That isn't a 'disease' it is lichen which indicates your air is pure and clean. Grows on a lot of trees/shrubs on our area, if you don't like simply remove but it is very likely to return! BTW it won't actually 'kill' you trees and shrubs!

That type of lichen does tend to be thicker on trees and shrubs that are under stress for other reasons. I notice that there is quite a bit of mulch piled around the base of that shrub. Is this your normal practice? Burying the bases of woody plants often results in the buried inner bark dying. That layer of bark transports sugar from the leaves down to the roots, so if that transport is interrupted, the roots slowly die, and the plants just slowly fizzle out.

Fascinating.....I love this lichen and have never known that it tends to prevail on ailing trees and shrubs. Great advice about the mulch Tugbrethil. Mulching isn't quite as simple as we would like it to be. I Always try to leave a gap between the mulch and the bark....it's not easy. :)

True, Karen! Mulch tends to walk about more than any inanimate object should! :)
The lichen isn't a cause of trouble, but is often a symptom. Growing plants with growing bark make a less steady substrate than the lichen likes. In the pine forests of northern Arizona, the old man's beard lichen can appear anywhere on the trees, but always grows most thickly on the dead or dying branches.