Rust would show up first as yellow spots on upper leaf surfaces, followed by orange/red dots on the undersides.

Rust: Caused by fungi of genera Puccinia and Melampsora... take off the affected parts, to improve the ventilation and spray with an appropriate fungicide.

Check at your garden center, there are several products available, i.e. Daconil, Banner, and Bayleton which is somewhat easier to use. Bayleton is absorbed systemically so you don't have to be quite so careful to spray every underside of every leaf...