Bacterial leafspot causes round, water-soaked spots on leaves and young stems. These develop into shrunken, brown areas about 1/8 inch in diameter. Destroy infected leaves and spray with basic copper sulfate.
Bacterial crown gall forms galls on the lower stems. There is no chemical control. Remove infected plants and do not replant in the same spot.
Shoot blight causes grayish to brown decayed spots on the leaves. The spots first appear at the leaf margins, then spread to the rest of the leaf. Infected flower clusters or twigs are killed. The disease is caused by Botrytis cinerea and can be controlled with sprays of benomyl.
A number of fungi cause leaf spots. If needed, spray with basic copper sulfate. Rake up and destroy infected leaves.
Powdery mildew causes a white powdery growth on the leaves. Spray with benomyl. Sulfur may cause defoliation.