Five Soybean Diseases to Scout

July 1, 2016 08:00 AM
Soybeans Miss

Protect your soybeans from yield-robbing pathogens this summer. Use this scouting guide to identify issues in your field.


“Rain and cooler temperatures create an environment favorable to the disease,” says Jon Bokmeyer, Monsanto product development manager. The disease is more likely in compacted or saturated soils.

While the disease typically infects the plant early in the season, even before emergence, it can attack the plant at any time according to University of Minnesota Extension.

Warning signs and management tips:

  • Brown lesions on roots
  • Dark brown discoloration from soil line up stem
  • Yellow, wilted leaves
  • Plant death in varieties without resistance or stunting and reduced yield in resistant varieties

Sudden Death Syndrome

sudden death syndrome sb

Cool and wet spring conditions can encourage soybean sudden death syndrome (SDS) development. The disease infects the plant after emergence but symptoms don’t show up until nearly flowering.

“SDS is second only to soybean cyst nematode as biggest yield robber in the Corn Belt,” says Andrew Ferrel, Mycogen commercial agronomist from Indiana. The disease overwinters in soybean residue and once you spot symptoms it’s too late to save yield with treatment.

Warning signs and management tips:

  • Yellowing and interveinal chlorosis starting with lower leaves
  • Discoloration when the stem is split with a pale-green colored cortex
  • Leaves fall off, leading to early plant death and yield loss
  • Seed treatments or planting SDS resistant varieties could offer relief for next soybean rotation since the disease will overwinter in residue

Brown Stem Rot

Brown Stem Rot (BSR) infects the plant in early stages and needs cooler temperatures (60° to 80°) while the plant is setting pods to show foliar symptoms. The disease can be more severe in fields with poor fertility, according to Purdue Extension.

SDS and BSR can be difficult to tell apart. Scouting and getting your boots dirty will help distinguish the difference between the two soil-borne diseases so you can plan for next year. There are no in-season treatment options for BSR but you can select resistant varieties next year.

Warning signs and management tips:

  • Pith will appear dark brown when stem is split
  • Leaf necrosis and chlorosis (similar to those of SDS)
  • Plant death
  • BSR risk higher in soil pH less than 6.5

Frogeye Leaf Spot

In contrast to many soybean diseases, frogeye leaf spot flourishes in hot, humid areas. Soybeans can be infected at any point in the growing season but most commonly after flowering. Spores from frogeye overwinter in soybean residue.

You can manage the disease in season, but you need to catch it early enough. Be mindful if you are having weather conditions that favor the disease so you can contact your aerial applicator faster.

Warning signs and management tips:

  • Initial symptoms are small, yellow spots about ¼” in diameter, centers will turn gray to brown and have reddish to purple margins
  • Lesions on stem will appear red when young and darken as they age
  • Spots on pods are reddish brown and can appear to be sunken and can infect the seeds inside the pod
  • Apply fungicide between R3 and R5 to maximize yield, according to Ferrel

White Mold

High-yielding soybeans are at the biggest risk of developing white mold. Spores survive in the first two inches of soil profiles and infect soybeans at canopy.  The fungus favors cool temperatures (no higher than 85°”) and moist conditions.

If your soybeans start showing signs of white mold infection it’s important to act quickly to slow the fungus, Bokmeyer says. In some cases it might be too late to save yield potential.

Warning signs and management tips:

  • Narrow row spacing, less than 20” could increase likelihood of white mold
  • Before disease symptoms appear small fungi-like apothecia can be found in soil (appearance similar to bird’s nest fungus, according to Purdue)
  • Initial symptoms show water-soaked stem lesions that eventually encircle stem and later cause stems to look bleached
  • Lesions can sometimes be found on stems, pods, petioles and leaves
  • Appears in patches throughout field
  • White cotton-like mycelia growth can be found on inside or outside of pods and stems and in some cases black raised spores can be found along stem
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