Could just be rust. What sun conditions is the canna grown under? What do the newest leaves look like?

Hey mantis,

4 hours of sun per day. The newer leaves are okay but it seems to be getting worse overall.

Do you think I should remove the affected leaves?

I would remove the affected leaves. Seems like rust to me. Try spraying with something like Daconil. I had a Shenandoah that did the same last year. Four hours isn't much.

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

Hey folks,

It's being grown in Northern Virginia. I don't have a spot with more sun available.

I'm surprised by all of this because the leaves have barely been wet at all over the last month and a half -- it has hardly rained, and when I water I don't wet the leaves.

Plethora87,
I don't believe any of us can give you a definitive answer but even though you have had little rain or do not water the foliage the photo indicates that the lower leaves seem to be the most affected? Do you have dew in the mornings? If so, the lower leaves will be the last to dry. Your limited availability of sun would contribute to the possibility of the lower leaves remaining damp longer.
Do a search for "bacterial leaf spot" and compare the damage to your plants. Bacterial leaf spot is more commonly seen in field grown cannas where the soil splashes or is blown onto the plants lower leaf surface but it can be transmitted in other ways. It is not typically seen in container grown stock unless the foliage remains wet for extended hours. I have seen it in greenhouse seedlings and small cannas where the water collects in the whorl. I have observed this condition often in the field (growing in the dirt).
We still can not rule out rust, virus or even a combination of these problems.
Best of luck,

Kent

Thanks Kent! Very much appreciated. I removed the affected leaves and sprayed Daconil so we'll see what happens. I'll be more vigilant about keeping things dry as well.