Gray Leaf Spot of St. Augustinegrass

Joey Williamson, Ph.D.
Home & Garden Information Center

Gray leaf spot is a fungal disease (caused by Pyricularia grisea) that occurs most commonly on St. Augustinegrass (Stenotaphrum secundatum), but occasionally may also be found infecting tall fescue, bermudagrass, centipedegrass and ryegrass in home lawns. On St. Augustinegrass the initial leaf spots are small and brown, but they expand rapidly into large, oval or elongate, tan to gray leaf spots, which are surrounded by purplish or brown borders. A yellow halo may surround some of the spots, or a general leaf yellowing may occur. During warm, humid weather the fungus produces a grayish mycelium (cottony fungal growth) and numerous spores on dying foliage, which gives a grayish appearance to the spots. Blades of grass will wither and die if leaf spots are numerous. Under favorable conditions for disease development, large areas of turf may appear to be scorched, resembling extreme drought stress.

Gray leaf spot of St. Augustinegrass caused by the fungus, Pyricularia grisea.
Gray leaf spot of St. Augustinegrass caused by Pyricularia grisea.
Photo by Lane P. Tredway, University of Georgia, Athens, GA

Gray leaf spot is favored when temperatures are between 77 and 86 °F during the day and above 65 °F at night, and is most severe during rainy, humid periods. The fungus survives dry summer conditions as fungal mycelium and spores on dead foliage and in the thatch layer. When favorable conditions of moisture and high humidity return during the summer, the fungus produces an abundance of spores that are spread by wind, splashing rainfall or irrigation, and the movement of lawn equipment on wet grass.

Management Tips:

  • Reduce thatch layer.
  • Irrigate deeply, but infrequently. This generally means one time per week with one inch of water. Always irrigate in the morning, which promotes quick drying of the foliage.
  • Avoid using weed killers on the lawn while the disease is active.
  • Avoid medium to high nitrogen fertilizer levels.
  • Improve air circulation and light levels on lawn. Limb up over-hanging trees and prune back nearby shrubs.
  • Mow at the proper height and only mow when the grass is dry. Bag and dispose of grass clippings if disease is present.
  • Control chinch bug infestations.
  • Use fungicide treatments as needed along with proper turfgrass culture. See Table 1.

For more information on proper St. Augustine care, see HGIC 1218, St. Augustinegrass Maintenance Calendar, and on other turfgrass diseases, see HGIC 2152, Leaf Diseases of Lawns, and HGIC 2150, Brown Patch & Large Patch Diseases of Lawns.

Table 1. Fungicides to Control Gray Leaf Spot.
FungicideExamples of Brands
1 Resistance to the fungicide by the gray leaf spot fungus will develop from continued exclusive use of either azoxystrobin or thiophanate methyl. Always alternate either of these fungicides with one of the others. Follow directions on product label for use. In general, azoxystrobin applications will control gray leaf spot for 28 days. The other three fungicides will control the disease for 14 days. Irrigate according to label directions after application of granular products.
2 RTS = A ready-to-spray hose-end product.
Landscape professionals should consult the 2014 Pest Control Guidelines for Professional Turfgrass Managers for recommendations.
Azoxystrobin1 Heritage G (granules, 0.31%)
Thiophanate methyl1 Lesco T-Storm 2G Fungicide (granules, 2.08%)
Cleary’s 3336-G (granules, 2.08%)
Scott's Lawn Fungus Control (granules, 2.30%)
Bonide Infuse Systemic Disease Control–Lawn & Landscape Granules (2.08%)
Southern Ag Thiomyl Systemic Fungicide (wettable powder, 50%; apply with pump-up sprayer)
Propiconazole Bayer Advanced Fungus Control for Lawns (granules, 0.51%; RTS2 2.42%)
Ferti-lome Liquid Systemic Fungicide II Containing Banner RTS2 (1.55%)
Bonide Infuse Systemic Disease Control–Lawn & Landscape RTS2 (1.55%) (NOT the same active ingredient as the Bonide Infuse Granular product)
Monterey Fungi-Fighter Conc. (1.55%; apply with hose-end sprayer)
Triadimefon Lebanon Turf Fungicide Bayleton 1G (granules, 1%)
Anderson Professional Turf Products 1% Turf Fungicide with Bayleton

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This information is supplied with the understanding that no discrimination is intended and no endorsement by the Clemson University Cooperative Extension Service is implied. All recommendations are for South Carolina conditions and may not apply to other areas. Use pesticides only according to the directions on the label. All recommendations for pesticide use are for South Carolina only and were legal at the time of publication, but the status of registration and use patterns are subject to change by action of state and federal regulatory agencies. Follow all directions, precautions and restrictions that are listed.