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.

Thanks, all, for weighing in. Yup, NHBabs, this does appear sedge-like (or nutsedge-like, which is the only sedge I'm all too familiar with). I'm fairly certain it's not nutsedge as the root structure is different. So, gyr_falcon, will happily forego those lucky stars today! :) Probably not kyllinga -- one article describes these having waxy leaves, and mine are definitely nutsedgy glossy. jekeesl -- I'm always very careful to use a roots-and-all approach when weeding (Mom and Dad taught us well -- we kids spent many, many a weekend pulling weeds in the yard!), as opposed to plucking the tops (which I have indeed read may stimulate growth). Because they're interspersed in the lawn, I can't use an herbicide. And leaving them untouched wasn't an option as it was apparent the St. Aug was struggling to gain territory. I will run this past the County Extension -- thanks again!

My only problem with Black Kow is the amount of sand in it. They should be charging much less in my opinion. Otherwise I believe it is a good product. You might want to check around for "materials suppliers." Generally they provide sand, rock, gravel, topsoil, subsoil, bricks, etc. to the construction industry. Call and see if they have compost products in bulk that you can haul away by the bag, bucket, or truck load. If you apply compost use no more than 1 cubic yard per 1,000 square feet. I've seen lawns smothered by it. Also rake and rake after applying. Any blade of grass that is buried will die. So rake to bring up the blades from under mini piles of compost. A leaf rake should work fine for this. Even a push broom works fine. If you apply compost then you could use corn meal in a week. Compost has all the beneficial microbes you need to restore a soil's health. I would only use it to restore from a chemical poisoning like fungicide or insecticide, but I would use it every time for those reasons. Another way to develop fungal disease in St Augustine is to leave stuff on it over night. My wife used to throw garden clippings onto the lawn and leave them for a few days. I had disease issues year after year until I got her to stop that. So leaving toys, tools, or anything on the lawn is going to be a problem. St Augustine needs to breathe, perhaps more than most other grasses. A lot of people in Fl have crabgrass issues. Do you have that?

John, if you're willing to spend a little upfront, Celsius WG will probably do what you want. Others have mentioned bonide sedge ender. In both cases there are some restrictions for st Augustine grass. I just switched to st. Augustine grass recently and now I use Celsius after help on forums. Google those products on this and other forums, or PM me and I could give you links.