I cut down a silver maple tree that was 5' diameter nine months ago and had the stump ground down 12" below the ground. I covered with top soil and seeded. The grass started growing ok but has now died over top of the stump and about 30' diameter circle around the stump has also died. The grass always grew well when the tree was standing. Its not dry and the other grass in the yard is green. What is causing the grass to die and how can I prevent it?