Numerous spots will cause leaves to yellow and drop off. On trifoliate leaves the disease will develop numerous, irregular, tan lesions that later turn dark brown. Leaf yellowing is usually prominent. Individual spots will frequently coalesce to form large blackish-brown blotches. Defoliation of severely diseased trifoliate leaves is common during wet seasons.
Defoliation typically occurs from the bottom of the plant to the top.
Early season brown spot will appear annually in almost every field in Kentucky. Late-season brown spot is much more variable in occurrence and severity. Cause The causal agent for the soybean disease brown spot is the fungus Septoria glycines . The fungus survives from season to season in crop debris and seed. Warm, moist weather promotes the sporulation of the fungus which is then spread by wind and rain.