Gray leaf spot usually attacks St. Augustinegrass, but it can also attack centipede. It is a summer disease caused by a fungus and is common in long periods of hot, humid weather. Newly sprigged or rapidly growing grass is more susceptible than well-established grass. Too much nitrogen fertilizer can make the disease more severe.
The disease causes irregular gray, dirty-yellow, or ash-colored spots with brown, purple, or water-soaked borders on leaf blades. Spots may be covered with gray mold in warm, humid weather. Lesions may occur on stems, spikes, and leaves. A yellow halo or general chlorosis may occur around some spots.
The disease is usually noticed first in shaded damp areas. In heavy areas, the grass may have a burned appearance. Seldom will this disease kill an entire lawn.
Fungus spores are spread by the wind, rain, irrigation water, and animals.
Avoid excessive nitrogen (water-soluble) fertilization in summer. Water in the day so foliage will not stay wet overnight.
For more information on this turf disease please follow this link to University of Arkansas Turf Website.
http://turf.uark.edu/publications/factsheets/Gray%20leaf%20spot%20of%20St.%20Augustinegrass%20FSA-7566.pdf