I'm guessing that you are watering too frequently, possibly daily, which has led to the lawn developing a fungal disease. Notice that there is a distinct arc to the brown blades from the bottom right of the picture to the middle left...that is another clue that it's a fungal disease and not caterpillars (worms). Quite often a fungal disease will form a near perfect circle.

Can you post another picture? Get about 3 inches away from some of the grass at the edge of the brown area. The best pictures are taken on a cloudy day or in the shade to reduce the contrast. Fungal diseases show up on the blades of grass.

If it is a fungal disease, my recommendation would be to apply ordinary corn meal at a rate of 20 pounds per 1,000 square feet. Corn meal attracts a predatory fungus which feeds on the disease fungus. It goes to work immediately but it takes about 3 weeks to see the improvement as the green grass returns. I have used corn meal successfully every year since 2002 to eradicate disease in my St Aug. Although, now that I have Floratam in the back yard, the grass fights off the disease faster than it can spread. I still see signs of disease...and then it's gone. If you don't have Floratam now, you might consider putting down one flat of it and letting it spread into the lawn. If you had Floratam in the lawn currently, it would be spreading into the thin (brown) areas and taking over even while the disease is present. Cool stuff.

Oh and as to the watering, this time of year you should be watering no more frequently than once every 2 weeks in your area. If you get high temps in the 70s, then you could back off to once every 3 weeks. If you are not doing that, please write back and tell us what you are doing so we can get you backed off to watering less often.