Violet Rust (Puccinia violae, #470)

Seeing some leaves on Dog Violet starting to get deformed with discoloured blisters I was expecting to find some kind of violet aphid or mite, but the root cause turned out to be a fungus. After a few days the undersides of the impacted leaves develop brown/orange pustules which will release fungus spores. Strange to see such a disease form so quickly on fresh green Spring plant leaves.