Bacterial blight is a very common foliar disease of soybean throughout the North Central region, but it seldom causes serious yield loss. Together with Septoria brown spot, it is usually the first foliar disease to occur on soybeans in most growing seasons. Diseased plants are usually widespread within a field. Infection can first occur on cotyledons. As plants grow, young leaves are most susceptible. Lesions or dead spots are most obvious on leaves in the mid- to upper canopy but can also occur on stems, petioles and pods. Lesions are small, angular and reddish-brown with water-soaked margins surrounded by a yellow halo. As disease progresses, lesions often grow together to produce large, irregularly shaped dead areas. Center of older lesions frequently fall out, causing leaves to appear tattered. On stems and petioles, lesions are large and black.
Bacteria survive winter in crop residue and seed and are spread by rain and wind. Bacteria can survive on leaf surfaces during the season and infect plants when conditions are suitable. Infections occurs through stomates and wounds caused by wind, hail, cultivation, etc. Thus, outbreaks are common after rainstorms with high winds. Cool, rainy weather favors disease development; high temperatures will slow or stop disease development.
This disease is best scouted at V2 through R6. Scout fields that have received heavy rains.
Variety selection: Use field-tolerant varieties. No resistance is commercially available, but field tolerance has been observed. Avoid planting extremely susceptible varieties where disease is potential problem. Check with a seed dealer for varieties that have field tolrance.
Crop rotation and tillage: Rotation and tillage reduce survival of Pseudomonas syringae. Non-hosts include alfalfa, corn and small grains; other legume crops can be alternative hosts. If tillage is considered to decrease pathogen levels, be careful to minimize soil erosion and maintain soil quality.
Photo by Daren Mueller and Adam Sisson