Pathogen(s) causing disease:

Septoria glycines

Symptoms:

Appear first on the cotyledons and unifoliate leaves early in the growing season. Angular, red to brown spots that vary in size from tiny specks to 1/4 inch diameter can be seen on the upper and lower leaf surfaces. Leaves with numerous spots rapidly turn yellow and fall to the ground. Spores produced on leaf spots infect the trifoliate leaves, stems, and pods of the soybean plant. Numerous tan lesions, which eventually become dark brown, appear. Later in the season infected leaves may turn yellow and drop prematurely. Defoliation proceeds from the bottom of the plant toward the top of the plant.

Conditions:

High moisture and moderate temperatures (60°–85°F) favor infection and disease development. Spread of the fungus is usually halted during hot, dry weather. Spores are spread by wind and splashing rain. Disease usually stops during hot, dry weather.

Management:

Crop rotation allows time for soybean straw with fruiting bodies to degrade. This disease is more severe in continuously cropped soybean fields. For fields with very high levels of brown spot, plow under soybean straw to promote rapid decay.

Sources:

U of MO
ISU
Ohio State