If you can't eliminate or treat the junipers (cedars) that are the hosts, the first line of defense is to plant resistant apple varieties. Unless all nearby junipers are removed, you cannot prevent the disease but you can attempt to control/minimize it. Since your trees are already established, you can try a fungicidal spray, however none are recommended for fruit you intend to eat.

Fungicides are available to protect trees and shrubs that are for ornamental purposes only.

• Fungicides with the active ingredient Myclobutanil are most effective in preventing rust. Copper and sulfur products can be used as well.
• Fungicides are only effective if applied before leaf spots or fruit infection appear
• Spray trees and shrubs when flower buds first emerge until spring weather becomes consistently warm and dry.

What? Can't use copper fungicide on fruit trees? I don't think that's right. I use it to keep the peach leaf curl in check, it's recommended for that. I spray in late fall when trees are dormant. I'd have to go find the bottle to know the brand, but it is a concentrate. Unless you mean copper with the Myclobutanil mixed in.

I don't know what to tell you. Check with your county extension agent for local advice. Different states/counties have different regulations but I'd assume spraying with what is registered for this disease while no fruit is present is acceptable. The MI extension website has several fact sheets on the subject but they link to OSU fact sheets and that link is broken.