October 5, 2007

Figure 1. An enlarged (20 X) view of a soybean rust lesion with pustules. Within the white circle the pustules have formed a volcano-shaped mass extending up from the lesion. (Photo by Seth Watson)

Soybean rust was positively identified for the first time this week in Nebraska. It was detected in a southeastern Nebraska soybean field. Identification was confirmed at UNL's Plant and Pest Diagnostic Clinic and the National Plant Diagnostic Network regional lab at Kansas State University Friday (Oct. 5).

The disease was found on soybeans in southeastern Richardson County near the town of Rulo and in Otoe County south of Nebraska City. Leaf samples were collected from scouting activity by Seth Watson, UNL soybean rust sentinel plot coordinator, on Oct. 3.

Samples of hundreds of leaves were collected as part of scouting activity with the national soybean rust sentinel plot system. The sentinel plot system is funded by the Soybean Checkoff program and the U.S. Department of Agriculture.

"As we are late in the season and many of our identified sentinel plots are mature at this time, leaf samples were being collected from fields with green leaves remaining starting on Monday of this week," said Loren Giesler, UNL Extension plant pathologist. "Based on where other reports of soybean rust had been confirmed in southern Kansas and near Des Moines, Iowa, we thought it should be here."

Giesler said the find is not unexpected as the disease has been found in many states this year and has been moving north from its overwintering location in the Gulf States and spread into Texas earlier this summer. Institute of Agriculture and Natural Resources faculty and staff are working to determine if other fields in the state also have rust.

"Fortunately for soybean producers this year, the disease is showing up too late for any impact on yield, and management will not be needed," Giesler said.

Figure 2. This enlarged (130X) view shows the volcano shaped pustules and the arrows point out the opening. Note around the edges and inside the pustule you can see the spores erupting from the top.(Photo by Seth Watson)

Further finds on the distribution and severity of rust occurrence this year will greatly help forecasting and modeling research, which is ongoing.

"This demonstrates the ability of the fungus to move northward, and had it occurred earlier in the year a totally different scenario could have developed," he said.

Updates on where soybean rust has been confirmed in Nebraska and other parts of the U.S. can be found on the USDA soybean rust Web site at http://sbrusa.net.

"As mentioned, no management will be needed this year for soybean rust in Nebraska, but growers should continue to stay aware of where soybean rust is being found in future years," Giesler said. "Each year the disease has spread north and west earlier, and this year we have observed a totally different distribution compared to last year.

"No rust was observed in the western soybean states in the Great Plains area last year compared to this year, and this year we are seeing movement north further west than last year's movement up the Mississippi Delta into Ohio."

If rust arrives earlier in the season in future years, management options will include fungicide application, but once the crop reaches the R6 growth stage (full berry stage), the impact of soybean rust is minimal and most fungicides cannot be applied based on the minimum pre-harvest intervals.

Producers are encouraged to contact a local UNL Extension office for more information on soybean rust, call the soybean rust phone hotline at (877) NEB-RUST or visit UNL's soybean rust Web site at http://soybeanrust.unl.edu.

Sandi Alswager Karstens
IANR News