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Some Soybean Fields Have a Big Purple Problem

Purple Seed Stain is affecting random soybean fields in 2018.

The Wildcat fans of either Kansas State University or Northwestern University may like the looks of purple seed stain in soybeans, but your grain elevator probably will not.  

Some soybean producers in the Plains states of Nebraska, Kansas, Missouri, and Oklahoma are finding signs of purple seed stain, or cercospora blight, during the 2018 harvest. A result of the fungus Cercospora kikuchii, it is also known by the names purple blotch, purple speck, purple spot, or lavender spot, according to Loren Giesler, Extension plant pathologist at the University of Nebraska.

Purple seed stain doesn’t reduce yield, but if more than half the load is discolored, growers may find excessive grain dockage, or may have to blend a bad load, says Doug Shoup, a crop consultant from Lyndon, Kansas.

Prevention in 2019

The inoculum source for purple seed stain is infected seed and debris from previous soybean crops. High humidity and late-season temperatures between 73°F. and 80°F. exacerbate the problem, add UNL plant pathologists Nick Arneson and Tamra Jackson-Ziems.

During the growing season, symptoms of purple seed stain-infected plants include late-season leaf bronzing.  

There are commercially available resistant varieties for Cercospora Blight, but no known sources of resistance for purple seed stain. There is also no clear relationship between the severity of Cercospora Blight and the severity of purple seed stain, Arneson and Ziems agree.

Crop rotation away from continuous soybeans, tillage to reduce spore population, and foliar fungicides to combat the disease all can help reduce the problem, Shoup says.

Extended crop rotations and residue incorporation will reduce the inoculum by breaking down infested residue. Foliar fungicides are registered for Cercospora Blight and can be applied during R3-R5 pod stages, which can reduce blight incidence and severity, but this may not affect soybean seed stain symptoms. Seed lots with a high percentage of infected seed should be treated with a seed treatment fungicide when used for seed.

What to do

Growers who find signs of purple seed stain in their fields should contact their preferred delivery point to see what steps should be taken. In some cases, elevators are accepting affected loads but will need to know ahead of time that the load has the disease. In other cases, farmers can blend compromised loads with clean loads to reach an acceptable threshold. Communication with the delivery point is crucial. 

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