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Cercospora Leaf Blight and Soybean Purple Seed Stain

Late-season warm and humid conditions lead to increased sightings of Purple Seed Stain, causing questions about its effect on yield. Purple Seed Stain, also known as Cercospora leaf blight, is caused by the fungus Cercospora kikuchii. While yield loss may occur if the disease infects leaves early in the season, the loss is usually less than 10%, and is dependent on how quickly the disease develops. However, seed discoloration may result in elevator dockage and grain quality may be downgraded. 

Cercospora leaf blight symptoms usually appear in the upper canopy leaves as a distinct purplish color and/or bronze highlights across the leaf margins. Disease symptom development on the leaves does not correlate strongly to symptom development on the seed. Even though Cercospora leaf blight may be in the field, it doesn’t mean that there will be Purple Seed Stain, and vice versa.

  • Infection occurs early in the season but is not usually visible until R4.
  • Upper canopy may have various-sized light purple spots with bronze highlights, which can be mistaken for sunburn later in the season.
  • Leaves may fall off the plant in cases of severe infections.
  • Symptoms may also be seen on petioles, stems and pods.
  • Infected pods show a noticeable, purple discoloration of the seed.
  • High humidity and warm temperatures trigger disease development.
  • The pathogen overwinters on crop residues and/or seed.
  • Temperatures averaging above 80° F greatly increase fungal sporulation.
  • Severity of the disease is related to environmental conditions at flowering, not the presence in planted seed.

  • Incorporate residue to help reduce inoculum, as the pathogen survives on crop residue.
  • Rotate crops to help lessen disease incidence.
  • Apply seed treatment fungicides to infected seed, but plant disease-free seed. 
  • R3-R5 foliar fungicide applications may help reduce blight and pod infections, help limit yield loss and improve seed quality.
  • No soybean variety is immune to this disease, but there are varieties known to have low susceptibility to Cercospora leaf blight.
Contact your Golden Harvest Seed Advisor with questions or for additional agronomic insights.

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