The diseases to which our trees succumb can be pretty befuddling to non-arborists. After all, it's tough enough to keep up with the constant changes in human biology with which science bombards us! Fungi are at the root of the most common diseases of trees. "Fungi lack chlorophyll and derive nourishment by feeding on (parasitizing) trees," states Steve Nix, About's Forestry Guide. Steve covers some of the most common diseases of trees in this piece.