Cool-weather fungal disease of St. Augustine. Brown leaves pull loose easily from the runners. Small spots in the lawn grow into large circles or free forms that look bad and weaken th turf but rarley kill the grass. Go organic to build the beneficial microbes in the soil. Spread a thin layer (1/2") of compost or for faster results corn meal at 20 lbs./1,000 square feet. Spray the problem area with Garrett Juice plus garlic tea. Potassium bicarbonate at 1 rounded tablespoon per gallon can be added for additional control.
Question: have crop circles in my St. Augustine. The grass in the area is "bladeless" though the "runners" are still mostly green and almost as abundant as those in non-affected areas. I am 100% organic with regular applications of whole ground corn meal, corn gluten, and lots of molasses (sandy soil). My last application of corn gluten and molasses was about three weeks ago. I have "treated" the crop circles to an additional heavy dose of whole ground cornmeal. C.P., Roanoke.
Answer: No more cornmeal. You may have used too too much already. Drench the area with garlic tea cut back on the irrigation.
See also: Dollar Spot Fungal Disease