Most of the major diseases that afflict trees—blights, cankers, root rots and leaf spots—are caused by fungi.
Beech Bark Disease results from the relationship between the beech scale insect, Cryptococcus fagisuga and the fungal pathogen Nectria coccinea.
Fabrella Needle Blight of Hemlock is caused by a fungus that infects the needles-Fabrella tsugae
Sudden Oak Death is caused by a newly discovered fungus-Phytophthora ramorum that also infects Rhododendron, huckleberry and a number of other trees and shrubs.
Thousand Cankers Disease. A tiny bark beetle creates galleries beneath the bark of walnuts, resulting in a fungal infection that causes a large numbers of canker—thus the disease’s name. Pennsylvania counties under a TCD quarantine as of August 2014.