Stewart’s leaf blight could give corn hybrids, farmers a sick feeling

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COLUMBUS – Stewart’s bacterial leaf blight, a corn disease that can cause significant yield losses in susceptible hybrids, could be severe this growing season.

The disease, also known as Stewart’s wilt, could cause problems for sweet corn and popcorn growers since many of the hybrids are not resistant to the disease, said Pat Lipps, Ohio State University plant pathologist.

“In field corn, yield losses are usually not extremely severe, but when the disease does damage the plants and they lose significant leaf tissue, a farmer can have yield losses approaching 10-20 percent,” Lipps said.

Yield losses. Yield losses in susceptible varieties can range from 40 percent to 90 percent.

“By far the No. 1 management recommendation we have is careful choice of varieties – to select a variety that has some resistance or tolerance to Stewart’s wilt,” said Celeste Welty, an Ohio State Extension entomologist.

Welty has listed 84 sweet corn hybrids with ratings for resistance to Stewart’s bacterial leaf blight on the Ohio State Veg Net Web page, accessed at http://ohioline.osu.edu/~vegnet/library/swsc00.htm.

The prediction of a severe Stewart’s bacterial leaf blight outbreak is based on data showing the possibility of high flea beetle populations this spring. The insect, a vector of the disease, overwinters and emerges in the spring to feed on young corn plants, transmitting the bacterium to the crop. Mild winter temperatures increase the chances of high flea beetle populations, thereby increasing the potential for Stewart’s bacterial leaf blight.

“A forecasting system developed many years ago looks at weather conditions – average monthly temperatures in December, January and February,” Lipps said.

“When these numbers add up to 90 or better, there’s a high probability that large numbers of overwintering beetles will survive and potentially spread the disease.”

Flea beetle index. Data collected from 10 sites in Ohio – Hoytville, South Charleston, Piketon, Jackson, Ripley, Oxford, Delaware, Kingsville, Wooster and Fremont – showed the flea beetle index exceeding 100.

“That would put the prediction into the severe area,” Lipps said.

“Flea beetles are normally not an economic problem in a lot of field corn most years,” said Bruce Eisley, Ohio State entomologist.

“It is almost impossible to keep the flea beetle from transmitting the disease, so all we can do is monitor populations in fields and treat when necessary.”

Welty said farmers have three choices when it comes to keeping flea beetles in check on sweet corn: plant seed already treated with systemic insecticide, apply a systemic insecticide into the soil before planting, or apply a foliar insecticide spray.

“If a farmer plants a resistant variety, he probably doesn’t have to do anything else,” Welty said.

“In some cases where a farmer might have to plant a susceptible variety, the easiest and most effective means of control is to plant pretreated seed.”

Seed treatment. The Environmental Protection Agency has approved a federal label for Gaucho, a new seed treatment product that previously was legally available only if the seed was treated in Idaho.

“It was allowed for end-use in Ohio, but seed was not allowed to be treated in Ohio,” Welty said.

“Now it has the federal label, which opens up some options for farmers and seedsmen.”

If flea beetle scouting is necessary, Eisley recommends farmers scout their fields to determine flea beetle populations as soon as corn plants begin emerging. Treatment may be necessary when 50 percent or more of the plants before the fourth-leaf stages are being severely damaged, and five or more beetles per plant are found.

Most of the common insecticides labeled on field corn can be used on flea beetles. In scouting flea beetles on sweet corn, the insect threshold is six beetles per 100 plants on susceptible varieties and two beetles per plant on resistant varieties.

“A farmer needs to scout a lot during the very young stages of the plant, up to the seventh-leaf stage,” Welty said.

“Also, we recommend farmers scout on warm, sunny days, since the insects like to hide during cold, windy conditions.”

Bacterium toxin. The bacterium that causes Stewart’s bacterial leaf blight produces a toxin that disrupts chloroplasts in cells, resulting in the formation of lesions that damage the leaves and affecting yields, since the plant loses the ability to produce carbohydrates to fill the grain.

“The earlier in the season you get damage to the leaves and upper portion of the corn plant, the greater the yield loss,” Lipps said.

Researchers estimate 15 percent of overwintering flea beetles and 45 percent of those that develop in the summer carry the bacterium that causes Stewart’s bacterial leaf blight.

The disease was negligible in Ohio in 2001, but warm weather the previous four years resulted in severe disease problems. It was during those years that the flea beetle index also exceeded 100.

“Based on our knowledge of past years, we would predict these beetles are probably going to survive pretty well this year and will be available to start feeding on plants as soon as the corn comes up,” Lipps said.

“Populations were lower than normal last year,” Welty said. “Although the temperature-based flea beetle index numbers look like it’s going to be bad, we didn’t have so many beetles overwintering, so hopefully flea beetle populations won’t be as severe as the model predicts.”

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