Thanks to cooperative weather for disease, gray leaf spot (GLS) was back with a vengeance in many parts of the Corn Belt last year.

After a couple of years of playing hit-and-miss over the corn-growing region from Nebraska to the Carolinas, the yield-stifling disease hit more than it missed last summer.

GLS is a foliar disease caused by the fungus Cercospora zeae-maydis. It's effectively controlled by clean tillage, so as long as corn growers moldboard plowed fields, GLS was of little note. In fact, GLS was first reported in 1925 in southern Illinois, but plant pathologists say it didn't begin to spread until the 1970s.

The move to reduced tillage probably is the most important reason GLS has become the No. 1 corn leaf disease. With plenty of susceptible corn and surface residue in which to survive, the disease thrived.

Another factor: Corn breeders initially developed hybrids with genetic resistance to most destructive corn leaf diseases, but paid little attention to GLS.

Research and extension plant pathologists tracking GLS movement believe it's migrating north about 50 miles each year and may infest as many as 20 million corn acres.

Recent studies suggest the disease can reduce yield by 30-50%. The time when the disease infects the upper leaves is important because the plant's top eight to nine leaves contribute 75-90% of the carbohydrates needed for grain fill. Losses are highest when it spreads to leaves above the ear soon after tasseling.

C. zeae-maydis spores can survive for more than two years in leaf sheath and ear husk tissues, which degrade slowly. GLS fungus spores can be dispersed by wind and rain as well.

Leaves infected with the disease develop characteristic long, rectangular, brown lesions that are still visible after harvest. When the infection is severe and photosynthesis is diminished, the plant may take nutrients from other tissues to fill the ear. This can result in weak stalks and even stalk rot, possibly causing additional yield loss from lodging or ear drop.

When the fungus is in a field, it doesn't always become a problem. To develop, GLS needs a prolonged period of high humidity or frequent rain, particularly in July and August.

One of the reasons GLS seemed worse in 1998 was that it got an early start. Pat Lipps, an Ohio State University extension plant pathologist, found GLS infections in early July. And Gary Munkvold, an Iowa State University extension plant pathologist, saw it in June and early July. Because of its early onset, late-planted fields were hit the hardest, says Munkvold.

While there's little growers can do to prevent the disease, there are a few management practices that can help reduce the damage.

First and foremost is planting resistant hybrids. Many are now available, but their resistance varies, so be sure to check the resistance ratings.

"The newer GLS-resistant hybrids have better resistance than those grown a few years ago, and they yield better," Lipps says.

Larry Dunkle, a USDA-ARS research plant pathologist at Purdue University, says studies in Indiana, North Carolina and other states suggest that the resistance of some hybrids may vary by location, too.

"Resistant hybrids that looked good in some areas were not as good in other areas," he says.

Crop rotation is the second phase of a good GLS management plan. Only corn is affected by C. zeae-maydis, so planting any crop other than corn can help reduce severity in subsequent corn crops.

Dunkle adds, though, that "One year out of corn may not be enough, because the fungus can survive in corn residue for at least two years."

Since residue is the key to survival, figure that one year out of corn is sufficient if surface residue is 35% or less. If residue cover is more than 35%, the experts suggest at least two years out of corn.

Tillage may still be one of the best practices for managing the disease. Reducing surface residue by tillage may be just as effective as crop rotation.

You may be doing a good job of managing GLS, but you need to watch what your neighbors are doing, too. Spores may be blown fairly long distances by wind or carried on debris moved by runoff water.

A fungicide is still the last resort for managing GLS. Tilt, from Novartis, has been used in high-value crops - like seed - for several years. A new fungicide called Quadris, from Zeneca, looked good in tests and may be registered in time to be used this year.