Rust is a fungus that is most prevalent on the West Coast in the United States, although it is seen elsewhere. Conditions that are favorable to it are rain, the more rain the worse the infestation, and temperatures over 65 F. Orange spores first appear on the underside of a leaf and become visible on the upper surface of the leaf as they become larger. The fungus resembles curry powder.
[From Roses, by Susan Bales, p. 84:] Rust takes the form of small orange or yellow bumps that form on the underside of leaves and then spread to the top of leaves and stems where they become conspicuous... Rust is a problem where summers and winters are mild.