Pathogen(s) causing disease:

Cercospora sojina

Symptoms:

Appear as small, yellow spots on the leaves. Spots eventually enlarge to a diameter of about ¼ inch. The centers of these lesions become gray to brown and have reddish-purple margins.

Conditions:

Infection can occur at any stage of soybean development, but most often after flowering. Wind readily transports the spores from one field to another. Spore production and infection requires warm, humid weather. Dry weather severely limits disease development. Leaves are most susceptible to infection when they are just emerging and become less susceptible as they mature.

Management:

Plant clean, pathogen-free seed. Burying infected soybean residue will help reduce the inoculum in a field. Rotating out of infected soybean for at least two years will help reduce your risk as well. Fungicide seed treatments can reduce the risk of infection. Spray applications of fungicides after growth stage R1 can reduce disease severity. But applications made at stage R3 are considered most effective in southern states.

Sources:

Purdue
U of Missouri