Your problem sounds similar to what I have in a small area. I, too, think it's fungal. I put down cornmeal over the whole lawn this weekend, and spread a light layer of compost over the affected areas. I'm keeping my fingers crossed...

Do you live in the area of SC. where mole crickets have taken up residence? Do another soap water test increasing the amount of soap by a bit. Pour it heavily from the center to the outside of one of the bad spots and after the angry worms have boiled out of the ground, turn your hose on gently, spray and water the soap suds watching carefully for mole crickets. They can be slow to emerge and disappear back underground quickly especially at this time of year when they are small. Hopefully mole crickets are not the problem.

Don't be afraid to use cornmeal as needed on and around brown patch areas of the lawn. Good idea to do a repeat treatment two weeks later. Remember to moisten it with water after spreading.

Belew,
You have described my lawn perfectly. I have been told it was grub worms. I recently purchased some beneficial nematodes and spread them around, then added a layer of very nice organic compost. I'm hoping for a better grass season in 2009.

You may want to dig about 1/2 foot in a bad area and see if you find any grubs. Also, if the grass appears to pull up easily, it's most likely grubs.

I had similar issues back when this thread was started. It apparently was a bug because it cleared up by itself. Without my doing anything the grass was back to normal by about Thanksgiving. For the OP it sounded to me like a fungus but bugs sometimes offer the same symptoms.

For bradley in Austin, there is one other thing that can cause yellowing. Excessive rain will wash the acidity out of the surface of our calcium rich soil. When that happens the iron that is already in the soil becomes unavailable to the plants until the acidity increases again. If your Austin turf is not dark green in April then something else is at work.

dchall,

If you could post a picture of this 'rain' your talking about, that may be helpful to me. Being in San Antonio, you should know it never rains in Austin. My grass was beautiful April through about August, then it started yellowing and coming up easily.

I had a tree planted and the guy dug about a 1-1/2 foot whole. He pulled out over 10 grubs just from that one spot so I have to lean towards a grub issue.

Bradley,
10 grubs in a 1.5 foot hole is not enough to get excited about. You'd need 20 grubs at least. Besides that the grubs are finished eating now. Let them rest and worry about it next summer.

Very simply, St.Augustine grass requires an inch of water per week to be healthy. An occasional rain does not get the job done. I have not seen any place in Texas, perhaps with the exception of Houston, that gets excessive rain especially enough to change the pH of the soil.

texasredhead, I don't know if this will help you understand, but in any 5-year period there seems to be a week when some place (not Houston) gets between 5 and 20 inches of rain. When your soil is white limestone with a high pH, any acidity that accumulates near the surface is enough to help grass turn dark green. But when the clouds drop too much water at one time, the resulting soup restores the high pH at all levels. Then the iron binds up and the grass yellows.

If you've had different experience that might be a good topic for discussion. Somewhere between Austin and San Antonio it seems to happen every year.