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Cercospora Leaf Blight: Kentucky Soybean IPM

Cercospora Leaf Blight (Purple Seed Stain)


Seedlings infected with Cercospora leaf blight are stunted and may die as a result of stem girdling. Infected seed leaves may shrivel, turn purple, and drop prematurely. Late in the season, upper leaves of plants may develop extensive blighting over large portions of fields. Blighting is the result of numerous pin-point spots to irregular blotches that are red-purple in color. Affected foliage defoliates prematurely.

Severe Cercospora leaf blight

Light Cercospora leaf blight

Purple stain

Small reddish-purple, slightly sunken lesions form on stems and leaf petioles. Infected seed will exhibit varying degrees of pink to purple seed coat discoloration.

Only the leaf blight phase of this disease affects yield. Seed discoloration is a cosmetic seed quality problem, primarily affecting marketability.


Cercospora leaf blight and purple seed stain are caused by the fungus Cercospora kikuchii. The fungus survives between seasons in seed and infested crop residue. Infected seeds produce diseased seedlings.

Infectious spores are produced by infected seedlings, infested crop residue and certain weeds. Spores are blown or splashed onto upper plant parts, and infection occurs during warm, wet weather.

IPM Techniques

  • Plant high-quality, disease-free seed or certified seed.

  • Rotate soybeans with other crops.

References and Additional Information

  • PPA-10b Kentucky Plant Disease Management Guide for Soybean by D.E. Hershman, Extension Plant Pathologist

  • Compendium of Soybean Diseases, J.B. Sinclair and P. A. Backman (The American Phytopathological Society Press)

This site was created and is maintained by Pat Dillon, Department of Entomology, University of Kentucky, S-225 Agricultural Science Ctr North, Lexington, KY USA  40546-0091 (phone: 859/257-3571). Please send questions or suggestions to: pdillon@uky.edu