Someone also recently posted regarding a possible fungus on their St. Augustine. Check out their pic (and posters' comments/advice) in case this is what you're experiencing. Good luck.

Generally you stop watering it frequently. Another cause of disease in St Augustine is covering it up with just about anything, letting it sit for a day or two, and then uncovering it. St Augustine needs air to breathe. Here is a pictures I took this weekend of St Augustine sitting on a pallet at a box store (which one doesn't matter, because they are all the same).

This piece of grass had been cut several days ago and sat under other pieces of sod until this happened. Corn meal is the solution for St Augustine. Apply at 20 pounds per 1,000 square feet and give it 3 full weeks to work. You will not see any improvement at 2 weeks. By 3 weeks you will have forgotten about it, but the grass will "suddenly" look very good.

Watering: Deep and infrequent is the mantra for watering. This is for all turf grass all over the place. Deep means 1 inch all at one time. Put some cat food or tuna cans around the yard, and time how long it takes your sprinkler(s) to fill all the cans. Memorize that time. That will be the time you water from now on. My hose, sprinkler and water pressure takes 8 full hours to fill the cans. Your time will likely be less. I like gentle watering. As for watering frequency, that depends on the daytime air temperature. With temps in the 90s, deep water once per week. With temps in the 80s, deep water once every 2 weeks. With temps in the 70s, deep water once every 3 weeks. With temps below 70, deep water once a month. Note that you have to keep up with quickly changing temps in the spring and fall. This deep and infrequent schedule works in Phoenix and in Vermont, so it should work for you. The reason for deep and infrequent is to grow deeper, more drought resistant roots and to allow the soil to dry completely at the surface for several days before watering again. If it rains, reset your calendar to account for the rainfall.