Downy mildew causes leaf lesions that are light green then yellow. A downy growth forms on both sides of the leaves in the yellow zone. Warm days and cool nights favor the disease. No chemical control is listed.
A wilt disease attacks the stems during wet weather and a cobwebby growth may develop. Parts of the plant above the infection wilt and die. The lower leaves rot, shrink, and shrivel. No chemical control is listed.
White rust causes pale yellow leaf lesions. The undersides of the leaves have smooth white pustules. Infected stems and flowers are seriously deformed. No chemical control is listed.
Club root causes deformation and lack of development of infected roots. Infected plants may wilt on hot days but recover at night. Avoid planting in soil where club root has been a problem. No chemical control is listed.
Powdery mildew causes a white coating on the leaves. If serious, it can be controlled with applications of benomyl.