Crown rot attacks the crown and invades the main roots. The plants yellow, wilt and fall over. Black streaks form in the stem tissue. Avoid infested soil. No chemical control is listed.
Verticillium wilt causes leaf drying and poor flowering. The water conducting tissues of the plants are blackened and infested plants decline over a period of years. Avoid infested soil. No chemical control is listed.
Bacterial spot causes spots on leaves. Remove and destroy infected leaves as noticed and cut and discard old stems in the fall. No chemical control is listed.
Powdery mildew causes a white powdery growth on the leaves. Applications of benomyl or sulfur control this problem.