I have brown spots in lawn mainly on the southern exposure areas. First thought was grubs, but turf is not loose on top & when i try to dig up, roots seem very intact. Lawn is dry now, but I began to notice these areas a week ago when it was not dry. Some one suggested rust, but I don't see any spots or powdery residue on blades, and it has not been overly wet. Has been quite humid lately, and the grass grew very quickly & got quite long (6") before I was able to mow. I cut to 3-1/2" long. I know I have limited window of time to treat for grubs, if that is the case.
Brown Spots in Lawn
You may have cut the lawn a bit short from a length of 6", but my first thought is urine from animals. Do you have a dog or cat? If not then possibly a neighbour's cat "leaving you overnight messages" If not cats then perhaps a squirrel or even a fox, again coming during the hours of darkness.