November 18, 2004

'Davis' soybean variety resists fungus

by K.C. Jaehnig

CARBONDALE, Ill. -- If you have a soybean fungus that blew in from the South, the best way to fight it may be with a bean variety that comes from the same place.

Jason P. Bond, a plant pathologist from Southern Illinois University Carbondale, has spent much of the year trying to get a handle on which soybean varieties can best resist frogeye leaf spot, a disease that appears to be gaining a foothold in the Midwest.

Both in the greenhouse and out in the field, Bond has found that a resistant variety called Davis, an old variety that is grown in more southern climes, came through unscathed.

"None of the (fungal) isolates were capable of reproducing on Davis -- that's the take-home message," Bond said.

For soybean breeders in the Midwest, that means that making crosses to Davis could produce varieties that would be resistant to all known strains of the fungus, Bond said, while researchers should begin hunting through other newer varieties for the same gene or a similar gene that confers that resistance.

"There are three known resistance genes, but only Rcs3 (found in Davis) can provide protection against all known isolates," he said.

Bond will be posting his findings on the Web at later this month. He already has sent the information on to the seed companies. Bond says the companies likely will make it available by the time farmers are making their spring planting choices.

The work with resistance and an effort to learn more about the fungus itself are part of a larger project that includes a study of charcoal rot, another fungus that attacks soybeans. A grant from the North Central Soybean Research Program is paying for the research, which involves plant pathologists and soybean breeders from seven states.

Frogeye leaf spot, so named because of the eyeball-like lesions that blight the leaves, reproduces easily out in the field -- mist on a summer morning will give it all the moisture it needs to begin manufacturing its spores -- and those spores spread through the air with no trouble at all. So you might think studying it would be a cinch. But no.

"This pathogen is very finicky when it comes to growing it -- it can actually die if you don't take care of it," Bond said.

"Some of its offspring grow faster than others, some colonies don't get as big as others, some decide every few cycles that they're not going to reproduce -- and the spores don't seem to like each other. You can never get a full plate of them. Most of the other pathogens we work with are much more forgiving in culturing."

While the project's researchers have become adept at getting the fungus to grow and reproduce, its finicky nature means they will never have large volumes to work with in their research plots. They've also discovered that its virulence can differ from year to year, which will make the goal of sorting the pathogen into different groups and assessing the deadliness of each a little more challenging.

"It's much more complicated than the other pathogens we work with," Bond said. "Fortunately, the person who knows most about the virulence of this pathogen is working on our project."

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