The disease is especially severe on Jonathan apple and can result in shoot dieback, death of flower buds, and russetting of fruit. The disease first appears in the spring on fruit buds and new shoot growth. Infected flower buds may be killed by low winter temperatures. Those that do develop typically break bud several days later than normal and are covered with a white, dusty growth of the fungus. The leaves on infected shoots also develop white patches and become distorted.
Diseased leaves are narrow, folded longitudinally, and become stiff and brittle with age.
On Jonathan apple, the fruit skin becomes rough and cracked and fruit size is reduced. The fungus overwinters on the tree in infected buds. In the spring, the fungus colonizes developing leaves and begins to produce spores on the leaf surface. These spores are disseminated by wind and can reinfect other leaves. Spore germination and infection is favored by a high relative humidity and temperatures between 60 and 80 F. Free water on the leaf surface is not necessary for infection and, in fact, inhibits spore germination.