Most of the diseases that attack St. Augustine grass are fungal diseases. These diseases occur because of too much moisture near or on the grass, over-fertilization and lack of essential nutrients. The only non-fungal disease that often attacks St. Augustine grass is the viral SAD (St. Augustine Decline), which is transmitted through infected equipment and air. Most of the fungal diseases that attack St. Augustine grass can be controlled with an application of a fungicide or by eliminating the causing factors, such as reducing the amount of fertilization.
Brown patch is a fungal disease that is caused by over-fertilization of the lawn during hot temperatures (above 80 degrees F in the day). Brown patch may be confused with SAD; however, the symptoms of brown patch are slightly different. The patches start small and gradually grow larger. The grass usually does not appear yellow. The patches may form circular rings around healthy green patches of grass.
The SAD (St. Augustine Decline) virus has several different symptoms. One of the main symptoms is large, yellowed circles spread across the lawn. Eventually, the grass circles will turn brown. The roots of the grass will turn brown and shrivel. Close inspection of the leaves will reveal brown tips and a mottled grass blade appearance with white spots.
Gray Leaf Spot
Leaf spot is a fungal disease that occurs due to high temperatures and moisture in the air. Gray leaf spot appears as small light gray or brown spots all over the leaves, as if the grass has chicken pox. Eventually, the spots will get bigger and may form a diamond shape. If left unchecked, the leaves will eventually split, turn yellow and die.
Leaf blight is a fungus that attacks St. Augustine grass. The symptoms include browning or graying leaf blades randomly scattered throughout otherwise healthy grass. Leaf blight typically occurs because of humid and hot weather and can sometimes occur because of over-fertilization.
Downy mildew is caused by cutting the grass when it is still wet or by keeping the grass wet for prolonged periods. The mildew appears as yellow clumps throughout the yard. The infected grass will be shorter than the surrounding healthy grass and will no longer grow.