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.

If it is St Augustine, daily water will lead to disease and dead lawn. 1. does look like crabgrass taken through a lens with Vasoline on it. 2. don't know 3. horse herb. This stuff will take over in St Augustine. 4. if the blade tips are blunt, then St Augustine. If they are pointed, then either zoysia or centipede. Centipede only grows under very bad conditions, so to get that dense it would have to be pretty neglected. If it responds well to fertilizer and water, then it is not centipede. 5. dunno 6. sand and thin St Aug - at least it looks like it from that altitude and focus level. If that is St Augustine or centipede, the herbicide I would suggest is this one. Read the directions carefully. This stuff is not something you want to get on you. You can only apply once a year. If it is too hot now, then wait until it cools off in the fall - read the directions. If you apply the chemical fert that lou suggested, you could go ahead and apply alfalfa pellets at the same time. App rate for YOU, first app of alfalfa, is 10 pounds per 1,000 square feet. That is because your soil has no nuthin in it. Wait 3 weeks and then you could apply at 20 pounds per 1,000 square feet thereafter. If you want it to improve at record speed, apply the alfalfa monthly for the rest of the season. Put some cat food or tuna cans out and time how long it takes your sprinkler to fill them. That is your target time for watering. The day you do that test is day 1. Then watch the grass carefully to see how long it goes before ANY part of it looks wilted. Then water a full inch again that day. Watch to see if it goes a day or two longer before wilting. As the grass matures it should go longer and longer between watering. Mow at your mower's highest setting.

Where do you live? (always my first question because it usually is a factor in the solution) The first picture didn't show me anything alarming. Second and third clearly show an advanced fungal disease process going on. How long did the grass sit on the pallet before you put it down? The disease you have usually results from insufficient air movement over the grass. Did you leave a pile of leaves in that spot or maybe other garden clippings? It doesn't take much when left there for a few days. The organic solution would be to apply ordinary whole ground corn meal at a rate of 20 pounds per 1,000 square feet over the entire lawn (because you don't know where it is and not showing up yet). Then reapply again in 3 weeks. You can get ordinary corn meal in 50-pound bags at a feed store. Call first. You do not want corn gluten meal. That has other uses. You want ordinary corn meal. It should be about $10 per bag. The chemical solution I cannot help you with. In these warmer zones when you read the fungicide labels carefully, they are only good for temps below 75 degrees and no rain for the following week. One of the reasons I went to a full organic program was due to the success I had with corn meal on this fungal disease.

Being a northern visitor I must ask.....does St Augustine grass really need that much water. I am aware it likes it moist....but hel* watering every day after its been down for a month and half, is that really necessry. We're told to give our lawns about an inch per week..rain notwithstanding, and I realize the southern parts of the country gets additional heat on a regular basis....but still, the soil has to accept all that water. What happens to sod when the water doesn't get absorbed from day to day....yet gets watered whether it likes it or not. If we study about how watering can benefit the roots of lawns, then we give what we feel the grass can use. Too much, the roots become saturated, and soon unable to take up any more. Water then sits, and can speed up rotting of the roots which, of course, cant feed the grass, the grass turns brown. On the other side, underwatering causes much the same. Roots starve of moisture, they die and the grass above dies with it. The word "water" might be a misnomer in this thread...perhaps the owner means she is sprinkling water over the sod, thus keeping it moist. But if she is really pouring on the H2O, then surely the sod might be in a position it thinks its a boat and want to float away. Does St Augustine stand up to winter cold as well as others...I have seen areas in the dead of winter with brown as the common, and yet around the corner, green as green can be.