Apple scab could be the problem. Sanitation is one of the tools.  Clean up all leaves and fruit as they fall and dispose of them in the garbage.  In the very early spring at what is referred to as green tip, when the leaves just barely are starting to show a tip of green, you spray with the appropriate fungicide.  You cannot cure it or fix the problem later in the year.  https://extension.umn.edu/plant-diseases/apple-scab