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Gray Leaf Spot Appears Hard to Resist

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Written Friday, September 29, 1995  

Battling gray leaf spot (GLS) will be tough for corn growers because many supposedly resistant varieties are not that resistant, according to an Ohio State plant pathologist.

"We have been evaluating a number of hybrids for resistance over the years, and most commercial hybrids sold as resistant are moderately resistant at most," says Pat Lipps.

Farmers are looking for ways to fight gray leaf spot, a disease that made headway in Ohio this year, while scourging Indiana and other Midwestern states. Premature plant deaths occurred in east-central Ohio, and almost every Ohio county had some level of plant infection, Lipps says. "Gray leaf spot will continue to be, or will become, an important corn production problem for growers in Ohio," Lipps says.

Pick a resistant variety by getting recommendations from seed dealers -- then check the degree of infection in field trial plots at farm shows and seed corn company field days, Lipps says.

Commercial hybrids have two kinds of resistance, which are distinguished from susceptible hybrids by either number of lesions or spore output. One kind suffers fewer brown lesions than on more susceptible hybrids. The other has yellow-to-orange lesions, which tend to be smaller and produce fewer spores than susceptible varieties.

After choosing a variety, plan next year's strategy to curb disease spread, Lipps says. Growers need to stall disease development until at least six weeks after the plant is most vulnerable at tasseling, he says. The objective is to prevent the disease from attaching to the leaves above the ear during this critical time. This should prevent yield losses greater than 10 to 15 percent.

One way to hold off GLS is to till crop residues, a main home of its fungus. This slows down the spread of early-season spores, which cause the most damage. Tillage should only be performed where allowed.

A secondary, slower, spread of gray leaf spot occurs from the production of thousands of spores in each necrotic leaf lesion two to three weeks after infection. Hybrids susceptible to cholorotic lesions slow secondary spread because these lesions produce fewer spores.

Where tillage is limited, Lipps says growers should consider crop rotation. A one-year rotation will work for tilled fields while a two-year break is advised for reduced-tillage or no-till.

A third control method involves planting problem fields first in order to shift the post-tasseling grain fill to a time when the disease does not spread as quickly. Shoot for an early July flowering to avoid August heat and humidity that favors disease spread and infection.

"There is little we can do about the weather, but it may be worth planting GLS fields first when field planting plans are made," he says.

Several factors seem to foretell continued problems in Ohio, Lipps says. These include federal conservation programs that limit or prohibit tillage; corn acreage base requirements of government programs, and lack of highly-resistant varieties.

"Gray leaf spot will obviously find production practices and environmental conditions conducive for its development in new areas of the state," Lipps says. "We can only wait to see which areas are affected economically.

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