is to get a soil test.

A regular soil test would also tell you if the pH is too high or low. Could also be compaction - you could use a screwdriver to see if it is the same density as the rest of the yard. I'd start with process of elimination.

If you are sure it's a fungus, you could also go right to the good stuff and use Headway G. It's a group 11 and group 3 fungicide that will address nearly any type of lawn fungus: