Several years ago, warnings circulated that a potentially devastating new disease called soybean rust had reached some fields in the southern U.S. and was causing problems in Brazil. A number of farmers as well as a number of agronomists feared the rust would someday infest areas as far north as Iowa. Some observers thought the rust would cripple soybean yields in Iowa and other Midwest states, but that has never really developed.
"We've identified three critical steps that must occur for the rust to get to Iowa," says Daren Mueller, an Iowa State University Extension plant pathologist:
What has to occur for soybean rust to get as far north as Iowa
(1) To survive winters somewhere in the south,
(2) Build up inoculum (spores) where survival occurs, and
(3) Movement of these spores to fields further north and for successful infection of soybeans to occur in those fields. These steps may need to reoccur several times for rust to get to Iowa.
In years past, soybean rust has survived in several states across the southern United States. 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. While we are all too familiar with how much snow and cold weather we had here in Iowa, freezing temperatures crept further south than normal.
The main overwinter source of soybean rust is kudzu, which was killed back in most places in the southern United States this past winter. 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. this year -- to date anyway.
Distribution of soybean rust in late May 2009 (top) and May 2010 (bottom). Note no known sources of soybean rust have been found in the U.S. in 2010.
During the 2010 growing season, ISU Extension will have five sentinel plots scattered across Iowa. These will be coupled with ISU's fungicide trials at the Northeast Farm (Nashua), Northwest Farm (Sutherland), Southeast Farm (Crawfordsville), Armstrong Farm (Lewis), and Curtiss Farm (Ames).
Iowa has a network in place to increase the number of mobile sentinel plots if the risk of soybean rust increases. Iowa plant pathologists also will continue to use ISU soybean disease expert X.B. Yang's predictive model to assess the risk of soybean rust getting to Iowa. 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.
Last Section 18 fungicide now gone
Flutriafol (Topguard, Cheminova) was the last of the Section 18 soybean rust fungicides to expire. While the Section 18 label has expired, Topguard received a federal registration on soybeans in late April. Cheminova is working on the state registrations for this product and hopes to have product in the field by June.
Topguard is in the triazole class of fungicides and has soybean rust, frogeye leaf spot, Cercospora leaf blight, brown spot and powdery mildew listed on its label as diseases that the product controls.
Seasonal updates on soybean rust are available online
As a reminder, you can view short, frequent updates on soybean rust on the ISU Soybean Rust webpage. You can also sign up to receive these updates through e-mail. Information on how to sign up for these e-mail updates also can be found on the Web page. Daren Mueller is an ISU Extension specialist with responsibilities for ISU's Corn and Soybean Initiative. Mueller can be reached at 515-460-8000 or by e-mail at [email protected].