North Okaloosa County’s recent wet conditions — including high humidity, heavy dews and frequent afternoon and evening rains — promote gray leaf spot, a common fungal disease of St. Augustine grass.
Individual spots or lesions first appear as brownish spots smaller than a pinhead. As they grow, they become circular and elongate lengthwise on the leaf. The spots turn brown to grayish in appearance. Numerous spots or lesions appear on an individual leaf. Heavily infected leaves begin to turn brown and wither, usually beginning at the blade’s tip.
Gray leaf spot can move rapidly during prolonged warm, wet periods. Overwatering or irrigating in the evening provides the prolonged period of wetness required for disease infection.
“Severity of the disease is enhanced by application of readily available nitrogen fertilizer and is proportional to the amount of nitrogen applied,” the Florida Lawn Handbook states.
Select fertilizers that are low in nitrogen or have slow-release nitrogen. Don’t overdo it in fertilizing your lawn, and do not apply a high-nitrogen fertilizer atop an already infected lawn. In addition, some lawn weed killers, such as atrazine, will increase grass’ susceptibility to gray leaf spot.
Irrigate during early morning hours to minimize the time the grass is wet. Water only as needed, and deeply.
Don’t irrigate when it is already wet from rain. With frequent rains, very little to no supplemental irrigation has been needed in most local lawns.
Avoid leaving your irrigation timer on after adequate rainfall. It’s best to operate your irrigation controller on manual so that you can monitor the lawn and provide irrigation during dry times.
You may need to use fungicide if the disease outbreak is severe. Look for products containing propiconazole, triadimefon or thiophanate-methy. Always follow the product label’s directions and precautions.
For more information on how to correctly water a Florida lawn, visit http://edis.ifas.ufl.edu/TOPIC_Lawn_Watering.
For more information on maintaining a Florida lawn, contact your county's UF/IFAS Extension Office or visit http://hort.ufl.edu/yourfloridalawn.
Larry Williams is an agent at the Okaloosa County Extension office in Crestview.