The American soybean industry has changed dramatically since soybean rust was found on our shores in late 2004 – and so has our understanding of Phakopsora pachyrhizi, the fungal pathogen that causes the disease.

Five years after soybean rust was discovered in Louisiana, more than 130 scientists at the American Phytopathological Society’s Fourth National Soybean Rust Symposium shared both good and bad news:

• Farms in the United States have not been devastated like those in Brazil, though 2009 saw the first major economic losses to soybean rust in hard-hit fields in Mississippi.

• An array of fungicides were registered and put to good use controlling soybean rust, but reports from Brazil indicate the soybean rust fungus may be getting more tolerant of the triazole class of fungicides in some areas.

• Some populations of kudzu are resistant to soybean rust, but the amount of infected acreage of kudzu that serves as a spore bank has been growing steadily.

• Geneticists are gaining an understanding of how some plants defend themselves against the fungus and are isolating genes breeders can cross into commercial varieties, but they point out that resistant varieties will not be a silver bullet for managing the disease.

Marking a Shift
“This is a celebration of accomplishment,” Anne Dorrance of The Ohio State University told the group in New Orleans, LA. “There has been a tremendous amount of research and sweat and driving hours and laboratory bench hours. We have focused a lot of brainpower on P. pachyrhizi.”

Scientists have published 170 peer-reviewed papers in European and American publications since 2004.The effort to understand and manage soybean rust is by no means over, but the 2009 symposium marked a shift in tone for the group. We’ve gone from the fearing this disease to being confident we can manage it if it becomes a problem.

Among the biggest changes growers will see in the coming year is a scale-down of the Sentinel plot system that has covered the nation’s soybean producing areas since 2005 with meticulously-scouted soybean plots. “With the end of federal grants that have supported the system, funding will drop 60 to 80 percent in 2010 compared to its 2006 to 2008 peak”, says Don Hershman, extension plant pathologist at the University of Kentucky.

Grower Commitment
The Sentinel plot system is a monument to soybean growers’ commitment to staying ahead of soybean rust, Hershman points out. The plots paid off handsomely – both in the value of the data they provided on how soybean rust moves and in the estimated $200 million growers have saved each year by avoiding unnecessary sprays or making properly timed treatments when needed.

Growers shouldn’t worry because if a “perfect storm” of spores and weather sets the stage for Midwestern outbreaks, or the pathogen shifts to a more Midwest-adapted race, they will be protected. “From a Northern perspective, we only need to know where soybean rust is from about April 1,” Hershman says. “The South is where the main event is. If things begin to change, if models indicate there could be a problem in the [Midwest], there will be ad-hoc monitoring. No soybean specialist will drop the ball on this.”