Disease Guide
University-rated fungicide efficacy, pricing per acre, and spray timing for gray leaf spot (Cercospora zeae-maydis). Based on Crop Protection Network 2025 data.
Gray leaf spot (GLS) is one of the most common and economically important foliar diseases of corn worldwide. It is caused by the fungus Cercospora zeae-maydis and is particularly prevalent in the eastern Corn Belt. The disease produces distinctive rectangular, gray-to-tan lesions on corn leaves that are bounded by leaf veins, giving them a blocky appearance that distinguishes GLS from other leaf diseases.
The fungus thrives in warm (75-85°F), humid environments with extended dew periods — 12+ hours of leaf wetness per day. Continuous corn fields with no-till or reduced tillage are at highest risk because Cercospora overwinters on infected corn residue. River bottoms, irrigated fields, and areas with poor air circulation are classic hot spots for gray leaf spot.
Yield losses from gray leaf spot range from 5% to 40% depending on severity and timing. When lesions reach the ear leaf and above before R3 (milk), the impact on grain fill can be severe. Gray leaf spot is highly responsive to fungicide treatment — most modern products provide Good to Excellent control, making it one of the diseases where fungicide ROI is most predictable.
Efficacy ratings from the Crop Protection Network 2025, based on multi-year university field trial data. Only products with tested ratings are shown — untested or not-listed products are excluded.
| Product | Mode of Action | Gray Leaf Spot Rating |
|---|
Source: Crop Protection Network 2025. Ratings reflect multi-year, multi-location field trial performance.
Efficacy ratings cross-referenced with real purchase prices. Products sorted by efficacy rating — best performers first.
| Product | Gray Leaf Spot Rating | Best $/Acre |
|---|
Prices as of early 2025 from AgChem, FBN, and Farmerceag. Actual pricing varies by geography, volume, and timing.
Apply at VT-R1. University trials consistently show the best gray leaf spot control and yield response with a single fungicide application at VT (tasseling) to R1 (silking). This protects the upper canopy during the critical grain fill period when leaf area matters most.
Scout at V10-V14. Begin scouting lower leaves for the characteristic rectangular lesions starting at the 10-leaf stage. GLS moves upward through the canopy — if you see significant lesions on the third leaf below the ear leaf by VT, a fungicide application is strongly justified.
Assess your risk profile. Gray leaf spot risk is highest in continuous corn, no-till fields, humid river bottoms, and with susceptible hybrids. In these situations, a preventive VT application is good insurance. In low-risk fields (first-year corn, resistant hybrid, dry forecast), you may be able to skip the application.
Nearly all modern fungicides work well. Because most products rate Good to Excellent on gray leaf spot, product selection can be driven by price and broad-spectrum needs rather than GLS-specific efficacy. A generic azoxystrobin at $2-3/acre provides Good GLS control for budget-minded operations.
Efficacy ratings for all 26 products across 7 diseases, pricing from 3 suppliers, decision frameworks, and an ROI calculator.
View full guide →Search and filter 9,196 yield records from 867 university field trials by year, state, and crop.
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View guide →Based on Crop Protection Network 2025 data, many products rate Excellent for gray leaf spot, including Quadris, Headline, Quilt Xcel, Trivapro, Delaro 325, Headline AMP, Stratego YLD, Miravis Neo, and Delaro Complete. Gray leaf spot is one of the most broadly treatable corn diseases — nearly all modern fungicides provide Good or better control.
Gray leaf spot is caused by the fungus Cercospora zeae-maydis. It thrives in warm (75-85°F), humid conditions with extended dew periods. Continuous corn and no-till or reduced-till fields are at highest risk because the fungus overwinters on infected corn residue. The disease is most common in river bottoms and areas with poor air circulation.
The optimal fungicide timing for gray leaf spot is VT (tasseling) to R1 (silking). Scout fields at V10-V14 for lesions on lower leaves. If symptoms are present on the third leaf below the ear leaf by VT, a fungicide application is strongly justified. In high-risk fields (continuous corn, no-till, susceptible hybrid), a preventive VT application is recommended.
Yes. Crop rotation is one of the most effective cultural controls for gray leaf spot. Rotating to soybeans or another non-host crop for at least one year allows infected corn residue to decompose, significantly reducing inoculum. Combining rotation with resistant hybrids and tillage to bury residue can reduce gray leaf spot pressure enough to avoid fungicide applications entirely in moderate-risk environments.
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