Lichen
By Jmh21
United States 
I live in the Pacific Northwest. I have noticed that lichen have been covering many trees and shrubs. Everything I read says that lichen are harmless, but whatever branch that the lichen is on appears quite dead to me. Are they harmful? Is there anything to do if they are killing my trees?
On plant
Lichen
7 Jan, 2009
Lichens won't kill your trees. They are essentially harmless. However, it is possible that you are seeing growths that are not lichens. For example, a virus infection will have symptoms much different than fluffy lichen or moss material on the outside of the plant. Virus symptoms are things like stunting, deformation, concentric ring-shaped spots, mottling, and yellowing. Some fungus diseases can look similar to lichen. There is also mistletoe infestation which can kill off trees. Not the Christmas type mistletoe but a different type of this parasite which is fairly common on conifers in the Pacific Northwest.
7 Jan, 2009