Source: Bruce Watt, University of Maine, Bugwood.org
The same fungus reportedly causes powdery mildew in peach, apricot, apple, pear, quince, and persimmon trees, and a few ornamental plants. This discussion will be limited to the disease as it affects plums and tart and sweet cherries.
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 midseason 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 fungus may overwinter on diseased, fallen leaves, but usually it does so in infected buds, as in apple powdery mildew. As infected buds expand in spring, new growth is overrun by the fungus. Much of the visible white growth consists of conidia, which are spread by wind to other new leaves and shoots.
Warm temperatures without rain, but with sufficiently high humidity for morning fog or dews, are ideal for rapid increase of the disease.
FRAC Group Code 3, 11, and M7 (dodine) fungicides may be applied as the disease develops.