Bacterial leaf spots are first small and dark green and appear water soaked. Later the spots turn purplish-brown. Leaf stalks and new growth may also be attacked. Infection of older twigs results in dieback. Prune out and destroy infected twigs. Sprays of copper fungicide may take care of the problem.
Anthracnose caused by Gloeosporium berberidis is sometimes listed as a leaf spot. Symptoms are round, brown spots with reddish margins on the upper leaf surface. Another fungus, Phyllosticta berberidis, causes purple spots on the leaves. Copper fungicides may provide some control of these diseases.
Powdery mildews of different genera cause a white coating on the leaves. The disease can be controlled with benomyl or sulfur.
Verticillium wilt causes wilting and eventual death. Nothing can be done but to keep plants healthy by watering and fertilizing. Severely infected plants do not recover.