Witches broom, also called black mildew, is caused by the fungus Apiosporina collinsii. The fungus infects the growing point causing the formation of many stems. The cluster of stems is called the witches broom. Another symptom is a black fungal growth, coating the undersides of the leaves. The damage to the tree is usually not serious and the brooms can be pruned off. No chemical controls are suggested.
Leaf blight caused by Fabraea maculata(Entomosporium maculatum) can cause leaf drop when a severe infection occurs. The disease causes small purple spots on the leaves. The spots enlarge and turn brown, later a small black dot will be seen in the center of the spot. Large numbers of spots cause infected leaves to drop.
Fire blight is characterized by the sudden wilting and death of branch tips. The blossoms wilt, blacken and hang on the twig. The bark is shriveled and has small bumps or blisters on it. Sometimes gum oozes out of the infected area and a crack forms between the diseased and healthy bark. Control with chemicals is difficult. Diseased branches should be pruned out. Make the cut at least 4 inches beyond the diseased area. Disinfect pruning tools with bleach between cuts. Fertilizing heavily with nitrogen increases susceptibility to fire blight.
Powdery mildews of several types cause white powdery growth on the leaves of amelanchier. Late in the season no controls may be needed. Early in the season spray with benomyl.
Fruit rot be a problem in wet weather. The fruits are often eaten by birds so may not be around long enough to become diseased.