Its a fungal disease that has a two year life cycle requiring Eastern Red Cedar trees to be in the vicinity. No Eastern Red Cedar trees, no cedar apple rust. But if you can't get rid of the cedar trees in your area then you need fungicide coverage between first pink and three weeks post bloom. There is no secondary infection event for this disease so that is the window...pink to 3 weeks post bloom.
Captan and Chlorothalonil are labeled for it.
Or if you want to go a more organic route, copper and sulfur can work on it.