Given that diagnosis, I would spend about $5 per 1,000 square feet to sprinkle ordinary corn meal on it. Corn meal leads to a cure for a surprisingly wide array of garden fungal diseases. The app rate is 20 pounds per 1,000 square feet. You can get corn meal at your local feed store or I saw it in 50-pound bags at Sam's Club last weekend. How often are you watering? And for how long? Was your lawn covered up with gardening clippings or anything else recently?

Pictures are usually needed but I think you have identified the culprit - fungal disease. I live in San Antonio, TX. I believe we have a similar issue with chemical fungicides. They must be applied when the temperature is not going to be above 80 degrees and no rain or irrigation for 7 days. Well, when is that??? January 3rd??? I had given up on finding a solution when I heard a guy on the radio suggest using ordinary corn meal on plants with fungal disease. I tried it on a rose plant and it was cured in a week. Nobody was more surprised than I was. Next week I listened to that radio guy for 3 hours. That day I went out and bought 50 pounds of corn meal at my local feed store. The rest is history. Since then I have never been afraid of a fungal disease. I have watched my garden carefully and have even identified the cause of my fungal disease issues. It is my wife leaving garden clippings on the lawn for a few days. Dew forms and the normal air flow is choked off by the pile of clippings. She had one favorite place to put the clippings so I just pretreated it with corn meal every spring. Now I have her putting them in the compost pile or on the concrete until they can be disposed of. The application rate to get rid of disease is 20 pounds per 1,000 square feet. You should apply to the entire lawn so you don't end up with green spots where you applied it. Corn meal is an organic fertilizer, too. Having said that and rereading your original message, if you have already used a fungicide, then corn meal may not work. Corn meal relies on the fungal health of the soil. If you have sprayed a fungicide then you have no fungal health left. You might want to apply compost first. Give the compost 2 weeks to work and then use corn meal. Then give the corn meal 3 full weeks to work. I would reapply after 3 weeks whether you have seen progress or not. The second app at the third week will thrive on the back of the first dose and should supercharge the antifungal effect. Most people wonder how corn meal works to kill fungus. The original TAMU work showed that corn meal contributes to the growth of the Trichoderma fungus. Trichoderma is a naturally occurring, predatory fungus that feeds on the cellular structure of other fungi. Normal soil has Trichoderma but not in high populations. Corn meal is decomposed by another fungus that multiplies the population of Trichoderma.

Aerators are Ok. St. Auguatine does not produce thatch and a dethatching machine will effectively destroy the root system of St. Augustine. What is effecting a lot of North Texas St. Augustine lawns is "take all root rot". Neil sperry has been talking about the situation as well as Texas A&M. You can find all sorts of info on the web. The cure that seems to work wonders is an application of peat mose over the lawn about 1" thick. This acid rich material seems to destroy this disease. However, you may have trouble finding peat moss as so many people have heard about the fix. Again, Again, Again, do not use a dethatching macnine on St. Augustine which spreads by runners and stolens. Also, our recent bad winter did not help St. Augustine.

Hmmmm... I was going to say you weren't watering enough. Three intervals of three minutes isn't very much. And in 100 degree temps, I'd say what little bit of water that's being applied is evaporating very quickly. But after after backing off you're finding the worst areas are still "WET" and in poor-draining, heavily-shaded spots, you've probably overwatered those areas. I'd continue to keep watering as you currently are. Good job waiting until you see signs of wilting. But I'd also only hand-water those areas that are showing signs of wilting. Hold off on the poor-draining, well-shaded areas until they start to wilt. The good news is that if your patches of brown aren't too large, the St. A will fill it in rather quickly once you get your watering dialed in. (I had a 10' diameter bare patch where the kids' pool was last year that filled in in about 8 weeks this spring.)