Several fungi cause cankers and, once the stem is girdled, the portion beyond the infection dies. Prune off infected branches.
Leaf spots are usually not serious enough to be controlled chemically. Rake up and destroy infected leaves.
Powdery mildew causes a white coating on the leaves. This problem is usually not serious. If controls are needed use benomyl.
Verticillium wilt causes branches to dieback and a severe infection may kill the tree. Fertilize infected trees with high nitrogen fertilizers.