How often do you water and for how long? How high/low do you mow? The weeds can be handled by something like Spectracide with Atrazine. But I would wait until the grass returns. I have gotten excellent results for fungal disease using ordinary corn meal. It is an organic approach that the experts at Texas A&M swear up and down does not work, but it has worked for me every year since 2002. It is a complete cure in 3 weeks. I am suggesting you try that because corn meal doubles as an organic fertilizer. Your spots don't look that much like a disease, but it could be. If you did not have the really healthy stand of grass right by the sidewalk, it would look more like disease. I would hit it with corn meal at 20 pounds per 1,000 square feet now and again in 3 weeks. If you don't see new grass coming up in 4 weeks, then get a commercial fungicide that works in your heat/precipitation zone - read the label closely to see how to apply and when. Corn meal has no restrictions on how or when. Then once you get the grass growing, zap the dollar weed (I assume dichondra or some other easy weed) and let it grow. And if you have not already raised your mower to the highest setting, do that yesterday.

I'm no authority on southern type grasses so what I suggest comes more from seasonal visits to your State than anything else. But....grass is grass, no matter where it grows and grass is not unlike any other plant....a lawn is just many plants growing together. You say you have a sprinkler system and your lawn gets plenty of water. "Plenty". That envisions lots of what that word means. As far as the type...maybe St Aug likes more than other grasses but up north we go by the rule..generally accepted as sufficient....one inch per week whether by rain or hose. You should understand that too much of anything is ..by definition...too much. Even a good thing. Grass has roots and if given care to bring it about grass roots should be deep so they can stand up to what is thrown at them; drought, heat, shade, drowning, poor mowing, or no mowing. I think St Augustine grass performs better when left to 3" to 4" high...shorter does allow more heat to penetrate into the soil and dries it out. Too much water can bring about the same condition as too little...the roots gow short and makes the grass open to attack. Too little water does exactly the same thing. Along with less moisture grass is subject to attack from pests and disease and loss of nutrition from not being fed. You didn't say whether you have given your lawn any fertilizer, or if you do, when do give it. To green up a lawn sufficient nitrogen in the fertilizer will bring this about. To make it grow its best the other two elements...phosphurus and potash (potassium) along with the other minor elements encourages grass to grow and keep it healthy. I noticed when I was in Florida this past winter practically every mention of fertilizer contained NO PHOSPHURUS in its contents of the bag. Nitrogen, yes....potash, yes.....but no phosphurus or very little. The bags were listed as having 20/0/10....along those lines.... I suppose that says that Florida soils contain sufficient phosphurus and doesn't need any further help in that way. Soils being what they are....one soil is not the same as the other. Plants use the elements in what soils they are given. To say one area doesn't need this or that...is suggesting the whole State has the same make-up. Without a soil test, how is one to know. Phosphurus is vital for healthy roots and where it matters, rich blooming. So one has to obtain better advice about the soil you have, in particular in your lawn, so that you can make informed decisions whether to give it phosphurus. You can do a soil test yourself or send samples to a local laboratory....undoubtedly Florida State University has an extension service that does this or you can enquire about a private lab to have it done. Since you are new to the area I do suggest this be done. Extension services by State funded colleges are listed in your telephone directory under "STATE" government or under the university itself. A phone call can quickly inform you whether the service is performed and how you go about delivering a sample.

