Gray leaf spot is a common fungal disease of St. Augustine grass. Recent wet conditions have promoted this disease. This includes high humidity, heavy dews, and particularly frequent afternoon and evening rains.
The individual spots of 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 may be found on an individual leaf. Heavily infected leaves begin to turn brown and wither, usually beginning at the tip of the blade.
Gray leaf spot can move rapidly during prolonged warm, wet periods. 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.” Select fertilizers that are low in nitrogen or that have slow release nitrogen. Be careful to not overdo it in fertilizing your lawn and do not apply a high nitrogen fertilizer on top of an already infected lawn. Also, some lawn weed killers such as atrazine will increase the susceptibility of the grass to gray leaf spot.
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. With the frequent rains, very little to no supplemental irrigation has been need in most lawns in our area. Avoid leaving your irrigation timer on when adequate rainfall has occurred. It’s best to operate your irrigation controller on manual so that you can monitor the lawn and provide irrigation during dry times.
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.
For more information on maintaining a Florida lawn, contact your County UF/IFAS Extension Office or visit http://hort.ufl.edu/yourfloridalawn.