My apple tree isn't very happy

So my apple tree currently looks like this








I have had a search online and in my books and I get several answers for what it could be and then nothing on what to do about it

so could someone help please and tell me what i need to do

I know i wont be getting any fruit this year from it now but I would love to get the problem sorted

btw it lives in a pot. It is one on dwarf rootstock