If hot, humid weather makes you wonder if your corn needs foliar fungicide for gray leaf spot, Purdue University plant pathologist Kiersten Wise has some answers. Wise says gray leaf spot has been confirmed in many corn fields in Indiana, including at high levels in some fields. That's particularly so for hybrids susceptible to gray leaf spot, Wise says. 

"The warm, humid weather over the last few weeks has created perfect conditions for disease development, and the heavy dews and intermediate rains we have received have also contributed to increased disease severity," Wise says in a recent issue of Purdue University's Pest & Crop Newsletter.

"Yield loss may depend on the number of lesions and how far up in the canopy they occur as the plant enters tasseling and pollination," Wise says. "If lesions have reached the ear leaf or higher during the 2 weeks before and after tasseling, yield loss could occur. If lesions develop on upper leaves later in the season, the economic impact will be less.

Deciding whether to apply fungicide seems straightforward, but not every corn field may need fungicide to manage gray leaf spot, she says.

"Research in Indiana indicates that strobilurin and strobilurin/triazole premix fungicides are most effective at preventing yield loss when applied in response to disease presence, and at the tasseling to early silking (VT-R1) growth stage," she says.

"There have not been consistent yield benefits from foliar fungicide applications in corn, according university researchers," Wise says.

The following factors affect decisions to treat corn.

1. Hybrid susceptibility

Hybrids vary in their susceptibility to foliar diseases of corn, and hybrids susceptible to diseases such as gray leaf spot are at a greater risk of disease development than hybrids with moderate or high levels of disease resistance.

2. Previous crop and cropping system

Most of the fungal diseases, such as gray leaf spot, survive from year to year on crop residue, and planting corn-on-corn, and planting corn into high levels of corn residue (no-till) will increase the likelihood that disease will develop.

3. Late planting

Research from Iowa State University has demonstrated that late-planted corn is at higher risk of gray leaf spot development.

4. Favorable weather conditions

Foliar diseases, like gray leaf spot, require high humidity, moisture and moderate to warm temperatures for disease development.

Wise says some level of gray leaf spot can be found in almost corn field in Indiana, but she cautions that deciding to apply fungicide should be based on the level of disease and the four factors she outlined. And she warns that many fields contain common rust.

"As rust pustules age and dry down, they are easily confused with gray leaf spot," Wise says. "Common rust rarely causes serious yield loss, and a few lesions of common rust on a leaf will not justify a fungicide application."

(Editor's Note: This article appeared in a recent issue of Purdue University's Pest & Crop Newsletter. It has been edited for length and clarity.)