When weather conditions are favorable for infection, however, it can cause some economic loss by reducing fruit quality. Powdery mildew is sometimes called rose mildew, for it seriously affects roses and other woody ornamentals. The causal fungus, Sphaerotheca pannosa , attacks young shoots, leaves, and fruit. Another powdery mildew fungus, Podosphaera clandestina , which causes common mildew on cherry, also infects peach. Infected young leaves may drop or fail to elongate and unfold normally, while those of new shoots become narrow, strap-like, and distorted.
Mature leaves are more resistant to the fungus.
This white, powdery appearance is the result of large masses of conidia that are produced on the leaf surface. Affected current year's twigs are stunted in growth and the lateral buds which differentiate into blossom buds may be destroyed. On developing fruit, the disease first appears as white, round spots two to four weeks after shuck fall. About the time of pit-hardening, the skin of the fruit under the spot turns pinkish and eventually becomes a dark brown, at which time the fungus and its spores disappear. The fruit surface becomes leathery and hard and may crack.