I have several large, beautiful Linden trees around the perimeter of my home. Last year I noticed many dead branches on one of them. Last fall I applied a "save a tree" type treatment as well as a treatment for aphids as advised by a lawn expert. This has not helped and an arborist has told me that aphids will not kill a tree. I am desperate to save this 20 year old tree. The leaves on the "sick" tree seem smaller and less green than my other Linden trees and as I said earlier many branches are dead. The tree gets ample water. We do use "weed and feed" one time per year on our lawn. Any advice?