Warm weather St. Augustine grass is planted throughout the southern United states as a lawn grass, primarily in humid regions along the Gulf Coast. It is preferred because of its weed resistance and easy maintenance, but it is prone to several diseases. These are mainly fungal, though viruses also attack St. Augustine. Most diseases are preventable, or can be treated if an infection is caught soon enough.
A disease that attacks mainly warm season grasses like St. Augustine, brown patch is most common in the spring and fall. Grass begins to brown and wither. Often the disease affects grass in a circular pattern ranging from several inches in diameter to several feet. The grass may recover and green back up in the center of the effected area but the edges normally remain brown. Prevention requires low-nitrogen fertilizers in the fall and avoiding excessive watering. Fungicide treatments have limited success.
Gray Leaf Spot
Gray leaf spot attacks St. Augustine in early summer. The blades develop light brown or gray spots with purple edges. Infected grass becomes thin and looks burned as the blades wither and brown. The gray spots are actually lesions caused by the gray leaf fungus and it is prone in shaded, high moisture areas of your lawn. Prevent the disease by watering early in the morning and only as needed. Fungicides formulated for leaf spot infections are successful in stopping existing problems.
St. Augustine Decline (SAD)
The SAD virus is prevalent in the Southern United States, primarily in Texas, Louisiana, Mississippi and Arkansas. The virus attacks the blades, causing them to yellow and die. SAD infections can kill off a St. Augustine lawn within three years. The virus is spread via lawn equipment such as lawn mowers. The only known prevention is to plant Raleigh, a SAD resistant St. Augustine variety.
Take-all Root Rot
Take-all root rot causes formally healthy grass roots to blacken and die, choking off the nutrient supply to the blades which then wither. The fungus may be present in the soil for a long time before it kills the grass, as it cannot not cause major damage until the grass is put under stress. Too much water, lime and herbicides lead to most infections. Fungicides formulated for take-all successfully stop the infection and must be applied in the spring or fall when the temperatures are below 80 degrees Fahrenheit.