Gray leaf spot is a fungus most often observed from late spring to early fall. Prolonged periods of rain, excessive applications of quick-release nitrogen fertilizers and applications of the herbicide atrazine increases the susceptibility of st. augustine grass to this disease.
Symptoms of gray leaf spot are small pinhead-sized spots that are olive green to brown in color. These enlarge and form circular to oblong spots. THe spots are tan to brown in color with distinctive dark brown outlines. In the rainy season, during periods of high humidity, the fungus will produce spores in the center of these spots giving them a gray appearance.
To reduce the risk of developing gray leaf spot use slow release nitrogen sources rather than quick release. Limit the use of atrazine herbicide applications to areas of weed infestations rather than the entire yard. Proper maintenance of St Augustine Grass is also an important factor in preventing disease. As you can see from this photograph the blades are ragged. This indicates the st augustine grass blades have been ripped versus cut making it further susceptible to gray leaf spot fungus. Be sure your irrigation clock is set to run at 4 am and not in the afternoons. The combination of heat and water during summer months will accelerate gray leaf spot fungus growth.
There are several fungicides available for homeowner use. Propiconazole and Strobilurin (QoI) fungicides. Fungicides such as Insignia and Heritage are among the most effective available and both treat a wide range of disease. Please be sure to read all manufacturers’ instructions when applying any chemical to your sod grass yourself and when in doubt hire a professional spray company in the Jacksonville and St Augustine areas to diagnose and treat your lawn.