Canna rust

Canna rust is a fungal disease of canna cause by Puccinia thaliae. Symptoms include orange spots on the plant's leaves and stems. In advanced stages of infection, the upper leaf-surface spots coalesce, turn dark brown-to-black and finally the infected leaves become dry and fall.

Control
When Canna rust first appears, the affected foliage should be removed and discarded, otherwise the fungi will propagate and destroy the whole plant. The affected foliage should not be composted, as that will simply spread the fungus further.

Commercially, a liquid copper-based spray can be sprayed on the stems and both sides of the leaves to prevent fungal problems before they occur. Although used commercially, copper is not recommended for home use because of its toxicity.

Links

 * - British Society for Plant Pathology report on Canna Rust