Grey leaf spot of corn (GLS) is a foliar disease that can impact yield when conditions are favorable. GLS is found most commonly in the Mid-Atlantic and Eastern corn belt states where favorable environmental conditions, periods of high heat and humidity, occur annually. Despite the potential for disease to become severe and limit yields in North Carolina, damages rarely impact grain yield. Local impacts may occur when susceptible hybrids are planted in areas at higher risk to infections, including low-lying areas of fields and no-till/conservation tillage situations where humidity may be high and inoculum is present.

GLS is caused by the ascomycete fungus, Cercospora zeae-maydis. This pathogen is favored by warm, humid conditions that are common to the Southeast production region. The fungus survives on the soil surface on crop debris from previous growing seasons. The sexual stage of the fungus does not appear to be important to disease. The pathogen reproduces via asexual spores (conidia) that are released from lesions on leaves or crop debris from the previous season.

This disease is found on the leaves of the corn, beginning with the lower leaves. The lesions are characterized by a rectangular spot on the leaf, that is yellow or tan in color with darkened margins (Figure 1). As lesions mature, they elongate and become more grey in color as spores are produced. Lesions may become several inches long, but are typically limited within the leaf veins, which gives the characteristic rectangular shape. Hybrids vary on lesion coloration and severity of symptoms.

In early stages GLS can look like:

Bacterial leaf streak

• This pathogen is not currently found in North Carolina, but may be transported here on infected plant materials. This pathogen produces rectangular lesions with a distinct yellow halo around the lesion when held up to the sun. Lesions are also devoid of fungal spores. Should bacterial leaf streak be suspected, contact local extension for assistance in identification.

Anthracnose leaf blight

• Early lesions are water-soaked and oval, but expand to irregular shapes.

Eyespot

• Lesions are small round, with darkened margins and produce a characteristic halo around the lesion, but do not elongate to rectangular lesions like GLS.

Common rust

• Early rust lesions may produce small yellow lesions before pustules form, which may be confused with GLS.

Figure 1. Rectangular lesions caused by the fungus Cercospora zeae-maydis.

NC State Plant Pathology

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Figure 1. Rectangular lesions caused by the fungus Cercospora zeae-maydis.

NC State Plant Pathology

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The fungus that causes GLS survives on corn debris left in/on the soil surface. When warm temperatures (70-90°F) and high humidity (≥ 90%) occur, spores are produced in the crop residue. These spores are blown into the crop canopy when plants are small. Initial infections are produced when warm conditions warm temperatures (70-90°F), high humidity (≥ 90%), and leaf wetness for at least 12 hours allow for spore germination on the leaf surface. As disease progresses under warm, humid conditions, abundant sporulation occurs allowing spores to be blown to new fields. The disease epidemic continues while conditions are optimal for disease spread. At the end of the growing season, the fungus remains in the crop debris sitting on top of or in the soil until susceptible plants and conducive conditions are present.

Several hybrids are available with partial resistance to GLS. Crop rotation and crop debris destruction are important to reduce inoculum survival and limit severity of epidemics. A rotation of at least two years is recommended for fields utilizing conservation tillage practices. If planting susceptible hybrids, it is important to scout early and often to monitor disease progression, fungicide applications provide the most impact to grain yield when disease is present. Fungicide applications may be valuable to protect yield when applied between VT and R2. For fungicide efficacy ratings, see information on the NC State Corn extension portal.

• The NCSU Plant Disease and Insect Clinic provides diagnostics and control recommendations
• The NC State Extension Plant Pathology portal provides information on crop disease management
• The North Carolina Agricultural Chemicals Manual provides pesticide information for common diseases of North Carolina. The manual recommendations do not replace those described on the pesticide label, and the label must be followed.
• The Crop Protection Network factsheet for gray leaf spot provides other resources for ID and management.

This factsheet was prepared by the NCSU Field Crops and Tobacco Pathology Lab in 2020.

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