"Turf just doesn't get sick on it's own," you think, driving out to the scene, "it was okay on Friday." 

You try to remember details, when you last heard the pump kick in, whether the spray calculations were double-checked, what the weather was.  When you get to the scene, you're going to hear the stories, versions of what may have happened.  Some of the people are going to be nervous, ready to suggest blame, or worse, slant their stories just a tiny bit.  "Ask the turf," you think jokingly, "at least I'll get a truthful answer."  Before you start accusing the usual suspects (fungus, insects, and exotic things like black layer) you will carefully look over the scene. 

Is the turf area brown in patches or streaks?  What about the individual leaves, do you see spots?  Are the base of the leaves rotten?  While you're down on your hands and knees, is there a smell?  Dig up the roots.  Are they firm and white, or brown and decayed?  Is the damage localized or distributed all over?  Was any chemical applied to the area?  And that's just a start.  Get ready to take some notes.