Don't bother, I think I found it, something called "canna rust," which has spots on leaves turning a brown-black color.
Canna rust, caused by the pathogen Puccinia thaliae, is a common fungal infection of Canna indica.

Read more: Common Fungi Responsible for the Spotting on the Leaves of the Canna Indica | eHow.com Common Fungi Responsible for the Spotting on the Leaves of the Canna Indica | eHow.com
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Common Fungi Responsible for the Spotting on the Leaves of the Canna Indica | eHow.com