I have looked at a number of lawns this year that looked like they had been damaged by Pythium Blight. I was always there after the fact, however, and was unable to diagnose it for sure. Pythium on golf course turf is a common problem, but it is fairly rare on lawns.
Here are some pictures from Jorden Kolpin in the plant and insect diagnostic clinic at Iowa State. These are pictures showing the mycelia early in the morning on Kentucky bluegrass lawns in Iowa. These have been verified to be Pythium by the lab.
This disease is also called "Cottony Blight". You will be able to see why in the pictures. The mycelia generally go away in mid day. Jorden makes a positive identification by looking under the microscope. The individual hypha lack cross walls and spores of the Pythium are generally present.
The hot, wet conditions are what brought this on. Wet conditions, with night temperatures above 68 F and hot days are required for its development. The problem should be over for the season, but watch for it next year.