Powdery Mildew of Cherry and Plum
This disease is caused by Podosphaera clandestina, one of the common species of the powdery mildew group of fungi. The same fungus reportedly causes powdery mildew in peaches, apricots, apples, pears, quinces, persimmons, and a few ornamental plants. This discussion will be limited to the disease as it affects plums and tart and sweet cherries, although it is rare on sweet cherries in the eastern United States.
The fungus attacks leaves and twigs, producing symptoms much like powdery mildew on apples. Infected leaves curl upward. Newly developed leaves on new shoot growth become progressively smaller, are generally pale, and are somewhat distorted. New shoots are shorter in length than normal. By mid-season, the whitish fungus can be seen growing over the leaves and shoots, sometimes in patches and other times covering most of the new growth. Such symptoms are especially common in nursery trees. The growth of sour cherry trees in the nursery and in young orchards is reduced by this disease.
The fungus can overwinter on diseased, fallen leaves, but more commonly it overwinters in infected buds as it does in apple powdery mildew. When infected buds expand in the spring, the new growth becomes overrun by the fungus. Much of the visible white growth consists of conidia, which are spread by wind to other new leaves and shoots. Dry summers with humidities high enough to produce morning fog or dews are ideal for the rapid increase of the disease.
Fungicides can be applied as the disease develops. Cultural practices such as pruning orchard trees and removing hedgerows located close to the orchard aid in the drying of foliage and fruit and help control this disease.