Bacterial pustule occurs mid- to late season when temperatures are warmer and more favorable for disease development. Symptoms may be mistaken for bacterial blight, Septoria brown spot or soybean rust. Diseased plants are usually widespread within a field. Lesions are found on outer leaves in the mid- to upper canopy. Lesions start as small, pale green specks with elevated centers and develop into large, irregularly shaped infected areas. Unlike bacterial blight, no water soaking is associated with lesions, but each lesion is surrounded by greenish- yellow halo. A pustule may form in the center of some lesions, usually on the lower leaf surfaces. Pustules crack open and release bacterial. Bacterial pustule will not cause leaves to tatter like bacterial blight.
Bacteria survive winter in crop residue and seeds are spread by rain and wind. Infection occurs through leaf stomates or wounds. Rain weather favors disease development. Unlike bacterial blight, high temperatures do not slow disease development.
The best time to scout is R1 through R6. Because of the small blister, this can be confused with soybean rust. Bacterial pustule does not cause leaves to tatter, distinguishing it from bacterial blight.
Variety selection: Avoid planting extremely susceptible varieties. Some varieties are marketed as resistant to this disease
Crop rotation and tillage: Rotation and tillage reduce survival of Xanthomonas axonopodis pv. glycines. Other legume crops may be hosts; non-hosts include alfalfa, corn and small grains. If tillage is considered, use proven conservation tillage practices to maintain soil quality.
Photo by Daren Mueller