Need more information.

Southern Utah? St George area?

What was your watering schedule for the summer? How often and for how long did you water?

What kind of grass do you have?

Sorry. Yes, St. George. Watered twice a week for 10 minutes until temps got above 100 degrees, then increased to three times a week. Irrigation ran at 6 am. The turf is a mixture of 90% tall fescue and 10% bluegrass. The front lawn received the same schedule and is lush and green, although it is an established lawn. The affected turf was installed last September.

Twice a week for 10 minutes is not enough for a cool season grass in the extreme heat. Having said that, it seems to work on your front lawn, so why is that? Are the sprinklers identical front and back? Same heads? Same number of heads per zone? Same apparent coverage? You might put out some tuna or cat food cans and collect water for the time your sprinklers run and see if you see any differences.

Generally we talk about deep and infrequent watering, but we seldom talk about proper plant selections adapted to different areas. You live in an area of extremely extreme weather with winter time temps in the teens being not that unusual and summer temps above 100 not that unusual. I would tend to lean toward non traditional lawn grasses like wheatgrass, blue grama. These are considered to be prairie grasses, but they do very well in your conditions especially when they are seeded to be dense. They need much less water to survive. If you let them go long enough they will develop seed heads, so some amount of mowing is required. But again, it is less frequent than a typical lawn type grass needs.

I think your yard is alive, and will improve after the summer heat breaks. It will be better next year after another year of root growth. Still for long term life it will need much more care than a prairie grass type lawn.

Thank you for your input. Same sprinkler heads, good coverage throughout with no dry spots. I did measure water in dead spots and those that were not affected. Approximately 1 inch; same as the front yard.

When I increased watering, the problem seemed to get worse, especially in areas that got the most sun.

My landscaper raked out the dead grass is the worst area, treated with fungicide again, put on some top dressing and overseeded. He set irrigation to every morning to keep seed moist. This was on Saturday. In just two days, I am now noticing green mold? on the stucco wall that is adjacent to the lawn area.

The areas that are the worst are almost like the turf is composting, with roots rotting.