Michelle Grabowski, UMN Extension Educator
Common blight is a bacterial disease of many kinds of bean including green beans, dry beans, and scarlet runner beans. This disease causes large brown blotches that are surrounded by a bright yellow halo on bean leaves. If disease is severe, browning of the leaves can spread, eventually killing the leaf. On bean pods, infections start as round water soaked spots, that become reddish brown with time.
Common blight is caused by the bacteria Xanthomonas campestris pv. phaseoli. Common blight bacteria often first come into the garden on infected seed. Once introduced, it can spread from plant to plant through splashing water or on a gardener's hands and tools. The bacteria can over winter in infected leaves and pods that fall to the ground. Common blight thrives in hot (82-90F) humid weather. The summer of 2012 in Minnesota has provided ideal conditions for the common blight bacteria.
Common blight is best managed by using resistant varieties. In addition, gardeners should avoid working in infected plants when they are wet. Bacteria are easily moved on hands and tools at this time. Use drip irrigation or direct water at the base of plants to avoid spreading the disease through splashing water. Remove weeds from the garden. Many weeds can harbor the bacteria. At the end of the season, remove infected plants and bring them to a municipal composting site or place them in a backyard compost that heats up. In large gardens, till under infected plant debris and avoid planting beans at that site for 2-4 years. Fungicides do not control common blight and should not be used. Although copper has been shown to be effective against some bacterial pathogens. Studies show that copper does not effectively manage common blight.