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Killing frost puts damper on Asian rust

Recent blasts of arctic air into the deep South caused some problems for travelers throughout the region, but hopefully provided some much needed breathing room for soybean growers in the Carolinas and Virginia.

As late as the first week in February, rust was still being reported on kudzu in south Georgia. In Cairo, Ga., located approximately 30 miles north of the Florida line, temperatures as low as 28 degrees F did not kill kudzu, which is a primary host for Asian Soybean Rust.

Though the jury is still out, it is hoped the recent waves of arctic air will push the freeze line for kudzu and other rust hosts further down into central Florida.

Rust didn't cause any significant problems in soybeans in North Carolina or South Carolina in 2005, though it was found in 23 counties in South Carolina and 18 in North Carolina — the northern-most counties were within 50 miles of the Virginia border.

“The worst thing soybean growers can do is take a cavalier attitude toward rust,” says John Mueller, Clemson University soybean researcher. “We know soybean rust can cause significant economic losses to soybeans in our environment, and if conditions in 2006 are different than 2005, the disease could be a problem for us,” Mueller adds.

Speaking at a recent South Carolina soybean production meeting, Mueller pointed out that rust was not found in the state until August in 2005, and not widespread until September. By September, much of the Carolinas plunged into an extended drought period, which further stopped the spread of rust.

Mueller points out two critical factors that made rust only a minor problem in 2005, may not occur in 2006. First, he says, there was no evidence of kudzu and other host plants providing a source of inoculum until March in the southern border states of Georgia, Florida and Alabama. That may not be the case in 2006, he warns.

Plus, given the option of managing drought or managing rust, a cheaper alternative may be to manage the disease. At any rate, Mueller stresses that growers in the upper Southeast can't count on a drought to provide a natural barrier to the spread of soybean rust.

In 2005 there were no dramatic movements of rust from Georgia and up into the Carolinas. In Brazil, dramatic movements of 200-300 miles per week have been recorded. “This is not Brazil,” Mueller stresses. “We have about 100,000 acres of soybeans in the Florida Panhandle and 240,000 in Georgia, which provides for lots of gaps between production from south to north — that's not the case in Brazil. Plus, we don't have inoculum in the field year-round like they do there,” Mueller explains.

Monitoring stations, which are scattered every 50 miles or so throughout South Carolina's 450,000 acres of soybeans, will be the early warning system for South Carolina growers.

“If rust will behave like it did last year — slow and erratic movements from field to field, our growers should have about a week's warning,” the South Carolina specialist says.

“Rust was not detected in any soybean-producing state until after flowering and it was always detected first in earlier maturing cultivars. So, when soybeans begin to flower, we will step up our monitoring program, which will be a key to managing soybean rust in 2006, Mueller says.

The South Carolina scientist also urges growers to avoid basing management decisions on rumors.

Mueller puts out an electronic newsletter that provides the latest information on rust movement — accurate information he stresses. Growers, he says, should monitor the USDA rust Web site and subscribe to his electronic newsletter by contacting him at

If rust does become a threat to Carolina growers, Mueller says the fungicides available to treat it have provided good protection in tests throughout the Southeast.

Several triazole and strobilurin fungicides performed comparably across the multi-state tests.

In general, if rust is already present in a field, triazoles are the optimum treatment. If rust is in the area, but a preventative is needed, the strobilurins are the best bet. A shortcoming of the triazole family of fungicides is a lack of control on other soybean diseases, particularly frogeye rust.

Frogeye rust makes identification of soybean rust much more difficult because of the similarity of foliar damage. For that reason, growers may consider planting frogeye resistant varieties.

“My suggestion for growers is to budget for $20 to spray for rust and to avoid planting soybeans in fields that don't have the yield potential to support the extra $20,” Mueller says.

With soybean carryover the highest on record, economists agree with planting fewer acres, regardless of the reason.

There is some promising news on the soybean rust front, according to Mueller.

Several soybean breeding lines have shown resistance to the disease. These lines are a long way from becoming recommended varieties, but the indication of natural resistance is promising.

Though sub-freezing weather and the Deep South aren't usually good bedfellows, a few strong blasts of arctic air may be just what soybean growers need to make a profit in what promises to be a challenging economic year for the crop.


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