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Gray Leaf Spot on St. Augustinegrass

Richard L. Duble, Turfgrass Specialist
Texas Cooperative Extension
Text and images copyright © Richard Duble.

Gray leaf spot, caused by the fungal organism Piricularia grisea, develops rapidly with abundant moisture and warm weather in the spring and early summer on St. Augustinegrass lawns. The disease is especially troublesome in shaded areas.

Symptoms. Gray leaf spot causes oval or circular, tan colored lesions with brown or purplish borders on the leaf blades of St. Augustinegrass. In severe cases of gray leaf spot, lesions develop on leaf sheaths and stems and the leaves wither and die. Under these conditions the disease causes serious thinning of St. Augustine turf. Unless the disease is controlled, a stand of St. Augustinegrass may be lost.

Control. To reduce the severity of gray leaf spot, avoid applications of soluble nitrogen fertilizers on moderately shaded lawns during summer months. Herbicide applications which may weaken St. Augustinegrass should also be avoided on shaded lawns. Apply water to the lawn in early morning only when water is needed. Avoid late afternoon and evening watering which keeps the leaf surface moist for long periods. Also, catch grass clippings in lawns where gray leaf spot is a problem.

Preventive fungicides for gray leaf spot include Daconil 2787 at 4 oz. per 1,000 sq. ft. at 7 to 10 day intervals and Acti-dione Thiram at 2 oz. per 1,000 sq. ft. at 7 to 10 day intervals. Curative rates of these fungicides are 6 to 8 oz. of Daconil and 4 oz. of Acti-dioneThiram at 7 to 10 day intervals.