I live outside Chicago with a blue grass mix. For the most part my lawn is healthy and we take care of it. Two years ago, during the winter my neighbors dead tree lost a large limb which fell into my backyard. Soon after, they took the rest of the tree down as it was clearly dead for some time.
The following spring, I found right next to the stump (or where the stump had been ground), on my side of the fence, my lawn was thinning quickly and turning yellow. I didn't really notice until mid summer and figured it was summer stress. This spring, the spot was immediately present and I tried to overseed to no avail. At this point, mid summer again, the thinning and yellow has spread in almost a straight line across the yard where the limb had fallen (or possibly where a root is). I noticed a similar line of dead grass in my neighbors lawn in a fairly straight line leading from the stump.

I would prefer to fix the problem myself, but am about ready to call in the professionals. Any suggestions out there? Originally I thought it was too much direct sunlight now that the tree is gone, but what possilities of fungus or other since the tree was clearly dead before it fell?