PLANT DESTROYER

The dark stains on the trunk of this alder tree are probably a symptom of a Phytophthora alni infection. You can see that the branches are dying because the bark and cambium have been killed.

  

Phytophthoras are organisms a bit like fungi but different.  They ae sometimes referred to as water moulds.   There are at least 200 known species. Phytophthora kernoviae produces similar symptoms in beech trees and Phytophthora infestans causes potato blight.  There is almost no known cure or antidote so they are indeed plant destroyers, which is what Phytophthora means.