Symptoms Approximately 2 to 3 weeks after planting, pin-point to small, angular, red-brown spots will form on unifoliate leaves. Spots are more pronounced on lower leaf surfaces but they develop on the upper and lower leaf surface. 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.

• When scouting for the disease brown spot, examine plants every four weeks from the vegetative stage until maturing.
  • At each site observe two rows of plants 10 feet in length.
  • The number of sites to check in a field is based on the size of the field.
  • See the field size and number of locations chart to determine the number of sites needed for your field.

• Report the severity of the disease according to the following rating scale:

  0 = no brown spot on leaves;

  1 = brown spot on lower leaves of many plants;

  2 = all plants have brown spot on lower leaves with some plants containing brown spot on middle and upper leaves, a little defoliation of lower leaves;

  3 = nearly all plants have brown spot on lower, middle and upper leaves, quite severe defoliation.

• All soybean cultivars are susceptible to brown spot. Rotate beans with other crops and plow under infested residue, where practical.

• IPM - 3 Kentucky IPM Manual for Soybeans

• PPFS-AG-S-1 Brown Spot of Soybeans by Don Hershman and Paul Bachi, Department of Plant Pathology, University of Kentucky

• PPA - 10b Kentucky Plant Disease Management Guide for Soybeans by Don E. Hershman, Extension Plant Pathologist

• Compendium of Soybean Diseases, J.B. Sinclair and P. A. Backman (The American Phytopathological Society Press)