URBAN AG
It’s that time of the year... Scouting for turfgrass diseases
by Alfredo Martinez, Professor of Plant Pathology, UGA College of Agricultural and Environmental Sciences Disease management
Gray leaf spot on St. Augustinegrass
Alfredo Martinez
Gray leaf spot
Gray leaf spot is a fungus disease that affects St. Augustinegrass, perennial ryegrass and tall fescue in Georgia. Hot humid summer weather and high nitrogen levels can make turf susceptible to this disease. The fungus causing the disease is Pyricularia oryzae (Magnaporthe oryzae). Symptoms: The symptoms of gray leaf spot vary depending on the grass cultivar. On St. Augustinegrass, gray leaf spot first appears as small, brown spots on the leaves and stems. The spots quickly enlarge to approximately ¼ inch in length and become bluish-gray and oval or elongated in shape. The mature lesions are tan to gray and have depressed centers with irregular margins that are purple to brown. A yellow border on the lesions can also occur. Conditions favoring disease: Gray leaf spot is favored by daytime temperatures between 80 to 90 degrees Fahrenheit and nighttime temperatures above 65 degrees F. High nitrogen levels, high moisture, and conditions that stress the turf, such as intermittent drought and soil compaction, also favor the disease. Gray leaf spot is most severe during extended hot, rainy, and humid periods. Shady areas with limited airflow tend to have more severe outbreaks of gray leaf spot.
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Cultural: Management practices that minimize stress and avoid rapid flushes of lush growth during the rainy season lessen the likelihood that severe gray leaf spot symptoms will develop. If irrigation is used to supplement inadequate rainfall, water infrequently but deeply. Proper irrigation regimens should protect against symptoms of drought stress without increasing disease pressure by extending periods of leaf wetness. Excessive soil moisture and leaf wetness promotes gray leaf spot. Proper mowing practices are most important for gray leaf spot management in St. Augustinegrass. This grass must be mowed frequently during the summer months to remove excess leaf tissue and keep the canopy open and dry. Mow the turf at the correct height for the designated turfgrass species and remove only one-third of the leaf blade per mowing. Collecting clippings reduces the spread of the disease when gray leaf spot symptoms are evident. Thatch layers should be removed if they are greater than 1 inch in depth. Vertical mowing is best to remove thatch. Topdressing with sand or soil is less effective than vertical mowing, but it helps with thatch removal. Controlling thatch can improve drainage, reduce drought and nutrient stress, and remove sources of gray leaf spot inoculum. St. Augustinegrass is especially sensitive to some herbicides. Chemical: Fungicides are more efficacious when applied as preventative treatments rather than after symptoms are evident. Fungicides in the chemical families of strobilurins, benzimidazoles, demethylation inhibitors (DMI), carboxamides, and nitriles are labeled to control gray leaf spot. These materials should be applied with at least 2 gallons of water per 1000 sq. ft. for a complete coverage of the turfgrass canopy. For a complete and updated list of available fungicides, refer to the commercial edition of the 2022 Georgia Pest Management Handbook at https://extension.uga. edu/programs-services/integrated-pest-management/ publications/handbooks.html or the 2022 UGA Turfgrass Pest Control Recommendations for Professionals at https://turf.caes.uga.edu/publications/pest-controlrecomendations.html For more information or photos of gray leaf spot: https://secure.caes.uga.edu/extension/publications/files/ pdf/C%201116_2.PDF
