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Corn+Soybean Digest

Green Stems, Harvest Blues

Soybean virus complex is the main culprit Unless you have high frustration tolerance, your religion may have suffered while you were combining soybeans last fall.

The culprit? Green stems in otherwise brown, ready-to-harvest beans.

"When you go into soybeans with green stem, it's definitely harder to combine the crop, and it takes a lot more fuel," says Glen Hartman, USDA-ARS plant pathologist at the University of Illinois.

Hartman heard rumblings of discontent from farmers in the very early harvest stage, but in central Illinois, where the problem was especially serious in 2000, growers got a reprieve. A hard, early frost killed those green stem plants, eliminating some harvesting headaches.

"Green stem has been a much bigger problem in Iowa the past three years," notes John Hill, Iowa State University plant pathologist. "And certainly there was a lot of it this past fall. I had a call, for example, from one of our extension people on his cell phone, and he said he was standing in a field just riddled with green stem."

Hartman and Hill agree with Craig Grau, a University of Wisconsin plant pathologist, that green stem is not a disease per se. "I think we have to treat the green stem problem as a symptom with multiple causes but especially viruses," says Grau. "It varies with the year, but it was very minimal in Wisconsin this past fall."

Hartman points out that the problem isn't a new phenomenon. Research papers from states farther south in the 1970s described green stem and its association with bean pod mottle virus.

The problem that has reared up in the North, especially in states like Iowa, Illinois and Wisconsin, has coincided with three recent abnormally mild winters and the explosion of known virus-spreading vectors like the bean leaf beetle.

New studies and observations of variety trials in Illinois last year showed why green stem has suddenly become a big problem in some Northern states some years.

"Several viruses and other stress factors, possibly even heavy insect feeding alone, may cause green stem," says Hartman. "But, recently, we're finding that it's primarily bean pod mottle virus. That association was 96% in over 100 plots with over 500 samples tested in 2000."

This winter started out much colder than recent ones, but several cold winters will probably be required to greatly reduce the insect vector and resulting virus problems. Only time will tell if the cold will nail these insects.

There is some good news. Hartman and a colleague inspected those 500-plus varieties and found some that show "some decent tolerance" to green stem.

Obviously, breeders haven't had time to breed tolerance or resistance into commercial releases yet, but they're starting to focus on it.

Hartman's advice: If you had the problem in 2000, talk to your seed suppliers, including the company breeders if you can, to find out which varieties show some tolerance. It could make combining a lot more fun next fall.

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