Diseases are common in hot humid climates......is it St Augustine ?

Assuming you have St Augustine, like 99% of the state, you should be doing the following in order of importance:

1. Watering deeply, 1 inch all at one time, and infrequently, once per week this time of year. When the temps fall into the 80s change to once every 2 weeks but keep the 1 inch. With temps in the 70s, change the watering frequency to once every 3 weeks. Measure one inch by setting out several tuna or cat food cans and time how long it takes your sprinklers to fill all the cans. That is your permanent watering time. Remember it. Mine is 8 hours with an oscillating sprinkler on full sweep. Yours will probably be less.

2. Mowing at the mower's highest setting. There is never a good reason to lower the mowing height of St Aug.

3. Fertilize on Memorial Day, Labor Day (coming up), and Thanksgiving. In FL you might add a dose around Washington's Birthday. I would make that one organic.

4. In April and late September you can spot spray weeds with an atrazine product.

If you post a very close up picture of your lawn we can look for fungal lesions. If you are watering more than once a week that could be your problem. Take the picture at about 6 inches from the brown grass. It is best to take the picture on a cloudy/overcast day or in the shade.