8/27/01

Contacts:
Gary Munkvold, Plant Pathology, (515) 294-6708
Tracy Petersen, Agriculture Communications, (515) 294-0706

SOYBEAN STUDY EXAMINES FACTORS INFLUENCING BROWN STEM ROT

AMES, Iowa -- Brown stem rot isn't the most damaging fungal enemy in soybean fields, but it does take a toll on yields.

The severity of the damage often is determined by factors that include tillage, herbicides and the presence of soybean cyst nematode. The effect on soybean yields from brown stem rot (BSR), and the factors that make it worse, are the subject of research by Gary Munkvold, associate professor of plant pathology at Iowa State University.

Munkvold, who is in the second year of a three-year project, said it takes careful observation by soybean growers to detect BSR, which works below ground and exhibits subtle above-ground symptoms. Though he noted that BSR rarely causes drastic yield reduction in individual fields, it is widespread and can reduce yields by a few bushels per acre, possibly on millions of acres in Iowa.

In his first year of research, Munkvold found that when BSR was present, some herbicides can worsen its effect. Herbicides that influence BSR's impact do so in two ways. "They can directly influence the fungus, or they can affect the plant's physiology, making it more or less susceptible to disease," Munkvold said.

"Our first year of research showed that the use of Blazer increased the severity of BSR, while the Roundup herbicide had no effect," he said. "Results with Pursuit herbicide have been contradictory. In Iowa trials, we saw more severe BSR with Pursuit. But in Wisconsin trials, soybeans treated with Pursuit had a lower incidence of BSR."

No-till practices appear to worsen the severity of BSR, Munkvold reported. And the presence of soybean cyst nematode can do the same. "Soybean cyst nematode seemed to reduce the effectiveness of plants that had BSR resistance," he said.

Munkvold encouraged soybean growers to monitor their fields for both soybean cyst nematode and BSR in order to make better decisions when it's time to select seed varieties and herbicides. "These decisions aren't independent of one another," Munkvold said.

Munkvold is continuing studies into the effects of BSR and its interactions with other factors. He'll take a closer look at the decreased plant resistance to BSR caused by soybean cyst nematode and will evaluate the colonization of soybean roots by Fusarium species, including the one that causes another disease, Sudden Death Syndrome. Munkvold said there have reports of interactions between herbicides and Fusarium.

Munkvold's research is part of the Yields Project, a multistate research program funded by Iowa and Illinois checkoff dollars through the Soybean Research and Development Council (SRDC). The Yields Project is one of the largest soybean research projects ever undertaken to understand factors that limit soybean yields. The project involves scientists at Iowa State, the University of Illinois, the University of Wisconsin and Southern Illinois University.

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