I have a small area in between the sidewalk and street that completely died about 10 years ago. Even since that time, the center area has had brown patches. I have treated with grub, insect and fungus treatments. None have helped. This area was perfect for the first 6-7 years after I layed the sod. No issues with the rest of my lawn and even the side edges of the easement are fine. There is an oak tree in this area that was planted by the city about a year after I put the sod. Maybe it's something with the tree?