Pseudomonas syringae pv. phaseolicola
Halo blight is a bacterial disease of snap and field beans. It first appears as small watery spots on the underside of the leaves and on the stem and pods. These slowly expand, drying out in the center (this part sometimes tears and falls away) and developing a characteristic greenish yellow halo around them (except in hot weather (above 80° F) when the halo may be very small or absent altogether). In severe cases these spots expand and merge to cover most of the leaf. Infected leaves may survive or they may eventually turn yellow and die. Like most bacterial diseases it thrives in warm wet weather.
The pods may develop small, shrunken brown spots. Depending upon the health of the plants and the severity of the infection, the pods may shrivel and not produce seeds, or they may develop normally.
Halo blight is usually caused by infected seed (even one infected seed can result in a widespread infection). The bacteria can also survive in crop debris (remove this or incorporate it into the soil where it can break down quickly). It needs moisture for transmission and reproduction and may be spread by rain, overhead irrigation, people and tools. Some bean varieties are resistant to halo blight.
Image: Howard F. Schwartz, Colorado State University, Bugwood.org