A potentially yield-robbing disease, gray leaf spot, has surfaced in east Kansas corn fields, heightening the need for producers to scout their acreage, a Kansas State University plant pathologist said.

"Over the past 10 days, levels of gray leaf spot have exploded in many fields in the eastern half of the state," said Doug Jardine, plant pathology state leader with K-State Research and Extension. "Much of the corn in the west is just starting to reach the maturity stage when gray leaf spot needs to be monitored."

Gray leaf spot is caused by the fungus Cercospora zeae-maydis.

Jardine said he scouted numerous corn fields in the western half of the Kansas River Valley (Manhattan to Rossville) the first week in July. Growth stages there ranged from tassels just emerging to pollen shed.

"In nearly every field, gray leaf spot lesions could be found within one to three leaves of the ear leaf. In a couple of fields, lesions were already present above the ear leaf," Jardine said. "It is my understanding that fungicide applications have begun. Still, every field should be checked for gray leaf spot progress and development."

Jardine surveyed southeast Kansas July 17-18 and found the corn there looks healthy, with little evidence of gray leaf spot.

"Most of the corn in southeast Kansas is mature enough that gray leaf spot is likely not to be a problem for the remainder of the season," he said.

If the forecast for continued humid weather and scattered thundershowers materializes in the rest of eastern Kansas however, the disease will continue to develop, he said. For fields where the disease is already above the ear leaf, producers should consider using a triazole or triazole-containing fungicide, such as Quilt, Stratego, Tilt, Bumper, or Propimax. In fields where the disease has made less progress, strobiluron fungicides such as Headline or Quadris can work well.

"The most severe disease outbreaks will occur where susceptible hybrids are being grown in a corn-after-corn or no-till situation," the plant pathologist said.

Producers can estimate returns that may come from applying a fungicide to a field with heavy gray leaf spot, Jardine said. With the following formula, for example, a grower might use these assumptions: Yield potential of 110 bushels per acre; fungicide application at a cost of $22 per acre; gray leaf spot at or above the ear leaf; and a selling price of $7 per bushel:

•5 percent loss to disease = ~ $16 net return

•10 percent loss to disease = ~ $53 net return

•15 percent loss to disease = ~ $90 net return

•20 percent loss to disease = ~ $129 net return

Higher yields would result in higher net returns, Jardine added.