Gray leaf spot is a common fungal disease of St. Augustine grass. Wet conditions promote this fungus. This includes high humidity, heavy dews and particularly frequent afternoon and evening rains. It’s primarily a disease of St. Augustine grass. The Florida Lawn Handbook States, “St. Augustine grass is the only important warm-season turf grass that is seriously affect.” I’ve seen St. Augustine grass and centipede grass growing intermingled with the St. Augustine infected with gray leaf spot and the centipede not infected.
The individual spots or lesions are first seen as tiny brownish spots smaller than a pinhead. As they become larger, they will be circular and then begin to elongate lengthwise along the leaf. The spots will become brown to grayish in appearance. Numerous spots or lesions might be found on an individual leaf. Heavily infected leaves begin to turn brown and wither, usually beginning at the tip of the blade.
This disease can move rapidly during prolonged warm, wet periods. Some people add to the problem by irrigating too frequently or they routinely water their lawn late in the day. Over watering or irrigating in the evening provides the prolonged period of wetness required for disease infection. For more information on how to correctly water a Florida lawn, visit http://edis.ifas.ufl.edu/TOPIC_Lawn_Watering .
According to the Florida Lawn Handbook, “Severity of the disease is enhanced by application of readily available nitrogen fertilizer and is proportional to the amount of nitrogen applied.” So be careful to not overdo it in fertilizing your lawn and certainly do not apply a high nitrogen fertilizer on top of an already infected lawn. It will be somewhat like adding gasoline to a fire, the disease will “explode” and rage out of control. Also, some lawn weed killers such as atrazine will increase the susceptibility of the grass to gray leaf spot.
The best way to manage this disease is to select fertilizers that are low in nitrogen or that have slow release nitrogen. Irrigate during early morning hours to minimize the period in which the grass is wet and water only on an as needed basis, watering less often but deeply. Avoid frequent, shallow watering and don’t irrigate when it is already wet from rain.
If the disease outbreak is severe, you may need to use a fungicide. Look for products containing propiconazole, triadimefon or thiophanate-methy. Always follow the product’s label directions and precautions.