Crown gall caused by Agrobacterium tumefaciens causes formation of roundish, warty-looking galls on the lower stem near ground level. There is no satisfactory chemical control. Avoid planting in infected soil.
Several fungi cause cankers and twig blight. The branch dies when the stem is girdled. Prune out infected branches below the point of infection.
Several genera of fungi cause leaf spots. Severe infections may cause leaf drop. Spray with basic copper sulfate.
Powdery mildew of different genera form a white coating on the leaves. Severely infected leaves turn yellow and fall. Use sprays of benomyl or sulfur.