It appears to be the beginnings of SAD or St. Augustine decline. Texas A&M Ag School has recommended applying spagnum peat moss to the affected areas about 1" thick.

It's not SAD, it's normal. It has happened to me for 15 of the last 20 years with no apparent permanent issues. Besides 1 inch of mulch over top of St Aug is GUARANTEED to suffocate it and eliminate the lawn completely. But I'm not surprised TAMU would come up with that bonehead diagnosis and worse solution.

That is a great picture. I've been trying to find a good lawn to use for an example of this problem. The problem is the heavy rains we had washed all the acidity out of the soil. When that happens, the pH at the very surface goes to 8.0 and the iron becomes bound up and unavailable to the plants. If you do nothing, it will remain that color for the rest of the season, and next spring, all will be great again. You may not remember that you had a problem. What seems to happen over the winter is the soil pH in the upper root zone becomes more acidic...just enough to release the iron for uptake in the plants. At this point, no matter how much iron product you apply, the high pH will never allow it to reach the plants.

One product that seems to work, and the reason is not crystal clear, is called greensand. The actual material is glauconite (GLAW co nite). It is mined from ancient seabeds in various places around the country. It is sandy and has a greenish tint. Apply at 40 pounds per 1,000 square feet and you have a chance of full recover this season. If you live north of Austin, I would not bother for this year. If you live south of a line from Corpus to Laredo, then you might want to go ahead and do it, because you have many weeks of active growing grass left this season. I live in San Antonio and have a house in George West - I'm not going to use it on my lawns.

At least you can live in comfort of knowing the problem happens all the time. It has no effect on the health of the grass or soil. The reason it happened to you and not your neighbors might have to do with the fertilizer they use. If they use one with ammonium sulfate, it could be their pH is lower. I use only organic fertilizers with no "acidifiers," so if it is going to happen, it seems to happen to me. Greensand clears it up after about 3 weeks. You should be able to get greensand at any good nursery or garden center.

This only happens in years with heavy rains. One year we had a low pressure system stagnate over top of us dumping 20 inches of rain in a week. At about day 4 of that event, I realized what was going to happen and applied the greensand while it was raining. It worked perfect! I had the only deep green lawn for the rest of the year while everyone around me went entirely yellow.