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Stewart's leaf blight odds wilt with fewer beetles

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Written Tuesday, March 22, 2005  

Stewart's bacterial leaf blight, a common corn disease found in Ohio, is predicted to be of low risk for most of the state this growing season.

Pat Lipps, an Ohio State University research plant pathologist with the Ohio Agricultural Research and Development Center, said that forecasting the severity of the disease, also known as Stewart's wilt, is dependent upon weather conditions and the mortality rate of the flea beetle -- an insect that is a vector of the disease.

"We look at the average monthly temperatures for December, January and February, and if they add up to less than 90, then the chances of having a problem with wilt is low," Lipps said. "We say this because the mortality rate of the flea beetle is high where the flea beetle index is 90 or lower. We expect low populations of the insect this year."

Several locations in Ohio were tracked to determine the risk level of the disease. Researchers recorded a flea beetle index of 91 in Wooster; 84 in Hoytville; 93 in South Charleston; 107 in Piketon; and 87 in Ashtabula. Index values less than 90 indicate a negligible disease threat, values 90-95 indicate low to moderate levels, values 95-100 indicate moderate to severe and values over 100 predict severe disease levels.

"These numbers indicate that the risk of Stewart's bacterial leaf blight should be low in northern and west-central Ohio, with the possibility of higher disease risk in south-central Ohio," Lipps said. "Although the flea beetle index has been a relatively good predictor over the years, we still recommend that growers scout their cornfields for the presence of flea beetles, especially if they know they have planted a hybrid that is susceptible to the disease."

The flea beetle, which becomes active in the spring when temperatures hit 65 degrees Fahrenheit, can carry a bacterium that is spread to the corn plant during feeding and favors disease development.

The bacterium produces a toxin that disrupts chloroplasts in cells. This results in the formation of lesions that damage the leaves, affecting yields, since the plant loses the ability to produce carbohydrates to fill the grain.

"If the disease develops in the growing points of the plant, it will kill the plant and cause seedling blight problems," Lipps said. "But the most common aspect of the disease is the leaf blight phase, where the disease is spread upward on the plant via leaf feeding."

Yield losses in field corn can range from 10 percent to 20 percent in mild cases, to as high as 90 percent yield losses in susceptible varieties. The disease is more problematic in sweet corn and popcorn production where most varieties lack adequate resistance.

The 2005 growing season continues the trend of lower disease risk seen in Ohio the past two years.


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