Mint rust is caused by a fungus, Puccinia menthae, which only infects plants in the mint family, especially spearmint and peppermint. It is encouraged by overhead irrigation, which frequently allows water to stand on plant leaves long enough for the fungal spores to germinate. Closely planted mints, or those that need to be thinned, are at a heightened risk due to increased humidity around the plants.
Rust on mint plants looks similar to other rusts in later stages, with orange to rust-colored spots covering the undersides of lower leaves in early spring. Mint rust symptoms may progress, manifesting as leaves that turn completely brown and drop from affected plants. In late summer and early fall, when these dropped leaves regrow, darker spots often appear instead. The very early stages of mint rust may appear as white bumps on mint leaves.