Agronomy

Septoria Brown Spot

Pathogen Facts

  • Septoria brown spot is a foliar disease of soybeans caused by the fungal pathogen Septoria glycines.
  • The first occurrence of Septoria glycines in the United States was documented in South Carolina in 1923.
  • Today, Septoria brown spot is widely distributed across the country and is especially prevalent in agricultural systems in which soybeans are grown continuously.
  • Although it is the most common foliar disease of soybean, Septoria rarely causes significant yield loss.
  • Septoria glycines primarily infects legumes, but can use velvetleaf as an alternate host.

Photo - Septoria brown spot on soybean leaf.

Identification and Symptomology

  • Septoria brown spot overwinters on infected soybean residue and infects new seedlings around V2 after spores are splashed from the soil surface.
  • Lesions appear as small brown flecks with indefinite margins, typically paired with chlorotic regions.
  • Lesion coloration can range from rusty brown to brown with a purple hue.
  • When lesions enlarge, they coalesce into irregularly-shaped brown areas.
  • Infected leaves can become chlorotic and drop off of the plant; this typically happens in the lower to mid canopy.
  • If rainfall is heavy and frequent later in the season, there is a potential that Septoria glycines can move to the upper canopy.
  • Lesions are often confused with bacterial blight (see below).

Photo - Septoria brown spot lesion compared to bacterial blight lesion.

Photo - Septoria brown spot on soybean leaf.

Conditions Favoring Disease

  • Warm temperatures (60-85 ºF) and humid conditions promote conidia sporulation of Septoria glycines.
  • Extended periods of leaf wetness are conducive for disease development.
  • Conidia are spread throughout the canopy via wind or rain splash.
  • Soybean monoculture, or rotation with other legumes, allows the pathogen to overwinter in crop debris.

Management Considerations

  • Under severe disease pressure, yield losses up to 9% may occur.
  • The potential effect on yield can be estimated by assessing the severity of infection during podfill, particularly at R6.
  • There is no variety that is completely resistant to Septoria brown spot, but partial resistance does exist.
  • Rotating to a non-host crop outside of leguminous species is effective at decreasing the inoculum in the field.
  • Tillage can effectively bury crop debris and cause a rapid decay of the fungus.
  • Foliar fungicides applied from R3-R6 can slow the development of Septoria glycines through the middle to upper canopy during podfill.

References

Author: Madeline Henrickson
August 2019

The foregoing is provided for informational use only. Please contact your Pioneer sales professional for information and suggestions specific to your operation. Product performance is variable and depends on many factors such as moisture and heat stress, soil type, management practices and environmental stress as well as disease and pest pressures. Individual results may vary.