I think that you will probably find that it is just algae or lichen. These don't harm the tree and are usually a sign of a lack of vigor or overcrowding of branches. You would not need to treat with anything as such but you could prune the branches so thety are less crowded and also feed and mulch the ground around the tree.

Looks like lichen to me in which case nothing to worry about - simply shows the air where you live is clean as lichen wont grow in areas of polluted air.

If it really bothers you then the best thing to remove it is a power spray set so that it removes the lichen without damaging the bark.

I have this over a philadelphus and I really like it. It is a harmless lichen as the others have previoulsy said.

Hallo all. That has answered a question that I was going to ask, a friend of mine has got lichen all over his gooseberry tree and apple trees, I was going to take a photo of it and then ask the question, but now I have no need to, I can tell him what it is and that he doesn´t have to worry about it, btw he had a superb crop of gooseberries and apples this year despite them being covered in lichen. Thanks GoY for solving the problem.