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Gray Leaf Spot


Gray Leaf Spot

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Symptoms of Gray Leaf Spot

Early Lesions

  • Early lesions of gray leaf spot are small, necrotic spots and may have a chlorotic halo (more visible when leaf is backlit)
  • Initially lesions may be tan or brown before fungal sporulation begins
    Mixture of gray leaf spot lesion types.
  • Early lesions may be difficult to distinguish from other fungal leaf diseases
  • Lesions usually appear first on lower leaves before silking
  • Lesions progress from lower to upper leaves

Fully Developed Lesions

  • Typical lesions of gray leaf spot are rectangular with straight edges
  • Lesion expansion is limited by parallel leaf veins
  • Lesion appearance may differ somewhat on different genetic backgrounds
  • Grayish color of lesions is due to sporulation of the pathogen
  • This symptom is usually diagnostic
  • With favorable weather, the lesions rapidly merge, killing entire leaves.
    Fully developed gray leaf spot lesions in corn.

Gray Leaf Spot Occurrence

  • Fungal disease caused by Cercospora zeae-maydis pathogen
  • Primary leaf disease of corn in the US
  • Disease of high residue farming - pathogen builds up in corn residue over time
  • Favored by warm temperatures and high humidity
  • Extended periods of leaf wetness (~13 hours) allow infection of leaves
    • Fog each morning for a week or more may initiate a severe epidemic
    • Disease may be first noticed on lower leaves before silking
  • Disease often spreads rapidly with favorable weather during late summer and early fall (during the grain fill period of corn development)

Disease Cycle

Gray leaf spot disease cycle.

Gray Leaf Spot Impact on Crop

  • Depends on leaf area loss relative to grain fill
    • If grain fill outpaces disease progression, yield loss may be minor
    • If significant leaf area is lost before grain fill is complete, yield loss will be greater
  • Stalk rots may increase
    • Loss of functional leaf area during grain fill may lead to stalk rots as carbohydrates are taken from stalk to fill kernels
    • Stalk lodging may increase harvest losses or slow harvest progress

Management of Gray Leaf Spot

GLS-resistant hybrids

  • Hybrids with genetic resistance can reduce yield loss due to GLS
  • Over the past 25 years, DuPont Pioneer researchers have continually improved parent lines and hybrids for resistance to GLS
  • Hybrids and parent lines are rated and screened in "disease nurseries" as well as locations with high levels of natural GLS occurrence
  • Disease resistance scores are provided to customers for all Pioneer® brand corn products to help them select appropriate products for their fields
  • Corn products are available with scores as high as “7” for GLS resistance (1 to 9 scale where 9 is most resistant)
  • No hybrid is completely immune to the disease - even the most resistant hybrids can suffer yield loss under high GLS pressure
  • Susceptible hybrids are more likely to benefit from a foliar fungicide application
    Average yield increase of hybrids susceptible, moderately resistant, and resistant to GLS due to foliar fungicide application.

    Figure 1. Average yield increase of hybrids susceptible, moderately resistant,
    and resistant to GLS due to foliar fungicide application in a 3-year
    University of Tennessee/DuPont Pioneer research study with very high GLS pressure.

Crop rotation

  • May reduce disease pressure, but rotation is only a partial solution
  • Compared to other residue-borne pathogens, gray leaf spot has longer survival time in debris

Tillage to encourage breakdown of crop residue

Fungicide application

  • Foliar fungicide treatment is the only management option available following planting
    Average yield response to foliar fungicide application as influenced by tillage and previous crop in on-farm trials.

    Figure 2. Average yield response to foliar fungicide application as influenced
    by tillage and previous crop in on-farm trials (374 trials, 2007 to 2014.)
    n = number of locations, * = insufficient data

    Table 1. Corn foliar fungicide efficacy against GLS
    (Wise, 2015).

    Fungicide Active
    Aproach® Prima
    picoxystrobin +
    Headline® pyraclostrobin excellent
    Headline® AMP pyraclostrobin +
    Priaxor® pyraclostrobin +
    very good
    Quilt® Xcel propiconazole &
    Stratego® YLD prothioconazole &

    Wise, K. 2015. Fungicide efficacy for control of corn diseases.
    Purdue Extension Publication BP-160-W. Purdue University,
    West Lafayette, Indiana.


Pioneer® brand products are provided subject to the terms and conditions of purchase which are part of the labeling and purchase documents. Product responses are variable and subject to a variety of environmental, disease and pest pressures. Individual results may vary. Fungicide performance is variable and subject to a variety of environmental and disease pressures. Individual results may vary. Always read and follow all label directions and precautions for use when applying fungicides. Labels contain important precautions, directions for use and product warranty and liability limitations that must be read before using the product. Mention of a product does not imply a recommendation.

The foregoing is provided for informational use only. Please contact your authorized 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.