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Bacterial fire blight causes leaves at the branch tips to wilt, turn brown, but remain hanging on the tree. The infected branches have a scorched appearance and infected bark discolors, shrivels and may crack or be shredded. There is usually a crack between diseased and healthy bark. The bacteria are washed down the branches and onto the trunk by rain. Once the trunk is girdled the tree dies. There is no reliable chemical control. Prune out infected branches. Use of high nitrogen fertilizers can increase susceptibility to fire blight.

Scab, caused by Venturia inequalis, can cause severe defoliation. Early in the growing season, olive brown spots form along the mid-ribs of the leaflets. Later the leaves turn yellow and the spots become more prominent. If the infection is severe, infected leaves drop.

Several fungi cause twig and trunk cankers. As cankers enlarge the part of the stem beyond the canker dies. Avoid wounding trees and keep them healthy with regular fertilization. Use of high nitrogen fertilizers for canker prevention can increase susceptibility to fire blight.

Powdery mildew forms a white coating on the leaves. Use benomyl if chemical control is needed.

Several rust diseases cause raised, light yellow spots on the leaves. Later, orange fruiting bodies appear on the spots on the lower leaf surface. No chemical controls are listed.

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Fireblight on mountain ash - 31K Fireblight on mountain ash - 66K
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