What is wrong with this apple tree?

It may be scab aka black spot. High temperature and rain causes it. clear up any fallen leaves in autumn and burn them as the spores overwinter on the leaves. Pick off the worst affected and burn and clean dissenffect pruning tools.
The apples will form baclk spot too, but they wont be completely ruined by it.
You can get sprays for it just read instructions as there are times when you cant spray, e.g. when the trees in flower.
Roses are affected by black spot too