Cortland, Idared, Jonathan, Monroe, and Rome Beauty.
Protective mildew sprays should begin at the tight cluster bud stage. Where the disease is severe, mildew fungicides should be continued until new shoots stop growing, or around the fourth cover spray. Choose varieties that are resistant to powdery mildew. Some scab-resistant apple varieties are resistant to this disease (see Table 4.1).