check into fungus problems. Over watering on top of the massive rain we have had this spring has fungus written all over it. (I live in Ft Myers, it rains about 1/4 to 1/2 inch a day) Ill try to find you some sites for a cure. You will find alot of people that will tell you to spread corn meal, it will work but its kind of pricy unless you know someone in Kentucky that is going to be driving down here. Copper based products will probably be the best soultion.

Where do you live? What is your watering regimen (frequency and duration)? Can you take a picture of the edge of the remaining grass from a distance of 3 inches? I need to see the blades of green grass and blades of any yellowing/browning grass up very close. You're getting plenty of light. St Aug grows very well in full shade as long as it gets light from somewhere. For example it will grow poorly under the low canopy of a magnolia tree but it will grow fine under the canopy of any tree with bottom limbs 6 feet high or more.

If you could get your camera about 3 inches away from the grass instead of 30,000 feet, I could likely confirm a fungal disease. The best pictures of grass are taken in the shade or on a cloudy day to minimize contrast caused by the sun. If you get close you'll see tan spots surrounded by dark brown spots. That's the disease. I'm guessing you live in Texas and have received more than your average share of rain in the past few weeks. If it is a fungal disease it will get better about 3 weeks after an application of ordinary corn meal at the rate of 20 pounds per 1,000 square feet. You can get corn meal in 50-pound bags at your local feed store. If you cannot get it there look for a grocery store catering to the Hispanic community. Ask for (or find it yourself) corn flour. Don't get the premix used for making tortillas. It should come in 25 pound bags. I scatter it by hand. Flour tends to blow away, so you might want to use a drop spreader to apply it. And it tends to clog a drop spreader which is why I apply by hand. If it is a fungal disease and you live where the temps are in the mid to high 80s already, don't use anything but corn meal. If you live where it's cooler, you could use a chemical fungicide. I've never needed one, so I can't give you any hints. Corn meal has worked for my St Aug for the past 13 years. If you want to reapply after 3 weeks it won't hurt a thing. In fact if you want to reapply the next day it won't hurt anything either. This weekend is an excellent one, timingwise, to apply fertilizer anyway. Corn meal is a weak organic fertilizer. If you only use it on the affected spots, you'll have dark green grass there in a month. Corn meal works by attracting a predatory fungus as part of the decomposition. That fungus consumes the disease fungus and your grass will recover. If your lawn is in full sun, you might want to throw a couple pieces of Floratam St Aug sod down. It's a very fast growing variety that can actually outrun this particular disease. I see the disease in my Floratam, but it never seems to die out like yours is. I still hit it with corn meal because the spots are ugly.

Because it has a fungal disease. Those dark spots on the dead blade are a dead giveaway. Where do you live? What is your watering schedule? How often and for how long?