Our Extension office frequently receives questions about lawns, mostly about how to care for them and why do they look bad. Dead spots in lawns are caused by several different problems, which can range from insect pests to broken irrigation systems to the neighbor’s dog urinating on your lawn! For residents with St. Augustine turf, the most common problem we see is a fungal disease with the common name of take-all root rot.
Take-all root rot, also known as bermudagrass decline or Gaeumannomyces graminis var. graminis, is a fungal disease that is very common on all warm season grasses. High rainfall and stressed grass are the most common triggers for the fungus to spread and damage grass. Otherwise healthy lawns can have the disease present without showing symptoms, and the fungus stays in the soil, so it never totally goes away. Symptoms include black or missing roots on St. Augustine grass stolons (aboveground stems), yellow or tan dead patches and eventually areas of completely dead turf. These symptoms closely resemble the type of damage that chinch bugs can cause in a lawn, which means that samples must be examined under a microscope to determine what the problem is. Blaming every dead spot in a St. Augustine lawn on chinch bugs commonly leads to a misdiagnosis and even worse lawn damage.
If your lawn does have a problem with take-all root rot what can you do? The key is to reduce the stress on your lawn. We cannot control how much it rains, but if there is adequate rainfall, especially during the summer months, you should turn your irrigation off. Be careful to not over fertilize your lawn- too much nitrogen can trigger the take-all fungus to really take off. And the most important point of all is to cut your lawn high! This means 3.5 to 4 inches high. If you have a service, you may need to walk outside and physically measure the height of the cut grass blades above ground level. Most samples of turf we examine for this disease are being cut at about 2.5 inches- this can kill a healthy St. Augustine lawn within a year or so all on its own. Chemical fungicides can help, but the best way to live with take-all is to care for your lawn properly and avoid putting any extra stress on it.
Watch a short video from our office lab that shows what we look for when diagnosing this disease: