The disease spores can spread quite rapidly under those conditions. The disease usually results in a heavy reduction in yield and quality of the fruit, but it can also blister and rupture the bark on twigs, defoliate the tree, and can result in the eventual loss of the tree. Apple, flowering crab apple, mountain ash and hawthorn are susceptible to this disease.
Symptoms occur on both the leaves and the fruit.
Look for pale yellow or olive-green spots on the leaves. The margins of these spots are not distinct, but gradually blend into healthy tissue. The discolored areas can darken nearly to black. Severely infected leaves may become curled, cracked, and distorted. On fruit, symptoms first occur as small, dark, raised areas.