Lichens is not harmful to any plant. It is not what is killing your trees and bushes. People have tried to power wash this off, but that action is what kills and hurts the plants. It grows mostly on shaded tree trunks.

there are many species of lichen, some prefer full sun but usually do little damage to the plants they grow on. Personall I wouldnt try to spray it off.

Lichens aren't likely to be the problem, Jytte. Some species are even beneficial. I would look for other factors, such as poor drainage, frost damage, too much shade, chemical runoff from street or power-washed house, etc. What are the specific symptoms, and can you post a picture? That's rich coming from me, I know, but some things are hard to describe.