Old crab apple tree

[modeller]

I've a 50 year old Malus which has bloomed and flourished every year until last year when all the leaves died in late summer and the whole tree was covered with grey lichen like growth. I decided the canopy foliage was far too dense restricting sun and air , so I removed 60 % of it and it worked. Within a month leaves were starting to grow but by that time it was Autumn and I had to wait until this Spring when I enjoyed another floral display and normal leaf growth, but now the leaves are once again looking poor and starting to fall also no tiny crab apples. Can anything be done or is it old age ?

[modeller]

It's been suggested that it might be powdery mildew but on Google the pictures show that it PM seems to cover the leaves whereas my lichen like growth only seems to be on the bark and requires a stiff brush to remove it just like thick lichen.