Powdery Mildew of Peach, Nectarine, and Apricot
Powdery mildew, sometimes called "rose mildew" (it affects some woody ornamentals), is often not serious. The causal fungus, Sphaerotheca pannosa, and rusty spot, a disease associated with mildew fungi, usually are rare in peach orchards. The fungus can attack leaves, twigs, and fruit.
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 can crack.
Diseased leaves often fail to unfold normally, while those of new shoots become narrow, strap like, and distorted. New shoots are shorter than normal and distorted. The white mycelium and spores of the fungus can cover infected leaves and shoots or appear as whitish patches.
The fungus overwinters in dormant peach buds. Flower buds of infected shoots often do not survive the winter. As leaf buds expand in the 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 also are more susceptible when young and become resistant at pit hardening.
Routine fungicides adequately control this disease. Most peach varieties are resistant to powdery mildew; however, Rio-Oso-Gem and Redskin are susceptible.