Fruit infections are common on severely infected trees. Apple blossoms emerging from infected buds may give rise to small, russetted fruit. When apples are infected during bloom, their growth is stunted, and a fine network of russetted cells that may merge into a solid patch covers their surface, and is visible at harvest. The powdery mildew fungus overwinters on apple as mycelium in dormant buds infected during the previous growing season. Conidia infect young leaves, blossoms and fruit, which in turn provide inoculum for secondary cycles as new leaves, shoots and fruit develop.
Leaves are susceptible for only a few days after they emerge.
Conidia from overwintering mycelium can be found as early as the tight cluster stage and are released during early bloom. Infection that causes fruit russet occurs from about 3 weeks before to about 3 weeks after bloom. Infection of lateral and fruit buds occurs within a month after they are formed, apparently before the protective bud scales suberize. After bud infection, the mycelium is quiescent until budbreak the following spring. The percentage of terminal buds infected can be especially high when growth resumes in late summer following terminal bud set.