Ames, Iowa
May 15, 2002

Iowa State University plant pathologists predict a high risk of Stewart's disease in this year's corn crop due to last winter's warmer-than-usual weather.

"Based on 30 years of historical disease and weather data, we are predicting that 10-15 percent of the seed corn fields in Iowa will have Stewart's disease in 2002," said Forrest Nutter, Iowa State plant pathologist, who added that the risk is moderate-to-severe or severe in the southern two-thirds of the state, but is low in northern Iowa.

The disease is especially devastating to corn being raised for seed because seed harvested from infected fields can't be sold to foreign markets. It also can take a toll on sweet corn and in severe cases, hybrid corn yields may be reduced.

The disease could begin to show up soon in what is called the seedling wilt phase, which can kill the plant. Producers should look for linear, water-soaked lesions on seedling leaves, followed by stunting and wilting. The seedling wilt phase of Stewart's disease occurs when over-wintering adult corn flea beetles infested with the bacterium feed on young corn plants, transmitting the bacterium to the plants.

Disease symptoms in mid-to-late season include yellow or water-soaked lesions or streaks on leaves
originating at the site of corn flea beetle feeding scars. These lesions will elongate along the leaf veins and coalesce, eventually blighting entire leaves.

During the season, the best control for Stewart's disease is to carefully watch flea beetle populations in
individual fields and apply an insecticide to reduce their populations. Thresholds for flea beetle control are based on the insect damage alone, not considering the damage due to Stewart's disease. But if corn flea beetle populations are high early in the season, they can damage corn plants even in the absence of the disease organism.

A computer model developed at ISU predicted that 1999 and 2000 would be bad years for the disease,
which is also called Stewart's wilt. "It just exploded in 1999 and 2000," said Nutter, who led the development of the computer model.

The model correctly predicted that the 2001 corn-growing season would have a low risk for Stewart's disease due to the more "normal" cold weather during the winter of 2000-2001.