http://www.pestproducts.com/chinchbugs.htm

We have a lot of St. Augustine in Texas and a common problem here is "brown patch" which appears as round shapes rather than irregular patches. It is a fungus that is carried by foot traffic such as the postman or dogs. It starts as a small round patch and grows larger in diameter. The grass is dead in the center and cannot be revived but, once treated, can serve as cover for new growth. It can be controlled with chemicals from your garden supply store.

Regards