Mission Statement of the Nebraska Crop Surveillance Network:
Mitigate the impact of crop diseases and insect pests and safeguard Nebraska's agriculture against threats of bioterrorism.

Plant Disease: Brown Spot

Taxonomy:

Causal Agent: Fungus
Common Name: Brown Spot
Scientific Name: Septoria glycines

Distribution:


  • Brown Spot is a common soybean disease that occurs most years on the lower leaves of soybeans.

Symptoms:


  • Irregularly shaped brown spots or lesions on soybean leaves.

  • Individual spots may enlarge and merge to form necrotic blotches.

  • The Septoria f ungus is common in soybean residue and spreads via spores, which can be airborne, or by splashing rain.

  • Infection occurs as early as the V2 growth stage on lower leaves.

  • Under favorable weather conditions (warm, wet weather), the disease may continue to spread to the upper canopy.

  • Late in the growing season, infected leaves may turn rusty brown or yellow and drop prematurely.

  • Spread of the fungus is usually halted during hot, dry weather.


Identification:


  • Look for irregular, dark brown spots on both upper and lower leaf surfaces.

  • Lesions typically will have a yellow or chlrotic halo when held up to a black light.

  • Adjacent lesions frequently merge to form irregularly shaped blotches

  • Leaves become rusty brown.

  • Symptoms of Brown Spot can also develop on stems and pods of plants approaching maturity.

  • Stem and pod lesions have indefinite margins, are dark in appearance and range in size from flecks to lesions several inches in length, but they are not distinct enough to be diagnostic.

Injury and Damage:


  • In some years the disease can cause premature defoliation, which can be confused with normal maturity.

  • Yield loss occurs most often in high yield environments and is related to timing and rate of defoliation.

  • Nationally, yield loss estimates are in the 8 to 15% range and occur when 25-50% of the canopy prematurely defoliates.

  • Disease severity at the R6 growth stage is predictive of yield.

  • Severe brown spot results in smaller seed size.

Management:

Soybean Variety

  • The onset of Brown Spot symptoms is influenced by the relative maturity of the soybean variety, and symptoms appear earlier in the season on an early-maturing variety.

  • Complete resistance has not been identified in soybean varieties or lines, but varieties do differ in partial or rate-reducing resistance that can be used effectively.


Crop rotation

  • Brown Spot is more severe in continuously cropped soybean fields.

  • The host range includes most species of Glycine, other legume species, and common weeds such as velvetleaf.


Fungicide

  • Application of fungicides to manage brown spot in Nebraska is not warranted, as the disease does not typically reach levels with reduced yield.


Tillage

  • For fields with very high levels of Brown Spot, plow under soybean straw to promote rapid decay.


Notes: