Without a photo, I can only guess as you do not give me any information.  If it is only the leaves, go to this link and see if it matches up.  If it is apple scab it is not fatal but you need to clean up all leaves and dispose of them as they fall.  In the spring, at green tip, when the leaves just barely show a bit of green on the branch, you spray with the fungicide labeled for Apple Scab.  There has been a lot of it this year.  Please check this link.  If that is not what it is, I need photos of the tree and where the trunk meet the ground and the sun/shade situation.  https://extension.umn.edu/plant-diseases/apple-scab