Cooperative Extension: Garden and Yard

You are correct in that it’s a type of Gymnosporangium rust, but it could be one of three distinctly different species of rust. Without a sample to look at more closely, I can’t tell for sure, but this site offers a great comparison that might help you narrow down the ID:  https://web.extension.illinois.edu/focus/index.cfm?problem=cedar-rust-differences. The fascinating aspect of this group of diseases is that they require two distinctly different host species in order to complete its life cycle. The most practical management advice is to prune out the orange areas to minimize the spread to the alternate host of this disease. Depending on the type of rust, the alternate host could be apple, hawthorne, quince, crabapple, pear, serviceberry. In some cases, it might make sense to remove the alternate host completely, but that may be too drastic or impractical for your specific situation.