Where are you in Central Texas? The town would be best, but basically are you east or west of I-35? North or south of I-20?

It's impossible to say from those pictures, but thanks for trying. The best way to take garden pictures is on a cloudy day or in the shade without a flash. Otherwise the contrast between sunlight reflections off the foliage and the dark shadows is too much. Also if you suspect a disease, then get down on your knees and take a picture from 3 inches away in an area where the grass is going from green to some other color. We need to look at the spots on the blades.

You should have been deep watering about once per month for the past month and every 2-3 weeks prior to that. If you were watering once a week or more frequently, then a disease is very likely. The following is the skinny on watering a lawn.

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.