It's a fungus, Greenfinch - curly leaf fungus. Some fruit trees are prone to it, notably peach and cherry.
There is a spray cure but, hang on a minute, if you leave it alone this year there is every probability that it won't be there next year. It's a sort of hit and miss fungus - some years your tree will get it and the next year it won't.
If you do decide to spray, the leaves which have already curled won't be saved, they've had it now I'm afraid.
The good news is that curl or no curl doesn't make a jot of difference to the fruit yield.
I used to spray when my cherry caught it but now I can't be bothered 'cos it makes no diffy to the tree itself