Powdery mildew, sometimes called rose mildew because it affects some woody ornamentals, is not often serious and occurs only sporadically. The causal fungus, Sphaerotheca pannosa, is usually rare in peach orchards. The fungus can attack leaves, twigs, and fruit, however, fruit infections cause the greatest economic loss.

Symptoms

On fruit the disease first appears as round, whitish spots 2 to 4 weeks after shuck fall. The spots get bigger until they cover much of the fruit. The white spots are produced by the fungus mycelium and its spores. Later the mycelium sloughs off and leaves a rusty-colored patch with dead epidermal cells. About the time of pit-hardening, the skin of the fruit under the spot turns pinkish, and the fungus and its spores disappear. Eventually the skin becomes leathery or hard, turns brown, and may crack.

Diseased leaves often fail to unfold normally, while those of new shoots become narrow, straplike, and distorted. New shoots are shorter than normal and distorted. The white mycelium and spores of the fungus may cover infected leaves and shoots or may appear as whitish patches.

Disease Cycle

The fungus overwinters in dormant peach buds. Flower buds of infected shoots often do not survive the winter. As leaf buds expand in spring, young leaves become infected and the spores produced on the leaves serve to infect young fruit, new shoot growth, and newly expanding leaves. Leaves are susceptible to infection when young but become resistant as they age. Fruit are also more susceptible when young and become resistant at pit hardening.

Disease Management

Routine fungicides adequately control this disease. These sprays should be initiated at petal fall and continued until pit hardening. Most peach cultivars are resistant to powdery mildew, however, Rio-Oso-Gem and Redskin are susceptible.