11/2/98

Contacts:
Forrest Nutter, Plant Pathology, (515) 294-8737
Brian Meyer, Agriculture Information, (515) 294-0706

ISU RESEARCHERS ABLE TO PREDICT SEVERITY OF DISEASE IN CORN FIELDS

AMES -- Last winter, Iowa State University researchers predicted that Stewart's disease would be a significant problem in corn fields during the 1998 growing season.

They were right. The 1998 growing season was one of the more troublesome for Stewart's disease in recent years. "By the end of the season, nearly every field in southern Iowa had Stewart's disease," said Gary Munkvold, ISU Extension plant pathologist.

Led by plant pathologist Forrest Nutter, ISU has been testing a computer model to forecast when Iowa fields would be at high risk for the disease. The model may give advance warning to seed companies so they can make more-informed management decisions.

Stewart's disease, or Stewart's wilt, is caused by a bacterium transmitted by corn flea beetles. The disease isn't a major problem in many fields, but it can be especially devastating for susceptible inbreds used to produce seed corn and sweet corn. In severe cases, yields can be greatly reduced.

But the greater economic damage comes when seed companies try to export their products. Many countries restrict the importation of seed corn produced from fields where Stewart's disease has been detected.

Working with colleagues in plant virology, entomology and agricultural meteorology, Nutter studied 26 years of data on the prevalence of Stewart's disease in Iowa seed-corn fields. The data is collected during annual inspections of hundreds of fields by the Iowa Department of Agriculture and Land Stewardship. The inspections are required to ensure there are no pathogens that could affect seed exports.

Since 1972, there have been eight years when prevalence of Stewart's disease was high, Nutter said. The researchers then studied weather data during those years. In general, mild winters, especially during the months of December, January and February, are favorable to the survival of flea beetles in the soil.

Nutter first tested a computer model developed more than 40 years ago for use in the Northeastern United States, and found that it did not work under Iowa conditions. Then, using the disease and weather data from Iowa, Nutter and his colleagues developed a new computer model to predict when the conditions were optimum for disease outbreaks. The researchers used special software to generate maps that showed which counties would be more at risk.

Using the model, the researchers predicted that 1998 would be a high-risk year for Stewart's disease throughout much of Iowa.

"With the success we had in predicting the severity of the disease in 1998, we believe seed companies could use this information to help decide where to plant, avoiding high-risk areas," Nutter said. "It also can help companies decide whether they need to apply insecticides to control flea beetles. And it should help them improve their management so they're only using chemicals when they're really needed, and only in high-risk areas."

Three seed companies -- DeKalb Genetics Corp., Novartis Crop Protection and Pioneer Hi-Bred International, Inc. -- are supporting additional research at ISU to fine-tune the model. "We'll study other variables that may help the flea beetles survive the winter," Nutter said. "With that information, I think we can do an even better job of prediction."

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