Source: Edward Sikora, Auburn University, Bugwood.org
Disease development is usually favored by rain and warm, humid weather.
Symptoms and Disease Cycle
The fungi overwinter on infected plants, plant debris, and weed hosts. In the spring, spores are produced and are discharged by splashing rain into air currents. They then land on and infect new leaves. Lesions or "spots" are more numerous on upper leaf surfaces and appear circular to irregular in shape. These lesions often have definite reddish-purple to rusty-brown borders that surround a necrotic area. Lesion size and appearance often are influenced by the host variety and the ambient temperature. The leaf spots sometimes cause severe problems, often depending on the variety planted. Susceptible varieties can be defoliated partly or completely by late summer. In years that are particularly favorable for disease development, they can be severely weakened.
Sanitation and the use of resistant varieties are advised, as well as adequate plant and row spacing and keeping plants out of the shade. The application of protective fungicides at blossom time and before fruiting will give adequate leaf spot control.