Over the course of 5 days I suddenly have this brown patch in my front lawn. There are no grubs when I dug up the grass and the roots seem strong. I haven't fertilized since the spring. The only other thing I can think is that we just had roofers out at our house and they ran a gas air compressor. Maybe the hot exhaust was blowing on it? Any guesses of what could be causing this? I live in Denver as well, incase that helps. Thanks for the thoughts!
My vote is on the exhaust from the compressor. I've seen similar damage to hedges where the compressor has been left too close. Also, the abrupt line with a fade to green at the far end seems much more like heat than chemical.
My recommendation would be to take line trimmer, scalp the area as best you can. If the grass starts to grow back quickly, you know it's heat damage. If it doesn't do anything in say a week, you know you have a different issue. You could also just mow the grass on the lowest setting and that would have the same effect.