Looks like summer patch or brown patch:
http://www.ampacseed.com/disease1.htm
You can buy a fungal treatment in liquid form. I would go to a place like Lesco and see if they have a small quantity. I think a quart of the concentrate stuff is around $300 or something and if you don't treat lawns then that's a lot of money on YOUR lawn!
Or go to a box store like Home Cheapo and get it there. This time of year try to only water in the mornings as watering during the heat of the day can cause it to get worse. Do the treatments on the lighter side when you mix the chemical. It’s better to do it several times than to burn more of your lawn once.