December 07, 2004

Researchers gaining ground on soybean diseases

by K.C. Jaehnig

CARBONDALE, Ill. - - Plant pathologists and soybean breeders from seven states are gaining ground in their quest for genetic control of frogeye leaf spot and charcoal rot, two soybean diseases invading the Midwest from the South. "We have identified somewhere around 130 public and private germplasm lines that appear to be resistant to frogeye leaf spot in our field studies," said Southern Illinois University Carbondale plant pathologist Jason P. Bond, who is coordinating the three-year joint effort with a $293,535 grant from the North Central Soybean Research Program.

Those lines will undergo molecular testing at the University of Georgia.

"If a small subset of those lines has the Rcs3 resistance gene, then we would be several steps ahead in the breeding and development of resistant cultivars," he said.

As for charcoal rot, researchers at the University of Illinois have developed a greenhouse test that can detect partial resistance to the disease.

"That's a big development," Bond said. "It's much harder to screen for resistance to charcoal rot than it is to screen for frogeye leaf spot.

"This greenhouse assay will let us identify which lines are promising and eliminate those that are susceptible, so instead of having to test several thousand lines, we can work with fewer than 300."

In addition, researchers at the USDA station in Stoneville, Miss., have been able to identify an experimental line among those they have developed that is partially resistant.

"That's in the process of being released, which will make the seeds available to public and private breeders to develop their own germplasm," Bond said.

"If someone had said at the start of the project that we would make this kind of progress with charcoal rot so early, I would have been very skeptical."

The group has also decided to take a look at fungicides as a possible means of controlling frogeye leaf spot - - something not part of the original plan.

"We didn't think to do that early on because normally soybean prices meant that it wouldn't be feasible, no matter how bad the disease got," Bond said.

"But going into this season, we had higher prices. There were reports of six- to seven-bushel yield increases with the use of fungicide, so we had farmers spraying for frogeye leaf spot in the southern part of the state this year and also in parts of Tennessee, Arkansas and southern Missouri. They'd call and ask for recommendations on which fungicide to use, and we had nothing to tell them."

The researchers plan to test the effectiveness of commercially available fungicides on fields in Southern Illinois, southern Indiana, Missouri and Tennessee.

"These are areas we know have frogeye leaf spot problems," Bond said. "They're probably also going to be good candidates for any Asian soybean rust (a costly disease that made its first appearance in the United States this year) that may blow in from the south - - they would probably see it first.

"We're hoping that next year at this time when we look at the fields where we sprayed the fungicide, we'll just be looking at frogeye leaf spot and charcoal rot and not soybean rust, but at this point, that's just a hope. It would be pure conjecture to say what, if any, effect rust will have next year."

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