Did you, or do you, sometimes lay things down on the grass in that area? It could be yard clippings, palm fronds, or anything that prevents the air and sun from hitting the grass. St Augustine is extremely susceptible to getting a disease in early spring when the air is cut off for a day or two.

I have had excellent luck using ordinary corn meal to kill the disease. Assuming you have not used any other fungicide, it should work. Apply at a rate of 20 pounds per 1,000 square feet to the entire lawn. Why? Because you don't really know where the edge line of the disease is. Also corn meal is an organic fertilizer which will leave a dark green spot where you apply it. If you don't apply to the entire lawn, it will look yellow where you missed.

Corn meal works because during the process of biological decomposition of corn, a predatory fungus called Trichoderma (try koh DER mah) comes along. The population of trichoderma builds up on the corn and then ventures off to consume other fungi, those being your lawn disease. Apply and wash it down into the soil. You don't need to flood the soil; that would make things worse. Just rinse it down off the grass. Then try to ignore it for 3 full weeks. At the end of 3 weeks you should see signs of recovery as well as a greener lawn in general.

I see a couple other things in your picture. The rust spots on your concrete look like Ironite stains. Ironite doesn't work in Texas due to the soil being basically limestone. You are much better off trying a material called greensand. It takes 3 weeks, too, but it works.

Your sapling looks like it is buried too deep in the ground. I do not see the tops of any roots. The only part of the tree that should be under ground is the root structure. When any part of the tree bark is underground, that bark will develop a disease which eventually disrupts nutrient transfer through the tree's phloem. The day that happens the tree will die - all in one day. If the tree was recently planted, I would dig it back up and put plenty of fill under it until the top of the roots show at the base. Then never mulch against the trunk of the tree as that has exactly the same effect as burying it too deep. Here is a picture of the aftermath.

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You can see the stain caused by the fungus and the rotten bark under the former surface. Now this is what it should look like when mature...

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I'm curious what this ended up being? I have this is my lawn on the edges.

jlperry3, would you mind starting a new topic for your question and tell us where you live and what kind of grass you have?

Yes, I will. I'm in San Antonio with St Augustine. I will try and start a new post.