I first notticed a lichen that developed on a Victoria Plum tree which seemed to cover the bark of the branches eventually killing them. I firstly cut off the affected branches but tree later died anyway.This same lichen has now spread to a number of other trees, including apple trees, ash and oak trees and blackcurrant bushes.I hope there is some advice you could give as to how to protect the trees and save those now affected.
Lichens appear on trees in the wetter parts of the country and elsewhere on trees that have poor leaf-cover due to poor growth for other reasons.
There is a disease of plum trees called silverleaf that produces fungus that looks like lichen, but does in this case invade the plum. It can affect apples too but not the others.
Nothing to be done in any case, except perhaps to feed trees and shrubs on poor soil with some tree and shrub fertilizer or general fertilizer.