Several fungi cause cankers but infected branches may be pruned off. If on the trunk, the tree will gradually dieback and die when the canker girdles the trunk. Keep the plants healthy by fertilizing.
Several fungi will cause leaf spots but are usually not serious enough to spray for. Rake up and destroy infected leaves at the end of the growing season.
Powdery mildew will form a white coating on the leaves. The disease is not serious but, if necessary, us benomyl.
Verticillium wilt can kill a few branches or the entire tree. Fertilize infected trees with high nitrogen fertilizers.