It affects the leaves and fruit. Depends on the variety how severe it may be and the weather conditions. Some apple varieties are resistant or might just have a few spots on the leaves. More serious cases can lead to defoliation of the tree and damage the fruit. I've seen recommendations to cut down red cedars within a few hundred yards of the apple trees to minimize the amount. There are a variety of rusts as well. Cedar apple rust, cedar hawthorne rust, cedar quince rust, and a few others. Can affect apples, crabs, pears, hawthornes, serviceberry, cottoneaster, mountain ash. All from the rosaceae family.
Doesn't mean you can't grow apples but if you have lots of red cedar but reading up on CAR resistance before deciding what to plant would be a good idea.