Leaf spots caused by fungi in the genera Gloeosporium and Marssonina cause leaves to die and drop off. Severe infections kill the twigs. Prune out and destroy infected tissue. Use basic copper sulfate sprays.
Crown rots of several types may be a problem in wet soil. There is no chemical control for these diseases.
Canker caused by Nectria cinnabarina causes stem death. The fungus may be identified by its reddish fruiting bodies. There is no chemical control for this disease. Prune out and destroy infected tissue.