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Avoidance and Management Tips for Summer Turfgrass Diseases

Gray leaf spot on St. Augustinegrass

Gray leaf spot is active and causing epidemics St. Augustinegrass. The fungus Pyricularia grisea also affects perennial ryegrass and tall fescue in Georgia. However, the disease is particularly aggressive in St Augustinegrass. Hot humid summer weather and high nitrogen levels can make turf susceptible to this disease. 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. In cool-season turfgrass, the symptoms are similar to those of melting out. Gray leaf spot is favored by daytime temperatures between 80°F to 90°F and night temperatures above 65°F. It is also found in areas with high nitrogen levels and that are stressed by various factors, including drought and soil compaction. This disease is most severe during extended hot, rainy and humid periods.

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. Irrigating in the late afternoon or evening should be avoided, as this prolongs periods of leaf wetness. 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.

St. Augustinegrass is especially sensitive to some herbicides. If possible, manage weeds using cultural management techniques and minimal amounts of herbicides. The timing of any atrazine application should be chosen carefully, as this herbicide can stress the grass, especially when temperatures may climb above 85 degrees F. Atrazine applications made before or during disease-favorable conditions increase the likelihood of severe gray leaf spot symptom development. Spot-treating trouble areas with the herbicide may also be considered. Herbicides should always be applied according to the label instruction Several fungicides are available for each of the diseases described above. Consult the Georgia Pest Management Handbook https://ent.uga.edu/extension/pest-management-handbook.html  or the Turfgrass Pest Control Recommendations for Professionals (www.georgiaturf.com) for proper fungicide selection and usage. Read the label and follow proper guidelines.

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

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

Spores of Pyricularia grisea (photo by Alfredo Martinez)

Brown patch (Rhizoctonia solani) and Pythium blight (Pythium spp) on Tall Fescue.

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. Symptoms are more severe when night time temperatures are above 62-65°F that coincides with 10 h of leaf wetness. 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.

Brown patch on tall fescue (Photos Alfredo Martinez)

Mycelium of Rhizoctonia solani (Photo Alfredo Martinez)

Several fungicides are available for each of the diseases described above. Consult the Georgia Pest Management Handbook https://ent.uga.edu/extension/pest-management-handbook.html  or the Turfgrass Pest Control Recommendations for Professionals (www.georgiaturf.com) for proper fungicide selection and usage. Read the label and follow proper guidelines.

For more information on Brown patch and Pythium visit https://extension.uga.edu/publications/detail.html?number=B1233#BrownPatch

Pythium blight has the potential to quickly cause significant damage to tall fescue. 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 each of the diseases described above. Consult the Georgia Pest Management Handbook https://ent.uga.edu/extension/pest-management-handbook.html  or the Turfgrass Pest Control Recommendations for Professionals (www.georgiaturf.com) for proper fungicide selection and usage. Read the label and follow proper guidelines.

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

Pythium blight on tall fescue (Photo Alfredo Martinez)

Oospores and Mycelium of Pythium (Photo Alfredo Martinez)