Gray leaf spot seems to have found its niche. In 1997, it was most prevalent where corn followed corn, particularly in high-management situations, and especially on low-lying fields or where irrigation was used.
Ten years ago, the disease was just beginning to move into the central and western Corn Belt. It was seen as a bonafide threat in the hot, humid summers of the early 1990s, when it seemed to run rampant over much of the region.
"It takes hot, humid weather early in the summer to allow gray leaf spot to develop into much of a threat," points out Gary Munkvold, Iowa State University extension plant pathologist.
There have been no widespread outbreaks since the 1995 epidemic that engulfed most of the Corn Belt. However, it has shown up in pockets, the largest of which stretches from south- central Nebraska south across Kansas.
The problem in those two states is greatly worsened by the practice of growing continuously irrigated corn. Irrigation combined with hot weather seems to create just the right environment for the disease. Add minimum-till or no-till so gray leaf spot spores from the previous year's crop are there, ready to pounce on the new crop, and you've got a volatile situation.
Because the fungus overwinters in corn residue, rotating an infested field to another crop for at least a year can reduce the severity in subsequent years.
"However, this may not be so dramatic in an area where no- tillage corn production is dominant," says Erik Stromberg, a Virginia Tech extension plant pathologist. "Spores from the fungus harbored in the infested residues can travel hundreds of yards to initiate disease in a newly rotated field."
If you can't rotate away from corn, your next best bet is tillage to bury infested residue. Burning is an alternative to tilling if you're in a no-till or minimum-till situation and don't want to disturb the soil surface. But be sure to check burning regulations in your township or county before you set fire to the field.
If crop rotation, tillage or burning isn't an option, your next move is to look for resistant hybrids. Despite what you might have heard, Stromberg insists that corn hybrids can be bred with resistance.
In fact, Virginia Tech and Garst Seeds jointly hold a patent on a method to rapidly move resistant genes into high-yield-potential corn inbreds.
Stromberg says the degree of resistance varies in hybrids currently available. Some, he adds, are moderately resistant.
"Within the next season or two, highly resistant, high-yielding, locally adapted hybrids will be available," Stromberg predicts.
Check with your local or state extension agronomist or corn specialist for information on gray leaf spot ratings from state hybrid trials.
Pat Lipps, an Ohio State University plant pathologist, cautions about getting too excited about resistant hybrids.
"While there are some that perform well in areas with moderate to low disease pressure, highly susceptible, high-yielding hybrids can produce more than some of the resistant ones," he cautions.
Another strategy where the disease was a problem in previous years is to plant earlier-maturing hybrids. The reasoning is that earlier hybrids will be closer to maturity before leaves are lost from gray leaf spot infection, so yield loss will be minimal.
Applying a fungicide after gray leaf spot symptoms appear is another option. Tilt, from Novartis, and two or three other fungicides, are labeled for use against the disease.
Paul Klemme, a Tilt marketing specialist, suggests that growers plant the highest yielding hybrids for their areas, and then scout for gray leaf spot.
"If it's a light year for gray leaf spot, you'll be bushels ahead with your high-yielding hybrids," says Klemme. "And if the disease does develop, it can be controlled with fungicides.
"Our recommendation is to use a fungicide as soon as you see two or more gray leaf spot lesions on the second leaf below the ear leaf on 50% of the plants," he advises. "If we can keep the ear leaf and the leaves above it free of gray leaf spot, we can minimize the impact of the disease on yield."
In yield trials in infested fields across the central and western Corn Belt, Klemme says fungicide treatment improved yields by 12 bu/acre or more, depending on the disease severity and hybrid.
"We even saw responses to fungicide on resistant hybrids," he adds.
The cost of one application will run $15-20/acre, so it takes a 6- to 8-bu yield increase to pay for the chemical, assuming a corn price of $2.50/bu.
Check with your extension crops specialist, crop consultant or farm supply dealer for approved fungicides and related information.