5/8/00

Editor's Note: Graphics illustrating the increase in gray leaf spot in Iowa can be found at:
1981 GLS map of Iowa
1997 GLS map of Iowa
Bar chart of increase from 1981 to 1997

Contacts:
Forrest Nutter, Plant Pathology, 515/294-8737
Susan Thompson, Agriculture Information, 515/294-0705

GRAY LEAF SPOT IN CORN INCREASES IN IOWA

AMES, Iowa -- If there is any good news to be found in Iowa's dry weather, it is that the incidence of gray leaf spot in the state's corn fields might be lower than if this were a wet year.

"Prolonged wet seasons or years with high humidities and above normal temperatures provide the most favorable conditions for the spread of gray leaf spot," says Forrest Nutter, Iowa State University plant pathologist. "It can be less of a problem in a dry year, but there is still reason for concern."

The disease is reaching epidemic proportions in Africa, causing severe reductions in grain yield and quality. It also has been reported in many other countries, including Brazil, Columbia, Costa Rica, Mexico and Venezuela.

The spread of the disease has been steady across the United States. Gray leaf spot was first identified in southern Illinois in 1924 and there were sporadic reports of it in several states over the next 50 years. It got more attention in the early 1970s when significant yield losses occurred in North Carolina and Virginia. Losses in the Corn Belt began to mount in the 1980s.

In Iowa, gray leaf spot was reported in four out of 28 counties inspected in 1981. By 1997, the fungus was reported in 44 out of 46 counties inspected. Those figures translate into statewide prevalence of the disease that increased from 8 percent in 1981 and to 95 percent in 1997.

1997 was considered a moderate year for the disease. Yet yield losses attributed to gray leaf spot in Iowa fields cost an estimated $123 million. In 1998, losses in seed corn production alone were estimated at $24-45 million.

Nutter says the fungus survives best from one year to another if crop residue remains on the soil surface after harvest. "Graphs showing the increase in conservation tillage match graphs that show an increased incidence of this disease," he says.

That fact doesn't mean Nutter is an advocate of more tillage. "It does mean crop rotation can reduce the impact of the disease because corn debris that is a breeding ground for the fungus is only left on the soil surface every other year," he says.

Monitoring for the disease is important so producers know what they're up against. Nutter says to look first for lesions on the lower leaves of a corn stalk. "Symptoms can occur in the seedling stage, but generally the lesions appear a few weeks before tasseling. The lesions are gray or tan in color and distinctly rectangular in shape," he says.

Control attempts are being made on two fronts. "Seed corn companies are working hard to develop resistant varieties," Nutter says. "There are some varieties that offer partial resistance. Some are better than others, but so far there isn't anything that is completely resistant to gray leaf spot."

A fungicide application may be economical, especially for seed corn producers who don't have the flexibility to choose resistant varieties. Gary Munkvold, ISU plant pathologist, is working on a modeling method for predicting gray leaf spot incidence so fungicide application decisions can be made more effectively. He's also evaluating experimental fungicides for improved control.

"Fungicides are most effective at preventing infection rather than curing it," Munkvold says. "The trick is to predict whether economic damage will occur in a field, and if so, when."

Munkvold says there are certain indicators that help suggest whether a fungicide will be necessary. These include susceptibility of the hybrid or inbred, the crop sequence in the field, the amount of corn residue on the field surface and the field's history of gray leaf spot damage.

Both Munkvold and Nutter say more research is needed to help provide answers on the effective use of fungicides and which varieties are the most resistant to this disease that is spreading in Iowa and around the world.

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