Fungal disease of some kind. How often do you water and for how long? How much rain have you gotten? Are you down in a low area or has that area been covered with brush or something? It looks like it was smothered or otherwise protected from sun and wind for a time.

I take an organic approach to lawn fungal diseases. I use ordinary corn meal at a rate of 20 pounds per 1,000 square feet. Corn meal attracts another fungus which is predatory on the fungus causing your disease. That fungus is called trichoderma (try ko DER mah). It takes 3 full weeks before you see any improvement. That can be the longest 3 weeks of your life, but after that you should see less yellow and much more deep green. Corn meal is also an organic fertilizer so the dark green color will be very noticeable at the 3 week point. If you have used a chemical fungicide recently, then the corn meal will not work against the disease but it will definitely green up the grass. And yes this is the same corn meal you cook with. You can get it in 50-pound bags at the feed store. And no, it will not attract varmints. Seems like it would but it does not. It does seem to attract birds but they don't eat it. They just come and visit. You can apply corn meal any day of the year, rain or shine, or every day of the year, without fear of hurting anything. If you wan to take a chemical approach, I cannot help you. Our temperatures, humidity, and rainfall are usually outside the recommendations on most chemical fungicides. I gave up on them years ago when I found out how well corn meal works.

On another issue you did not ask about:
You are mowing your St Augustine much too low. Raise your mower all the way to the highest setting. It will take a month or so before all the grass gets tall enough to be mowed but it eventually will. Mowing it low favors the bermuda which is mixed in with it. Mowing it high can have the effect of choking out the bermuda. Tall St Augustine looks very lush.

living in the very same town as you do I have noticed this on some lawns. I have the very samething on my saint augustine and my guess is that it is due to iron defiency. might i also suggest that you sharpen your mower blade. let me get back with you later and i can ask my neighboor that is a greenskeeper so I can get you a soultion to your problem.

Looks fungal to me.

Culturally, you should make sure you are watering deeply but infrequently - once a week or so. Also, you need to sharpen your mower blade. The jagged edges don't heal as quickly and invite disease problems. Finally, you should bag the clippings until the fungal problem is under control, so you don't accidentally spread it around.

I'd personally try the cornmeal approach first. It is cheap and easy and you may have it in the pantry already. If that doesn't work, they sell chemical fungicides.

Yellowing from the bottom up is iron deficiency (chlorosis). Yellowing from the top of the blades is fungal disease. I just took this picture a minute ago. It shows what chlorosis looks like in St Aug.

{{gwi:86010}}

Note that the grass is healthy with no spotting on the leaves. Near the bottom of the blade it is yellower and near the top there is some darker greening. This is much easier to see in 3-d by looking at the grass. Pictures of grass are hard to take.