Bacterial blight occurs every year in Iowa without causing significant yield losses. It's most commonly found in areas that have received a lot of rain because the bacteria are spread by rain and wind. Although yield loss has not been documented for bacterial blight except in unusually rainy growing seasons, bacterial diseases should be monitored carefully in seed production fields because they are seedborne and can affect seed quality.
Bacterial blight is often confused with brown spot, a leaf spot disease caused by the fungus Septoria glycines (Figure 1). The presence of a yellow halo around the lesions of bacterial blight, and lacking in brown spot, can be used to distinguish between the two diseases. Bacterial blight tends to infect new leaves, while brown spot infects older leaves.
Symptoms of bacterial blight
Lesions caused by the bacteria are usually first observed in rainy weather on younger, top leaves of soybean plants. The lesions are small, angular, water-soaked, yellow-to-brown spots. The angular lesions enlarge in cool, rainy weather and merge to produce large, irregular dead areas. The bacteria also can infect stems, petioles, and pods.
Management of bacterial blight
Avoid the use of susceptible varieties. Crop rotation also reduces disease risk.
If you find severe infection this season, you should not use the seeds for next year because this disease can be seedborne.