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 purple 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 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.
- 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 post-emergent weed killers on the lawn while the disease is active.
- Avoid medium to high nitrogen fertilizer levels.
- Improve air circulation and light level 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. Augustinegrass 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.
|Examples of Brands|
|Azoxystrobin1||Good||Heritage G (granules, 0.31%)
Scott’s Disease EX (0.31%)
Headway G (Granules, 0.31% with propiconazole, 0.75%)
|Pyraclostrobin||Good||Pillar G Intrinsic Fungicide (Granules, 0.38% with triticonazole, 0.43%)|
|Fluoxastrobin1||Excellent||Disarm 0.25G (Granules, 0.25%)|
|Thiophanate methyl1||Good||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%)
|Propiconazole||Fair||Bayer Advanced Fungus Control for Lawns 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
Martin’s Systemic Fungicide RTS2 (1.55%)
Bayer Advanced Fungus Control for Lawns Ready to Spread (Granules, 0.51%)
Spectracide Immunox Fungus Plus Insect Control for Lawns (1.45%) RTS2
|Myclobutanil||Fair||Spectracide Immunox Multi-Purpose Fungicide Spray Concentrate (1.55%; apply with hose-end sprayer)
Spectracide Immunox Fungus & Disease Control for Lawns RTS2 (1.5 %)
Monterey Lawn Fungicide RTS2 (2.0%)
Ferti-lome F-Stop Granules (0.39%)
Lebanon Eagle 0.62G Specialty Fungicide (Granules, 0.62%)
Lesco Eagle 0.39% Granular Turf Fungicide (Granules, 0.39%)
Ferti-lome F-Stop Lawn & Garden Fungicide RTS2 (1.00)
|Triadimefon||Fair||Lebanon Turf Fungicide Bayleton 1G (Granules, 1%)
Anderson Professional Turf Products 1% Turf Fungicide with Bayleton
|1 Resistance to the fungicide by the gray leaf spot fungus will develop from continued exclusive use of either azoxystrobin, thiophanate methyl, or fluoxastrobin alone. Always alternate any of these fungicides with one of the others. Alternatively, choose products, such as Headway G or Pillar G, which contain 2 active ingredients. Follow directions on product label for use. In general, azoxystrobin, pyraclostrobin, or fluoxastrobin 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 2018 Pest Control Guidelines for Professional Turfgrass Managers for additional recommendations.