ID cited as key to Asian soybean rust control

Soybean disease experts agree that Asian soybean rust offers ample reason for concern across the United States, but widespread outbreaks are not likely if growers, consultants and university personnel take appropriate precautions.

“Why is it a concern?” asked University of Illinois soybean pathologist Wayne Pedersen during a soybean rust management seminar at the recent Commodity Classic in Austin, Texas.

Pedersen, who works with the National Soybean Research Lab, said the disease “is devastating.” Losses may range from 10 percent to 80 percent.

The good news, he says, is that the pathogen is unlikely to overwinter north of Orlando, Florida, “most years. Cold in Florida is a good thing (for soybean producers),” he said.

“So if we get it again, it has to blow in.”

He says the unusual hurricane activity last fall brought the disease up from South America and into southern soybean fields, as far north as the Missouri Bootheel.

He compared the event to a 1970 spread of corn blight. The disease had been identified in specific corn varieties in the South in 1969. In 1970, observers identified a little of the disease in Florida. Then three hurricanes, Alma, Becky and Celia, hit and carried the pathogen as far north as Southern Minnesota and over to East Texas.

“Can that happen with Asian soybean rust? I don't think so. We have sentinel plots in the South and all the way into Minnesota. If we know where it is, I think we can manage it. If we don't find it in sentinel plots, we probably will not see it that year.”

The pathogen has moved considerably: from Africa and into South America and last fall, into the United States.

“Pedersen said the disease usually hits double-crop soybeans. “That's a concern.”

He said two strains of rust may show up, but one is not as aggressive as Asian soybean rust. “We have to do an assay to differentiate the two. We can't identify it in the field.”

Another concern is the number of host species on which Asian rust can survive. Hosts include 95 species, including the indomitable kudzu.

“But the disease will not kill kudzu. It's a perfect host,” Pedersen said.

“Identification is the key to control,” he emphasized. “Early rust infection, at first glance, may look like soybean downy mildew. But looking through a hand lens shows it's different.”

A lens brings out pustules and other distinctive features of rust disease. The pathogen also infects soybean pods. “It takes a significant effort to train farmers and consultants to look for Asian soybean rust,” Pedersen said.

Ideal growing conditions include a warm, humid environment. In seven to 10 days, infection moves to spore production. Infection may occur on the lower plant tissues at first.

Pederson said crop rotation is of little value in managing the disease. Row spacing and plant populations also appear to have limited effect. Early maturity groups and early planting may help.

“We're also looking at plant resistance and fungicides,” he said.

Finding resistance comes with significant challenges. “We screened 1,000 varieties and none had a high level of resistance,” he said. “We screened the ancestor set and still found no high level of resistance.

“We looked at 16,000 lines from germplasm and found 5 percent with promise. That makes us optimistic that material is out there with a level of resistance.”

He said partial resistance also might be possible. He labels that level of resistance as “slow rusting,” in which the disease causes less injury or severity of the infection may be less. He said researchers also may identify plants with tolerance and will show less yield reduction with infection.

“Until rust gets to the United States, we can't test for it,” he said. “For now, the only thing we have is fungicide control. Fortunately, we have a number of products that do a good job. And soon, we expect to find varieties with some level of resistance.”

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