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 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.
| Fungicide | Examples of Brands |
|---|---|
| Azoxystrobin* | Maxide Professional Grade Dual Action Disease Killer Granules |
|
Thiophanate methyl* |
Green Light Systemic Fungicide Ferti-lome Halt Systemic Fungicide Scott's Lawn Fungus Control |
| Propiconazole | Ferti-lome Liquid Systemic Fungicide Ortho Lawn Disease Control |
| Triadimefon | Bayer Advanced Fungus Control for Lawns Green Light Fung-Away Systemic Lawn Fungicide Spray RTS Bonide Fung-onil Lawn Disease Control Hi-Yield Lawn Fungicide Granules with Bayleton |
| *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. For example, use azoxystrobin, then use either thiophanate methyl, propiconazole or triadimefon. Or use thiophanate methyl and alternate with propiconazole or triadimefon. | |
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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.