HI Jerry,

I'm sorry to hear about your lawn!  

A fungus is possible, but the pattern doesn't quite match for our most common fungal problem, necrotic ring spot.

I would check first that the irrigation system is applying water evenly on the lawn, following the directions found here: https://extension.colostate.edu/docs/pubs/garden/irr-audit-tutorial.pdf

If you can rule out watering, the next step will be to rule out the presence of weedy grass species that do not handle heat as well as Kentucky Bluegrass.  Does the brown grass pull up easily, or does it still seem well rooted?  

Lastly, we can check for fungus.  You could bring a sample of the lawn (about the size of a small shoebox, including both green and brown areas) to the Douglas County Extension office in Castle Rock, or you could request a site visit (call<personal data hidden>).

I hope this is helpful!

Cordially,

John