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Scouting for Turfgrass Diseases….is That Time of the Year

1. Time to scout for 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.

Disease Management:

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 2021 Georgia Pest Management Handbook at https://extension.uga.edu/programs-services/integrated-pest-management/publications/handbooks.html  or UGA Turfgrass Pest Control Recommendations for Professionals at https://turf.caes.uga.edu/publications/pest-control-recomendations.html

For more information or photos of gray leaf spot https://extension.uga.edu/publications/detail.html?number=C1116

Gray leaf spot on St. Augustinegrass (photos Alfredo Martinez)

2. Time to scout for Brown patch (caused by Rhizoctonia solani) and Pythium blight (caused by Pythium spp).

These diseases are often the most serious diseases on cool season grasses, especially on tall fescue, ryegrass and Kentucky bluegrass.

Pythium blight: has the potential to quickly cause significant damage to turfgrass. The disease starts as small spots, which initially appear dark and water-soaked. Affected turfgrass dies rapidly, collapses, and appears oily and matted. White, cottony mycelia may be evident early in the morning.  The disease is driven by hot-wet weather, which correlates with an increased stress on the turf. Similar environmental and cultural factors that encourage brown patch also promote Pythium. Therefore, cultural practices for control of brown patch will also help to minimize Pythium blight development. A correct diagnosis is important because Pythium control requires specific fungicides.

Several fungicides are available for Pythium management including fungicides used contain strobilurins, phosphonates, cyazofamid, propamocarb, fluopicolide + propamocarb, mefenoxam among others. Consult the 2021 Georgia Pest Management Handbook or the 2021 Turfgrass Pest Control Recommendations for Professionals for proper fungicide selection and usage. Read the label and follow proper guidelines

Pythium blight on tall fescue and Kentucky bluegrass (photos L. Miller)

Brown patch can cause a foliar blight, which results in necrotic leaves and circular brown patches up to 4-5 ft in diameter. High soil and leaf canopy humidity, and high temperatures increase disease severity. Higher than recommended rates of nitrogen in the spring promotes disease. Management options includes avoid nitrogen application when the disease is active, avoid infrequent irrigation and allow the foliage to dry, mow when grass is dry, ensure proper soil pH, thatch reduction, and improve soil drainage. An array of fungicides are available for brown patch. Including fungicides in the chemical families of strobilurins, benzimidazoles, demethylation inhibitors (DMI), Succinate Dehydrogenase Inhibitors, carboxamides, phenylpyrrole, nitriles among others. Consult the 2021 Georgia Pest Management Handbook or the 2021 Turfgrass Pest Control Recommendations for Professionals for proper fungicide selection and usage. Read the label and follow proper guidelines

Brown patch on tall fescue (Photos Alfredo Martinez)

For more information on Brown patch and Pythium visit

https://extension.uga.edu/publications/detail.html?number=B1233#BrownPatch
https://extension.uga.edu/publications/detail.html?number=B1233#PythiumRootRot

3. Centipede green-up problems? Several centipede samples have been submitted to the UGA Plant Disease Clinic recently (spring 2021). Check this publication out https://extension.uga.edu/publications/detail.html?number=C1003 for more information.