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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.
- Causal Pathogen
- Identification
- Crop Damage
- Management Considerations
- References
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.
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%
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 Forvico™ 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 SCFlutriafol + 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.
Dependable Disease Control
Disease moves fast to destroy crop yields. You need a fungicide that works just as fast.
Disease moves fast to destroy crop yields. You need a fungicide that works just as fast.
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Aproach® Prima is not registered for sale or use in all states. Contact your state pesticide regulatory agency to determine if a product is registered for sale or use in your state. Always read and follow label directions.
Not all products are registered for sale or use in all states. Contact your state pesticide regulatory agency to determine if a product is registered for sale or use in your state. Always read and follow label directions.
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.