St. Augustine (Stenotaphrum secundatum) is a warm season perennial grass that thrives in southern parts of the United States. It spreads easy and produces a green carpet of grass that requires little maintenance. The grass, however, is susceptible to a black fungus caused by different types of mold that appear after warm and rainy weather or excessive and consistent irrigation.
Black fungus on St. Augustine grass is caused by three fungal diseases called gray leaf spot, slime mold and brown patch disease.
Symptoms of gray leaf spot appear as small brown spots with purple margins that enlarge to become elongated or diamond-shaped. Sections on leaf blades may also turn yellow. In extreme cases, lesions on spikes and stems develop and the leaves die. Gray leaf spot also causes turf grass to appear scorched or burned and thin.
Slime mold appears on St. Augustine grass as tiny pustules that resemble oil or ash deposits. The pustules develop very quickly and are orange, yellow, gray white or purple. Infected blades appear yellow.
Brown Patch or Rhizoctonia blight starts as a tiny round spot that enlarges in size to become a 2-foot-wide horseshoe or circular patch. Sometimes the inner section of the circle recovers, leaving a brown outline that resembles a smoke ring.
These three diseases affecting St. Augustine grass are water-related. Overwatering exacerbates the problem. Late evening or night turf grass watering does not allow the moisture or droplets on blades to dry until the next morning when the sun comes out. Thatch on grass that prevents air circulation also makes the grass susceptible to disease, and using too much or too little nitrogen-rich fertilizer assists in fungal growth. Shaded sections of St. Augustine grass, areas with frequent water runoff, and low-lying spots are most vulnerable. Climate conditions favorable for the development of these diseases include consistent daytime temperatures between 70 to 85 degrees Fahrenheit, and nighttime temperatures below 68 degrees.
Proper irrigation practices prevent St. Augustine grass from becoming susceptible to the fungal diseases. Water the grass in the morning so the moisture dries up by evening. Remove nearby trees to reduce shade, and aerate often to remove impenetrable thatch. Follow a proper fertilization schedule to avoid feeding the grass excess nitrogen.
The East Texas Gardening website recommends fungicides to treat gray leaf spot include those that contain myclobutanil, azoxystrobin and thiophanate-methyl.
Although slime mold does not harm the grass, removing it becomes necessary because it looks unsightly. No fungicides are required. Rub the blades or direct the flow of water from a hose over infected areas to sweep the powder off.
Treat brown patch with fungicides containing thiophanate-methyl, quintozene, myclobutanil and mancozeb.