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Corn+Soybean Digest

Put A Stop To Gray Leaf Spot

The choices seem almost limitless as you start to select hybrids for next year. It's doubtful, however, that you'll make your picks solely on yield.

That's because you have to factor in which pesky diseases and insects could spell trouble in '04. But be sure to pay close attention to gray leaf spot resistance when making your picks, say corn plant pathologists.

Gray leaf spot is a foliar disease caused by the fungus Cercospora zeae-maydis. Lesions on corn leaves are typically tan and rectangular.

Gray leaf spot wasn't as big of a problem this year in Ohio as it usually is, says Pat Lipps, plant pathologist at the Ohio Agricultural Research and Development Center. “We didn't have the yield losses we've seen in previous years. Still, a lot of fungi will overwinter. Farmers should look for hybrids with disease resistance.”

Several pockets in Ohio have been plagued with gray leaf spot this year. If you're in one of those areas, says Lipps, rotate to soybeans next year. If you use reduced tillage, it's best to rotate out of corn for two years. Or, at least step up your tillage and get the stalks buried.

Gray leaf spot appeared late in Indiana this year, so yield reduction wasn't a big concern, says Greg Shaner, Purdue University plant pathologist. “The earlier it shows up, the more damage it usually causes.”

“If I was growing corn, I'd plant hybrids with resistance to gray leaf spot, even if it was just moderate resistance,” Shaner says. “I'd also look for hybrids with resistance to anthracnose stalk rot.”

Although you might not have identified gray leaf spot on your farm this year, you're not off scot-free. “It's still worth worrying about because the fungus can blow from farm to farm,” Shaner explains. “It produces spores on residue and then blows onto new corn plants from fairly long distances.”

So even though you're doing a good job of managing for gray leaf spot, you need to watch what your neighbors are doing too, he adds.

Rotation and resistant varieties are your best line of defense, Shaner says. However, foliar fungicides applied during the growing season do work and are especially cost-effective on specialty crops like seed corn, sweet corn and high-oil corn. He questions the payback on regular dent corn though, when material and application costs run from $16 to $30/acre.

The amount of residue cover generally reflects the survival of the fungus. “So if you've got 30-35% residue, you'll likely have more of a problem with gray leaf spot,” says Jim Perkins, Monsanto corn pathologist.

Perkins doesn't worry too much about the disease causing economic damage if farmers are on a strict corn/soybean rotation in reduced tillage. “But if you're on a corn/corn rotation and see the disease, look for resistant hybrids. If you're on a corn/soybean rotation and see some gray leaf spot, pick a moderately resistant hybrid. And if you've never seen it, select a moderate- to low-resistant hybrid,” he advises.

Jim Stack, Kansas State University plant pathologist, agrees. “Definitely look at resistant hybrids. Every company now has at least one excellent hybrid for gray leaf spot and they really make a difference,” he says.

Also, if you've got a problem, rotate out of corn and allow the residue to decompose. “Most inoculum is on leaf blade tissue and decomposes in one year if there's adequate moisture and freeze-thaw cycles,” he says.

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