Rust doesn't just mean oxidized metal. It is also the name of a deadly plant fungus.
Rust occurs when temperatures are warm and damp, like most spring mornings. It is easy to identify. Simply look at the underside of leaves and you will see bright orange or red spots of growth.
There is no known treatment for rust and it is easily spread by ants, aphids, and garden tools. If you find a leaf infected with rust, remove it and throw it in the trash. Always be sure to disinfect your tools after removing rust-infected leaves, to avoid spreading the fungus to healthy plants.