Powdery Mildew

Areas of white powdery fungal growth, roughly circular in shape, develop on the fruit in spring. These infected areas later become scabby and dry. In late summer and fall, similar fungal growth appears on leaves. Occasionally, symptoms may develop on fruit and leaves in spring.

Sphaerotheca pannosa attacks the plum fruit whereas Podosphaera tridactyla attacks the foliage. An unidentified species, possibly of Podosphaera, attacks fruit and leaves of certain plum varieties (Red Beaut and Black Beaut) in spring; other varieties may be affected in some years as well.

Sphaerotheca pannosa is known to survive as mycelium on roses and in infected buds of peach trees, and these plants may serve as a source of inoculum for plum trees. This pathogen is not known to overwinter on plum, but recently cleistothecia were discovered on peach trees, which suggests that this pathogen may also produce cleistothecia and survive on plum trees.

Podosphaera tridactyla overwinters as special spore-forming structures called cleistothecia on the surface of shoots, on dead leaves on the orchard floor, and on bark. Spores are produced from these structures during spring rains, and they infect the developing foliage on plum trees. Growth of the pathogen is favored by cool, moist nights and warm days.

Watching for the disease during routine monitoring helps to determine the need for possible action the following year, but by the time it appears on the fruit it is too late to spray during the current season. If there are roses infected with powdery mildew near the orchard, these bushes are potential sources of inoculum, and it may be beneficial to control the disease on the roses or to remove them.

Apply a fungicide at full bloom and make additional applications on a 10- to 14-day interval as needed. The fruit is thought to be resistant to infection after pit hardening. It is important to alternate fungicides of a different chemistry to prevent the development of resistance.