More Soybean Rust, But Mostly Too Late

By Amy L. Fahnestock
Senior Associate Editor

The number of counties reporting Asian soybean rust by Aug. 31 this year is more than double findings on the same date in 2008, but in many states, the soybean crop is far enough along and yields will not be affected. Soybean rust has been found in seven states and 84 counties in the U.S. so far in 2009, and in two states and five municipalities in Mexico.

“Clearly, things are much further ahead this year, and this is probably due to the early finds in Louisiana a few months back,” says Don Hershman, University of Kentucky Extension plant pathologist.

On Aug. 29, rust was detected on kudzu in Jackson County, FL.

Aug. 28 was a prolific day for soybean rust confirmations. All told, rust was reported in Drew, Lincoln, Desha, and Lee counties in Arkansas; LaFayette, Morehouse, and West Carroll parishes in Louisiana; Attala, De Soto, and Madison counties in Mississippi; and in Greene County, AL.

Louisiana: The positive findings of soybean rust in the state the last week of August were in commercial fields, according to Clayton Hollier, Louisiana State University Extension plant pathologist. Two of the fields were late R6 and R4, while the other two were at the R7 stage. Louisiana has 20 confirmations of soybean rust in 2009.

Mississippi: Soybean rust was found at low levels in a commercial soybean field at the R6 growth stage in Tunica County on Aug. 26, report Tom Allen, Mississippi State University (MSU) plant patholgist and Trey Koger, MSU Extension soybean specialist.

More rust was detected three straight days, Aug. 21-23, in commercial soybean fields in Quitman, Tallahatchie, Calhoun, and Coahma counties on soybeans in the R7-R8 growth stages. The overall count for affected counties in the state is now 19, and the pair are not recommending any management strategies because of the late growth stages and low spore incidence.

Alabama: Rust was detected at very low levels in a commercial soybean field in Greene County on Aug. 28. This followed soybean rust confirmations on Aug. 20, 23, and 26. “Wet weather patterns earlier this month appear to have spread soybean rust to a good portion of the state,” sayd Ed Sikora, Auburn University Extension plant pathologist. Rust has now been found in 15 counties in Alabma.

“Growers with soybeans in the R3-R4 growth stages with good yield potential that have not been sprayed previously should consider applying a tank mix combination of a stobilurin and a triazole fungicide or a prepackaged tank mix of the two products,” Sikora says.

“Growers with fields at the R5 growth stage with good yield potential that have not been previously spreayed with a fungicide should consider an application or a triale fungicide,” Sikora adds. “Soybeans that have reached the full pod R6 growth stage do not require a fungicide application for the purpose of preventing soybean rust.”

Hershman expects to find Asian soybean rust in Kentucky fairly soon. “The fact that the disease has been detected in north Alabama is significant, and means that we and Tennessee will be the next states to report soybean rust. I truly believe that our soybeans crops ar no longer at risk for damage.”

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