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Thursday, October 13, 2011

Purple Stained Soybean Seed and weather damage

Several fields of newly harvested soybeans are showing up with purple staining on the seed and/or the seed is materially weather-damaged or shriveled and rotted. The purple coloring is most likely due to the fungus Cercospora kikuchii, a late season disease that shows up when humidity is high and temperatures warm like occurred during the extended rainy period in late August thru mid-September (in few cases if black nightshade was present at combining staining could be due to berry juices). If a significant number of seeds are infected it can lower oil percentage (but actually could increase protein) and lower germination should seed be intended for planting next year.
Lots of these beans are normally rejected for food use.
US Grading Standards for soybeans suggest a grade 3 or lower can be assigned but is not specific to amount needed to cause that grading. Depending on the severity, the beans can often be used for livestock feed if used in limited amounts and properly processed and are also not materially weathered. Such weathering however is showing up in a few cases and here more caution is needed. Such beans should be further examined for pathogens and possibly toxins (although normally more rare in soybeans compared to weathered corn) to determine potential use. Such severely weather beans may receive a grade 4 at the elevator. If farmers plan to plant soybeans in the same field next year, tilling under the residue before planting would be a good cultural practice to reduce potential damage to next year's crop. More information and a photo are available at http://pdc.unl.edu/agriculturecrops/soybean/purpleseedstain

Zane R. Helsel

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