Xanthomonas axonopodis pv. phaseoli
Bacterial blight affects green, snap, field, lima, scarlet runner, mung and tepary beans and is most problematic in hot, humid conditions. The first symptoms to appear are small, angular, pale green, water-soaked spots on the leaves. These gradually enlarge and merge to form large brown blotches with dry centers (commonly surrounded by a narrow yellow zone). In extreme cases the leaves may become scorched and withered and drop off. The stems may also develop brown blotches, as well as cracks or water-soaked cankers that ooze a yellowish liquid (this is very infectious). Spots also appear on the pods, starting small, but enlarging to form dry, brown sunken patches. Depending upon the health of the plants and the severity of the infection, the pods may shrivel and not produce any seeds at all, or they may develop normally. Some bean varieties are resistant to bacterial blight.
Bacterial blight is usually caused by infected seed (a single infected seed can result in a widespread infection). Minimize the spread of this disease by removing infected plants as symptoms show themselves (and don’t touch other plants before washing your hands thoroughly). The bacteria can also survive in crop debris, so this should be removed, or incorporated into the soil, where it can break down quickly. This disease needs moisture for transmission and reproduction, so avoid getting the leaves wet (use drip irrigation) and avoid touching the wet leaves. It can also be spread via soil or sap on tools.
To minimize the effects of this disease, give the plants plenty of water and nutrients, good air circulation and well drained soil. Rotate bean crops every 4 years.
Image: Howard F. Schwartz, Colorado State University, Bugwood.org