Wow, I am very aware of the brown spot problem and it has absolutely NOTHING to do with water your St Augustine. It is grubs that are laid in the lawn by the Japanese beetle in the spring. At the end of summer towards fall the grub from the eggs feeds on grass roots. It looks like round brown spots like your dog pissed there. If you pull on the brown spot and it comes up like a patch that's further proof. I treated my back yard and it stopped. I didnt treat the front and it has brown patches and bare spots. Dig around before it gets too cold and you will find grubs......Organic treatment is Milky spore

Especially a large area under 2 oak trees Well that sounds like your problem, at least that is where I would start. Saint Augustine is the most shade tolerant of the warm season grasses, but it has limits as to how much shade it can tolerate. If the tree keep that area in shade most of the day, not even SA will tolerate it. Even the variety of SA has varying degrees of shade tolerance. For example FloraTam, the most popular SA in Florida does not have very good shade tolerance when compared to say Palmetto SA. IF it is not shade, you live in Florida, I bet your neighbors have Saint Augustine right? Well if that is true then you also have Chinch bugs which SA is on top of their menu and preferred food. Chinch bugs can wipe out a SA lawn in no time.

You could do 50/50 of soybean meal and alfalfa pellets (not meal) at the rate of 30lbs per 1000 sqft to feed the microherd in the soil. You will have to spread each separately with rotary spreader. Use the highest setting for alfalfa pellets and probably halfway between highest and lowest for soybean meal. Do you have ortho-dial? You could get unsulfured blackstrap molasses and APPLE VINEGAR (not the fake one). again 50/50 at the rate of 4oz per 1000 sqft after you've spread soybean meal and alfalfa pellets. Mow at the highest setting. Water deeply whenever large area are wilting. Try to go as long as you can between watering.

Barry I'm so glad you bumped this back up. Not sure how I missed it (repeatedly), but I disagree with much of what has been said. I don't mean to confuse you, but differing opinions usually do, so sorry in advance. St Aug should be very easy to grow in Houston. It will plow over under around and through any other grassy plants you might get including bermuda. All you have to do is mow it at the mower's highest setting every week or two. Currently it looks like you're mowing at the lowest setting. St Aug never needs to be mowed at anything lower than the highest setting. Some riding mowers will go up to 6 inches and some only go to 2-3 inches. If you're mowing at the mower's high setting now, you need a different mower. St Aug can grow up to 32 inches high and look fantastic (if you like the flowing wheat look). The taller it is the deeper the root system it gets and the (MUCH) better drought resistance you get. There is the occasional weed that will get into a thin spot, but usually you can spot spray those out with atrazine. How many thousand square feet do you have to mow? This looks below average for a new lawn, but when the lawn is so big, anything can happen. The big problem is mowing too low, but it does look like the sod went in with some bit of fungal disease. The 3rd to last picture shows some fungal lesions. I have been using ordinary corn meal to take care of fungal disease in St Aug for the past 13 years. It works for me every time. The application rate is 20 pounds per 1,000 square feet. I get it at the local feed store. It takes 3 weeks to see an improvement, but since you've already applied a chemical fungicide, corn meal won't work. Chinch bugs? You absolutely cannot see them from a photo. However, you can wipe your hand across the surface then they will flick up in the air. Put a piece of paper down and see if you can flick them up on it. There are some good YouTube videos showing how to do that. They hit the hottest part of the soil first, so that's usually next to asphalt or concrete. Then almost never begin in the middle of the yard unless you have watering issues. The first place to check would be in your last photo. And just because you might have one insect pest does not automatically make you a candidate for the full catalog of insects. And don't apply insecticide because you see bugs. Insects are not only normal, they're required for healthy soil and turf. You do have watering issues. I assume for such a large lot you have a system in the ground. It needs to be tuned up, because it is missing spots. Place some tuna or cat food cans around in the dry spots and in the green spots. Then time how long it takes to fill the cans. Some high flow systems can fill them in 20 minutes, so keep an eye on them. But some of your cans are going to be pretty dry for a long time. Eventually you want them all to fill at about the same time. Then use the time it takes to fill them as your new watering time for every zone. That's 1 inch of water. The idea is to apply a full inch all at one time. Then wait until the grass is ready for the next drink. Generally speaking when the temps are in the 90s, water once per week. With temps in the 80s water once every 2 weeks, temps in the 70s water every 3 weeks, and once a month the rest of the year. This also promotes deep roots. You don't have clay soil. You might not have perfect soil, but clay is not the issue. Usually the problem is a salt imbalance that causes soil to act like clay. Even sand can act like clay. If you really want to know all about your soil, send a sample to Logan Labs (not TAMU). For $25 they will send you a detailed report. Post that report here and morpheuspa will read it for you telling you what you need to apply, where to find it, how much, when, and how. That's another $200 value for free. Fertilize in May, late September, and again in late November. Don't try to jump the gun in the spring. If you want info on organic lawn care, let me know. I think you should, but not everyone agrees. If you go to a full chemical program, then I would suggest at least once a year to use an organic fertilizer (ordinary corn meal, again) to keep your soil biology happy. So in the long run, taking care of St Aug is easy. You water it depending on the temperature, mow at the highest setting, and fertilize 3x per year. If you do these simple things, you should never need herbicide, insecticide, or fungicide. If you see weeds in the early spring, don't use a weed-n-feed. Use a herbicide with atrazine. Read the label 2x and follow the directions carefully. Photo tip: Take garden pictures on a cloudy day to minimize contrast between bright and shadow.