Lawn grass blade infections are sometimes hard to diagnose, and unfortunately we can't see the symptoms on the pictured plants very clearly. It resembles rust, a common fungus that favors mild temperatures and damp conditions, typical of this time of year. Management tips are given in the Purdue web page that the page above links to, but rarely is use of a fungicide warranted as the turf eventually outgrows any setback. If the lawn wasn't fertilized last autumn or this spring (though spring fertilization of a healthy, vigorous lawn usually isn't needed), then a bit of supplemental nitrogen may help it to recover faster.

Miri