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Brown Spot of Soybean

Causal Agent:

Septoria glycines

Symptoms and Signs:

  • Begin as small, brown lesions on the cotyledons or unifoliate leaves
  • Reddish-brown to dark brown spots on lower canopy leaves will appear
  • Lesions can coalesce to form large brown areas on leaves
  • Tissue surrounding lesions will be chlorotic
  • Defoliation from bottom of the plant upwards
  • Brown lesions may appear on stems, pods, and petioles

Disease Cycle:

  1. Overwinters on infected plant debris as fruiting bodies called pycnidia.
  2. Warm weather will encourage spore development, then spores will be spread by wind and rain to stems and leaves.
  3. Spores will infect through stomatal openings in the lower canopy of leaves.
  4. Severity increases in prolonged periods of leaf wetness.
  5. Mature lesions produce pycnidia, which can produce secondary inoculum throughout the season.

Disease Management:

Yield losses due to brown spot are rarely severe. Cultural practices focusing on limiting plant debris build-up will help reduce losses.

Host Resistance: It is important to use a cultivar with resistance towards S. glycines to avoid significant yield loss. In productions with no-till or narrow rows, host resistance is increasingly important.

Crop rotation: Rotating with non-hosts allows time for soybean straw containing fungal fruiting bodies to degrade. This disease has been known to be more severe in continuously cropped soybean fields.

Tillage: Plowing under soybean straw can promote rapid decay of debris.

Scouting: Losses of 2 to 4 bushels per acre have been reported in Ohio in very carefully controlled field studies, rarely is it economically feasible to manage this disease. For seed companies, monitor levels of defoliation near R6 to be sure that varieties still have good levels of resistance.  Discard varieties when defoliation of the lower canopy reaches the mid-level of the plant. Proper identification of this disease will allow for more effective management.