identify grass disease
This is rust disease on your turf. It crops up when weather conditions are just right, in this case, lots of rain and temps in the 70's.
There is nothing that we recommend you treat it with, and it will either recover as weather dries/warms, or that area will thin, in which case you can overseed in the fall.
Be sure that you are not over-fertilizing, especially in the spring.
Here is our page on lawn diseases, with a link to rust disease: