Soybean South


No Yield Loss
Caused By Late Rust

All but three counties in Mississippi reported Asian soybean rust in 2008.
State specialist says conditions were conducive for rust,
but the disease showed up late.

By Bonnie Coblentz

Mississippi soybean growers wrapped up another year’s battle with soybean rust without yield losses to the disease, even though it was found in 79 of the state’s 82 counties.

Tom Allen, an Extension plant pathologist at Mississippi State University’s Delta Research and Extension Center in Stoneville, has been part of the team monitoring the disease since 2007.

He announced Nov. 1, 2008, that soybean rust had been found in all of the state’s counties except Hancock, Harrison and Stone Counties located in south Mississippi.

“We can safely say we escaped almost completely unscathed with regard to soybean rust for the soybean season,” Allen says. “I don’t think we lost a single bushel to soybean rust in 2008. That’s four years in a row that the state has had zero bushels lost due to soybean rust.”

For the last four years, the U.S. Department of Agriculture, through the Risk Management Agency, has provided funding to support the extensive, national scouting program conducted for soybean rust. That funding ran out Dec. 31, and no other funding has yet been found to replace it.

In 2007, Mississippi had soybeans growing in 77 counties, and rust was found in 76 of these. Rust was found only on kudzu in three counties, and Hancock was the state’s only soybean-growing county where rust was not found.

Record number of rust-positive counties
In all, more than 1,200 unique locations were scouted across all 82 counties.

“This is the highest number of positive counties we’ve had to date, and it is one county more than three times what we had in 2007,” Allen says.

“The question can be asked if there has been better scouting for rust or more rust, and I can safely say there is more rust out there.”

Alabama, Louisiana, Arkansas and South Carolina each had more counties positive for rust in 2008 than in 2007, but several other states had fewer instances of rust found last year than in the previous year.

Soybean rust appeared late enough in the season that the Extension service never suggested blanket fungicide applications across vast numbers of acres. Last year, the Extension experts suggested applying an appropriate fungicide in a few key locations in the state with late-planted, post-flood soybeans infected with rust.

In 2008, a few producers decided on their own to spray certain fields with fungicide for the disease, but in most cases, the beans were well past the growth stages where rust could have impacted yields.

“Last year was the first time we’ve ever found rust on soybeans younger than approximately R5.5 in Mississippi,” Allen says. “Last year we found rust on R3/R4 soybeans and on beans in the V-stages. Soybeans are more tolerant to rust in the V-stages, but if there is a large amount of inoculum present in the area, the soybeans can become infected.”

Rust predictions for 2009
The 2008 season was the first time rust was found on commercial soybean fields before scouts detected it in sentinel plots. One reason is because wet weather prevented sentinel plots from being planted as early as planned.

Trey Koger, Extension soybean specialist, says last year’s later-planted soybean crop and cool, wet weather in August and early September provided good conditions for rust.

“We had quite a bit of rust in the state by the end of September and October,” Koger says. “But because it developed late, the disease had no significant impact on the state’s soybean crop.”

There is no way to predict what will happen in 2009 based on what happened last year. Soybean rust cannot overwinter in Mississippi because cold weather kills kudzu – a rust host – and any stray soybeans left in the state. Each year, the disease must blow into the state on weather currents from further south.

“Just because we have found soybean rust in a particular location, either a soybean field or kudzu patch, does not mean that it will be back in that location next season,” Allen says.

Bonnie Coblentz is with MSU Ag Communications at Mississippi State University in Starkville.
Contact Dr. Tom Allen at (662) 686-3272 or