5/9/2025

Gray Leaf Spot of Corn

Crop Focus
Written by Mark Jeschke, Ph.D., Pioneer Agronomy Manager

Key Points

Gray leaf spot (GLS) is a common fungal disease of corn that overwinters in corn residue.
Cropping systems with reduced- or no-till and/or continuous corn are at higher risk for gray leaf spot outbreaks.
Planting hybrids with genetic resistance to GLS can help reduce the risk of yield loss due to infection, and foliar fungicides can be used to manage gray leaf spot outbreaks.

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Causal Pathogen

Gray leaf spot (GLS) is a common fungal disease in the United States caused by the pathogen Cercospora zeae-maydis in corn.
Disease development is favored by warm temperatures, 80°F or 27°C; and high humidity, relative humidity of 90% or higher for 12 hours or more.
Cercospora zeae-maydis overwinters in corn residue, allowing inoculum to build up from year to year in fields.
Cropping systems with reduced- or no-till and/or continuous corn are at higher risk for gray leaf spot outbreaks.
Conducive weather conditions encourage the rapid spread of disease near the end of summer and early fall, when corn plants allocate more resources to grainfill.

View a larger image.

Identification

Early Symptoms

Gray leaf spot lesions begin as small necrotic pinpoints with chlorotic halos, these are more visible when leaves are backlit.
Coloration of initial lesions can range from tan to brown before sporulation begins.
Because early lesions are ambiguous, they are easily confused with other foliar diseases such as anthracnose leaf blight, eyespot, or common rust.

Cercospora zeae-maydis spore.

GLS lesions begin as small necrotic spots with chlorotic halos.

Later Symptoms

As infection progresses,lesions begin to take on a more distinct shape.
Lesion expansion is limited by parallel leaf veins, resulting in the blocky shaped “spots.”
As sporulation commences, the lesions take on a more gray coloration.
Entire leaves can be killed when weather conditions are favorable, and rapid disease progression causes lesions to merge.

As GLS develops, lesions become blockier and more gray in color.

As GLS progresses, lesions will coalesce and form larger necrotic areas.

Crop Damage

Gray leaf spot lesions on corn leaves hinder photosynthetic activity, reducing carbohydrates allocated towards grain fill.
The extent to which gray leaf spot damages crop yields can be estimated based on the extent to which leaves are infected during grain fill (Table 1).
Damage can be more severe when developing lesions progress above the ear leaf around pollination time.
Because a decrease in functioning leaf area limits photosynthates dedicated towards grainfill, the plant might mobilize more carbohydrates from the stalk to fill kernels.
This can result in a higher risk of stalk lodging and stalk rots due to a loss of structural integrity.
Table 1. Estimated yield loss based off of percent of tissue infected by gray leaf spot (Lipps, 1998).

Percent Leaf Area Affected at R5
(Early Dent Stage) Approximate Yield Loss
5% or less 0 - 2%
6 - 25% 2 - 10%
25 - 75% 5 - 20%
75% - 100% 15 - 50%

Percent Leaf Area Affected at R5 (Early Dent Stage)	Approximate Yield Loss
5% or less	0 - 2%
6 - 25%	2 - 10%
25 - 75%	5 - 20%
75% - 100%	15 - 50%

Management Considerations

Cultural Practices

Cercospora zeae-maydis overwinters in corn debris, so production practices such as tillage and crop rotation that reduce the amount corn residue on the surface will decrease the amount of primary inoculum.
Crop rotation away from corn can reduce disease pressure, but multiple years may be necessary in no-till scenarios.

Hybrid Resistance

Planting hybrids with a high level of genetic resistance can help reduce the risk of yield loss due to gray leaf spot infection.
Pioneer^® brand corn products and parent lines are improved through a screening process in areas with a high incidence of gray leaf spot and specialized “disease nurseries.”
Pioneer brand corn products are rated for their genetic resistance to gray leaf spot on a 1 to 9 scale, with most current products rated between 4 and 7.
Susceptible hybrids are more likely to benefit from a foliar fungicide application, but resistant varieties may benefit as well under high gray leaf spot pressure (Figure 1).

Figure 1. Average yield increase of hybrids with different levels of resistance to GLS due to a foliar fungicide application in a three-year research study with very high GLS pressure (Jeschke and Luce, 2009). View a larger image.

Fungicides

During the growing season, foliar fungicides can be used to manage gray leaf spot outbreaks (Table 2).

Farmers must consider the cost of the application and market value of their corn before determining if a fungicide is likely to be an economical solution to gray leaf spot.

Table 2. Fungicide products rated very good to excellent for control of gray leaf spot. (Wise, 2025).

Trade Name	Active Ingredients	GLS Rating
Adastrio® 4.0 SC	Flutriafol + Azoxystrobin + Fluindapyr	VG-E
Aproach® Prima 2.34 SC	Cyproconazole + Picoxystrobin	E
Delaro® 325 SC	Prothioconazole + Trifloxystrobin	E
Delaro® Complete 3.83 SC	Prothioconazole + Fluopyram + Trifloxystrobin	E
Fortix® 3.22 SC Preemptor™ 3.22 SC	Flutriafol + Fluoxastrobin	E
Headline AMP® 1.68 SC	Pyraclostrobin + Metconazole	E
Lucento® 4.17 SC	Flutriafol + Bixafen	VG-E
Miravis® Neo 2.5 SE	Pydiflumetofen + Azoxystrobin + Propiconazole	E
Priaxor® 4.17 SC	Pyraclostrobin + Fluxapyroxad	VG
Revytek® 4.44 SC	Mefentrifluconazole + Fluxapyroxad + Pyraclostrobin	VG-E
Stratego® YLD 4.18 SC	Trifloxystrobin + Prothioconazole	E
Topguard® EQ 4.29 SC	Flutriafol + Azoxystrobin	VG
Trivapro® 2.21 SE	Benzovindiflupyr + Azoxystrobin + Propiconazole	E
Veltyma® 3.34 SC	Mefentrifluconazole + Pyraclostrobin	VG-E

References

Jeschke, M. and G. Luce. 2009. Foliar Fungicides for Gray Leaf Spot Management in Corn. Pioneer Crop Insights Vol. 19 No. 9. Corteva Agriscience.
Lipps, P. 1998. Gray leaf spot and yield losses in corn. Crop Observation and Recommendation network. Issue 98-23.
Wise, K. 2025. Fungicide Efficacy for Control of Corn Foliar Diseases. Crop Protection Network CPN-2011-W.

Stop the Clock on Crop Disease

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The foregoing is provided for informational use only. Please contact your Pioneer sales professional for information and suggestions specific to your operation. Product performance is variable and depends on many factors such as moisture and heat stress, soil type, management practices and environmental stress as well as disease and pest pressures. Individual results may vary. Pioneer^® brand products are provided subject to the terms and conditions of purchase which are part of the labeling and purchase documents.