Brown Patches in St. Augustine Grass?

My back yard started forming brown patches recently and I can't figure out what is causing it. Some of the spots look like someone poured gas on them. The spots are very defined, dark brown, totally dead and dryer than a popcorn fart. Other areas are just slightly brown and/or the blades of grass are turning yellow. I dug up a few spots looking for grub worms but didn't find any. I then sprayed some fungus stuff on the yard from Lowes last month and it hasn't helped. Can anybody recommend a product that they have had positive results from? Please hurry because it's spreading to my front yard!