Bacterial blight occurs over most of the United States. Several different strains can infect both snap and lima beans. Common blight (pictured) causes large, water-soaked, light green spots on leaves that later turn brown. Water-soaked spots appear on pods and may produce yellowish ooze in wet weather. Leaves of plants infected with Halo blight have many small dead spots with yellow halos around them. Spots on pods produce cream-colored ooze when the weather is humid. Cool, wet weather promotes the spread of this disease. Brown spot blight, more common on limas, causes small, reddish-brown spots with distinct margins on leaves and pods.
Use a 3-year crop rotation for beans. Don't work in the garden when plants are wet. Don't save seeds from infected plants. Copper fungicides are effective as a preventive when applied as directed on the product label. It's best not to depend too heavily upon this method of control, as copper can build up in soil to levels toxic to earthworms and microbes.
Photo courtesy of Howard F. Schwartz, Colorado State University, Bugwood.org
Article published on June 23, 2008.