My apple trees are both about 2 years old and this season the leaves started to look like this. These pictures are from a couple weeks ago. Now every single leaf is like this and they're all really browning and a lot have fallen off. What is this!?
It looks like a really bad case of apple scab. You would have needed to preventatively spray the tree to prevent it.
At the end of the season get rid of as many fallen leaves and apples as you can, or your tree will be reinfected next year. I have a tree that is resistant and one that is not. I find two different compounds to spray it with (one being sulphur) and I alternate it with another substance every three weeks in spring. That way it does not build up resistance to one substance.
If you do not control this disease it will significantly weaken your tree.