Gray leaf spot, caused by the fungus Pyricularia grisea. The disease is of particular significance on the warm season St. Augustine grass and has been common here in the south since first being reported in 1971.
Infections and subsequent symptomatic tissue can appear quite quickly. Damage is usually noticed during the warmer months of August and September. Conditions favoring infection include hot days over 80°F, nights with prolonged cloud cover when humidity is high, and prolonged leaf wetness. Infected leaves may have water soaked lesions and appear chlorotic. The youngest leaves often take on a characteristic fishhook shape. The disease is most severe on young seedlings. Gray to brown lesions range in size from 2-5 cm. At times, a yellow margin may surround the lesion and the leaf blades may have dark brown borders. The large amount of spores produced by the fungus creates a "felted" look to the leaf blades.