It looks like fungal disease. Mine looked just like yours when it started. I tried using cornmeal like some recommend but it didn't work for me. I had to use a chemical fungicide. If you can't find it locally this place has good prices. http://pestcontrol.domyownpestcontrol.com/search?w=fungicide

You picked a doozy of a topic to reference. I don't think I've ever seen so many people piggybacking onto one thread. The spots you see on your grass are the same as those I posted in that other topic. The corn meal should help. Give it 3 full weeks. This is not a shade issue or a watering issue. St Aug will be thinner in the shade but it should be healthy looking with zero spots or yellow blades. Here's a picture of my lawn in San Antonio growing in deeper shade than you had. The point of this picture is 1) no spots, and 2) lots of shade. I took this picture to illustrate chlorosis in St Augustine caused by too much rain. The yellow blades in the center are from the rain. The point you cannot see is that this area looked horrible a month earlier from disease. You could see the soil, leaves, stolons, and spots on the few living blades of grass. Note that you can still see through the turf a little to the stolons underneath. If this was in full sun it would be so packed in the blades would be growing straight up. I'll get a picture of my new lawn in Bandera looking straight down. It's very hard to push a mower through that stuff. Okay I have to comment on the TAMU pamphlet on growing St Augustine. Generally when I see something from TAMU I read it and do the opposite. Truth! Here's my experience this year. I live in Bandera now, so think Texas Hill Country. Our temps ran in the mid to high 90s this year. We got our first rain on Memorial Day and it nearly washed my belongings out of the garage. It was 11 inches in 3 days with most coming the first night. That's too much rain so I was expecting to get some disease. I did but only about 1 square foot was affected. But since Memorial Day I have watering the actual grass one time. Fortunately we got some rain relief last week, so from June 1 to Aug 19 I did not water. Had I followed TAMU I would have watered it every 5 days. Had I followed my own advice (below) I would have watered it every 7 days. What the TAMU people did was over simplify watering, but then they went into too much detail on how to achieve the simple watering. Watering is slightly harder than they suggest but setting it up is easier than they suggest. Here's my take on watering after reading and moderating lawn forums for 10 years. Watering: Deep and infrequent is the mantra for watering. This is for all turf grass all over the place. Deep means 1 inch all at one time. Put some cat food or tuna cans around the yard, and time how long it takes your sprinkler(s) to fill all the cans. Memorize that time. That will be the time you water from now on. My hose, sprinkler and water pressure takes 8 full hours to fill the cans. Your time will likely be less. I like gentle watering. As for watering frequency, that depends on the daytime air temperature. With temps in the 90s, deep water once per week. With temps in the 80s, deep water once every 2 weeks. With temps in the 70s, deep water once every 3 weeks. With temps below 70, deep water once a month. Note that you have to keep up with quickly changing temps in the spring and fall. This deep and infrequent schedule works in Phoenix and in Vermont, so it should work for you. The reason for deep and infrequent is to grow deeper, more drought resistant roots and to allow the soil to dry completely at the surface for several days before watering again. If it rains, reset your calendar to account for the rainfall. I guess I should add that if it rains 11 inches, reset your calendar back by a month at least. Mowing height. If you read the topic you referenced you saw pix of my dog in the tall grass. (We found her and she's up at the new house in Bandera). From those pictures you can see that tall St Aug is pretty healthy. I seriously did not water, mow, fertilize, or weed that lawn from October 2011 through Feb of 2015. All it got was rainfall. 2011 and 2013 were drought years, and it thrived. Point is keep your st Aug as tall as you can stand it. Set the mower to the highest setting and weld it in place. Contrary to what TAMU says, mowing it low does not make a better turf. Fertilizing. If I was on a rigorous fertilizer schedule I would use organics 5x per year following the federal holiday schedule (Washington's birthday, Memorial Day, 4th of July, Labor Day, and Thanksgiving). If I was on a casual schedule I would fertilize on Memorial Day, Labor Day, and Thanksgiving. If you decide not to use organic fertilizer, I would still strongly suggest you use it at least once a year in addition to, or in lieu of, one of your fertilizer apps. The general experience of the universe of lawn care websites is that lawns that get no organics for 10 years or so will develop issues where the chemical fertilizer seems to not be effective. One dose of organics will reset the soil biology and the chemical fertilizers will work again. I think you can avoid that issue by preempting it. Organic fertilizer is much less expensive per 1,000 square feet than compost. The fertilizer is also much easier to apply than compost.