Tuesday, November 3, 2009

Soybean Purple Seed Stain Widespread in Kentucky

A month or so ago I blogged about the prevalence in KY of Cercospora leaf blight, a fungal disease caused by Cercospora kikuchii. I mentioned that a phase of the disease - purple seed stain - was also likely to be prevalent once harvest got underway. Well this is now the case and many questions are being raised about purple seed. First of all, the purple coloration is caused by a plant toxin called cercosporin. Cercosporin is red-purple so any tissue it is produced in has a purple-tinted color. But the degree of infection is not always related to the extent of the purple coloration. Actual infected tissue may be considerably less than would be suggested by the purple color.

Typically, purple seed stain is a minor production problem. Yields are reduced by the foliar phase of the disease, Cercospora leaf blight, but purple seed stain is strictly a seed quality issue. Rarely, planting heavily infected seed can produce diseased seedlings, and reduce stands, in subsequent crops. More commonly, purple seed may be discounted at the point of sale in some, but not all, markets.

Other than that, I think the purple seed is kind of pretty. But you have to be a plant pathologist to see any redeeming qualities in a plant disease. All in all, I do not think anyone should get too worked up over purple seed stain.

Photo: Dr. Anne Dorrance, The Ohio State University, Wooster, OH.

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