Apple Disease

I can't believe it's 11.30am already and i've acomplished nothing except this blip!

I'm rather concerned about my apple tree. It was yanked out of the ground as a twig 6 years ago, travelled 100 mph up the M6 tied to the roof of the car, then shoved in a hole in our garden.
It looked extremely worse for wear, but the following year it flourished and gave us some rather nice apples.

Mum came up the other weekend and told me it had a disease. I thought it looked pretty with blossom on so hadn't actually looked at the branches in detail, but yep... funny yellowy green stuff in certain places??
Not sure what to do, maybe i'll just spend the day picking it off!!