Rust spots on ornamental rowan trees-advice needed

Art Works

Hi I have 5ft ornamental rowan saplings planted in my garden -The Sheerwater seedlings and Joseph Rock variety have rust spots on their leaves- does this require  treatment?- also what is it/ does it harm the tree? -also should they be fertilized-if so what do you suggest-?

-I would appreciate any constructive insights and suggestions-Thank you