My guess would be heat stress and bug damage. When a portion of lawn becomes stressed it starts to weaken and bugs take full advantage of those areas. Im sure most of the damage is already done, but pay close attention to the area to see if its spreading. For now you could apply some Quality Compost in and around the trouble spots @ 1/4 to 1/2" thick. Brush or lightly rake the compost off the grass blades and down closer to the existing soil. As for the grass blades wilting/curling: This is perfectly normal. The grass blades curl/reduce surface area during the heat of the day in order to conserve water from evaporation through their leafs. If by the following morning the grass blades are still wilted, the grass should be watered within 24hrs. Are you sure your apply 3/4 to 1" of water with each irrigation cycle? If not do the tuna can test(link below) Here is a link that might be useful: How to calibrate your sprinkler

I have three live oak trees in the front on my north side and have no trouble getting grass to grow. It might not be the thickest stand of St Aug, but it doesn't die out every winter. Sand is my favorite St Aug substrate, so no problems with that. What is your watering regimen? How often do you water and for how long each time. My guess, based on years of reading posts from Floridians with St Aug problems, is that you water far too frequently, but I'd like your confirmation before I launch off into corrective action. I further suspect that your lawn dies out from a fungal disease exacerbated by the watering regimen. How high/low do you mow? Again, my guess is that you mow too low. If you are not using organic fertilizer, you should start. The main reason sand works for me is that I've been using organics for 12 years. By organics I mean something like corn meal, alfalfa pellets, soybean meal, or other readily available ground up animal feed that you can find at your local feed store. The application rate for these organics is 15 to 20 pounds per 1,000 square feet. You can apply them any day of the year, or every day of the year, without fear of harming the grass. What these feed types of fertilizers do is develop a healthy family of microbes in your soil. Then the microbes will feed the actual plants and grass growing there.

I'm going to ask for another picture taken from 3 inches above a spot that shows both brown grass and green grass. Please take it in the shade like the area between you and the wagon in the second picture above. Generally with watering we talk about deep and infrequent. Deep means 1 inch all at one time. You can time how long it takes your sprinklers to deliver 1 inch by placing some cat food or tuna cans around the yard. Put them in the green spots and the brown spots. The time it takes to fill them is your watering time. My oscillator sprinklers, with my hoses and water pressure, take 8 hours to fill the cans. High flow in-ground systems can fill the cans in 20 minutes. Your time will be different. Infrequent, the other part of the watering mantra, means you wait until the soil dries out from the previous watering. There are many factors in determining when to water but they all simplify down to air temperature. Now that the temps are in the 80s, you can water once every 2 weeks. With temps in the 90s water once a week. With temps in the 70s, water once every 3 weeks. When it's cooler, water once a month. This is the single most important key to success with St Augustine. Mowing height is the second important factor. Always mow at your mower's highest setting. Hopefully that is 4 inches rather than 2.5. My neighbor's riding mower only goes up to about 2.5, so I quit using it. I'll mow weeds at that height but I steer around my actual grass (I've moved to a weedy lawn in Bandera). Tall St Augustine grows much deeper and more drought tolerant roots. These two factors account for at least 90% of success with St Aug. Fertilizer is much less important. It counts, but if you're trying to improve your current lawn, watering and mowing are the most important. Nice to see you're using organics. I can save you some money on that if you like. Rather than paying $20 for 40 pounds I buy the raw materials at the feed store for more like $10 for 50 pounds. The ingredients in the Gardenville product are unequaled; however, not always needed. If you used that once every year or two, along with 3 total applications per year, you'd be fine. I fertilize with either corn meal ($8) or alfalfa pellets (rabbit chow, not horse cubes ($12)). The application rate is 15-20 pounds per 1,000 square feet or about the same as the Gardenville fert. You can use these organic products any day of the year, or every day of the year, without any fear of hurting anything. They don't need to be washed in like chemical fertilizers. There is no reason I can think of that the grass over the septic fields would not be growing. The grass over top of the concrete tank might need more water, but the rest should be fine. I would suspect lack of water (or too much water). If you get your water and mowing height adjusted, you should see it turning around. Please post another close up picture so I can look for signs of disease. Rather than writing an essay on disease when you don't need one, I'll wait for the pic.

First of all it is a lawn that is mowed too low. It should be mowed at the mower's highest setting. There is (almost) never a good reason to mow St Augustine low. What is your watering schedule? How often and for how long are you watering? I don't see the direct evidence of fungal disease in your pictures. Usually if you have a disease there will be new grass popping up in the middle of the dead grass, and lesions on the blades of the oldest of the new grass. The second picture might be showing that, but it was taken from 6 feet away. If you can take one from 6 inches away, that should show whether it is disease or drought. I suspect disease, so I'm going to suggest using ordinary corn meal (under $10 per bag at your local feed store). Apply at a rate of 20 pounds per 1,000 square feet over the entire lawn. I also suspect overwatering. You should be watering deeply no more than once per week in Houston, especially with all the rain. So if you could verify your watering schedule, I can tell you how to tune it up. Also what kind of sprinkler(s) do you have?