I would recommend Daconil, but remove infected leaves.

One of problems is I could not determine where the canna is growing. This is often important because certain pathogens are more prevalent in specific zones. The necrotic symptoms appear to me to be more closely aligned with a bacteria leaf disease (specifically-bacteria leaf spot).
These symptoms could be the result of a virus infection but does not appear to be either CaYMV or CaYSV.
Canna rust (fungal) is most prevalent in the Southern areas of the U.S. but can occur further north but most often in late summer and the fall season. Rust always begins on the lower side of the leaf surface with the onset of red postulates. Rust is typically easy to diagnose by observing the underside of the leaf prior to necrosis.
If possible, it would be better if your plant received more hours of sun and prevent overhead watering late in the day. Wet foliage for extended time will augment the onset of both bacterial and fungal disease.
I cannot personally rule out any of the three possibilities.

Kent