I have a customer with a willow oak tree that was planted by the nursery at the home about 5 years ago. When the homeowner purchased it, it looked healthy for the first year. The second year the bottom third had browned leaves and dropped them. The third year the bottom two thirds browned leaves and dropped them. The fourth year the entire tree browned leaves and dropped them, but the bottom started to fill-in with new branches. This year about two thirds of the tree has filled in nicely with new branches and healthy looking leaves. What are your suggestions of happend to this tree? I have not opened any books to incestigate it, because I just got back from the customer's house. She was told that it was due to poor irrigation, however this is the only tree out of 4 that was affected.