In years past, soybean rust has survived in several states across the southern U.S., but droughts and other obstacles have prevented the overwintered spores from building up and moving north until late in the season.

This past winter was a different story, says an Iowa State University Extension specialist. Freezing temperatures crept further south than normal.

"The main overwinter source of soybean rust, kudzu, was killed back in most places in the south," Darren Mueller says. "As a result, there have been no known locations in the U.S. where soybean rust survived the winter.

"Kudzu has leafed out and soybeans are growing, but there still have been no known soybean rust finds in the U.S. to date."

Mueller says three critical steps have been identified for rust to get to Iowa and many other parts of the Midwest.

• Asian soybean rust must survive winters somewhere in the south
• A buildup of inoculum (spores) must occur for survival
• Movement of spores to fields further north and successful infection of soybeans in those fields must occur.

"These steps may need to reoccur several times for rust to get to Iowa," Mueller says.

Mueller says a network is in place to increase the number of mobile sentinel plots if the risk of soybean rust increases. Iowa State will continue to use X.B. Yang’s predictive model to assess the risk of soybean rust.

"According to early results from his predictive model, the risk of rust getting to Iowa is the lowest it has been since 2005, which was the first full year of soybean rust being in the U.S.," Mueller says.