Gray Leaf Spot

In-Depth Downloads

Symptoms of Gray Leaf Spot

Early Lesions (image below)

  • 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
  • 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.

Typical fully developed gray leaf spot lesions

Facts on Gray Leaf Spot

  • 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
  • Gray leaf spot (GLS) often spreads rapidly with favorable weather during late summer and early fall (during the grain fill period of corn development)

Disease Cycle

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

Misture of GLS lesion types in same field

Management of Gray Leaf Spot

  • GLS-resistant hybrids from Pioneer
  • Hybrids with resistance can reduce yield loss due to GLS
  • Over the past 25 years, 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
  • Scores are provided to customers on all Pioneer® brand hybrids to help them select appropriate hybrids for their fields
  • Hybrids are available with scores as high as "7" for GLS resistance (on Pioneer's 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
  • 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. Common fungicides include Headline, Quadris, Quilt, Propimax EC, and Stratego
  • Read and follow all fungicide label instructions.

Response to Foliar Fungicide Application

Average yield response of Pioneer® brand hybrids to foliar fungicide
application according to GLS disease rating (300 trials, 1999-2008).

All products are trademarks of their manufacturers.