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Asian soybean rust threat to dry and snap beans is uncertain
Lincoln, Nebraska
June 23, 2005

While Asian soybean rust could enter Nebraska this summer, it's uncertain whether this fungal disease poses a threat to dry edible and snap, or green, beans in the state, a University of Nebraska-Lincoln plant pathologist said.

Depending on storm movement and wind currents, soybean rust spores could move from southern states to the Midwest and High Plains in the coming months, said James Steadman, plant pathologist in the university's Institute of Agriculture and Natural Resources.

If environmental conditions are favorable, it could become established in commercial dry edible or snap beans. The disease does best between 65 and 80 degrees and requires moisture for development.

"With its wide range of host plants, including legume weeds like kudzu and some edible legume crops, the disease could pose an unknown but potentially serious threat to future crops of soybeans and possibly common beans (dry edible and snap)," Steadman said.

Soybean rust was first detected in the U.S. last fall in several southeastern states. Soybean rust defoliates soybean plants and can reduce yields up to 90 percent in untreated fields.

However, scientists don't know how susceptible dry edible beans are to soybean rust, said Loren Giesler, another UNL plant pathologist.

"With soybeans, you know it is susceptible. With dry edible beans, the producer may have a crop that's not susceptible and make an unnecessary and expensive fungicide application," Giesler said. "This makes management much more difficult."

The long-term survival of this tropical disease in temperate climates, such as in Nebraska, is another unknown, Steadman said.

Tests at U.S. Department of Agriculture quarantine greenhouses in Maryland and bean plots in South Africa and Brazil suggest that dry bean varieties vary in their reaction to soybean rust and are not affected as severely as soybeans, Steadman said.

In addition, preliminary observations suggest that infection severity of susceptible dry beans is lower if they were located more than 10 feet from infected soybeans, he said.

"The potential impact on dry edible and snap beans is of more concern if dry edible beans are planted with or contiguous to soybean fields," he said.

Steadman said some dry edible beans could be planted near soybeans in the northern Sandhills, but where dry edible and snap beans are planted each year varies across the state.

He added gardeners' green beans should be safe.

"First of all, snap beans usually aren't grown in the heat (of the summer) and are finished producing by mid-summer," he said. "Even so, gardeners may want to keep watch for symptoms if they plant a fall crop of snap beans in August."

If soybean rust hits Nebraska, it most likely would be later this summer.

Steadman said USDA and industry research is under way to determine whether soybean rust in other bean species could be a potential problem.

"Right now, we really have no idea if this is going to be a problem, but we have sentinel plots and, hopefully, this will give us a better idea of how soybean rust affects other types of beans," he said.

Soybean rust symptoms include tan to dark brown or reddish lesions one-twelfth to one-sixteenth of an inch in diameter and are most abundant on the underside of leaves in the lower to mid-levels of plant canopies. The disease starts at the bottom of the plant and moves up.

Producers need to carefully scout fields to detect foliar infection as early as possible.

Several fungicides are labeled for use on soybean rust. For more information about soybean rust and dry edible beans, visit Crop Watch, extension's crop production newsletter, at http://cropwatch.unl.edu/archives/2005/crop05-14.htm#rust.

For information about fungicides and the latest information about soybean rust throughout the growing season, visit UNL's Targeting Soybean Rust Web site at http://soybeanrust.unl.edu.

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