Many fungi cause leaf spots, but chemical control is rarely warranted. If necessary, use basic copper sulfate.
Leaf blight caused by Herpobasidium deformans results in dead leaves. No chemical control is listed.
Various genera of powdery mildews form white powder on the leaves. Use sprays of benomyl or sulfur.
Bacterial crown gall causes round, warty-looking growths on the stems near the soil line. Dig out and destroy infected plants.