Aliens & Pests

A hemlock tree, planted in a yard 14 years ago, died suddenly. The homeowner left town with the tree apparently healthy and returned two weeks later to find the tree had lost all its needles. On inspection, all the above-ground parts of the tree were dead. We excavated the tree with an Air Knife to see if there were below-ground signs or symptoms that might indicate the cause of death. We found extensive injury to the cambium, phloem and xylem below the ground level of the tree.
 

Stem of a dead hemlock showing extensive injury.

What caused all this damage? Read more for the solution.