Without more information my first guess is that you might have rust. Years ago I had rust on roses, cannas, hollyhocks, and a few other plants. If so, here's a couple things to try.

First, cut off all the infected foliage and get rid of it. Personally I would cut the whole clump down to the ground and dig and divide while you're at it. Don't put the clippings into a compost pile. Make sure that you clean up all the debris on the ground. You might check other plants and see if you have the same problem.

Rust is a soil borne disease and is usually spread by rain and sprinklers splashing it up onto leaves and stalks. You'll see it mainly on the undersides of leaves.

If you dig and divide, amend your soil with a good compost. Mulch with a good compost and keep an eye on the emerging plants. I kept my beds constantly mulched with homemade rough compost to keep the soil from splashing on the plants and buried soaker hoses under the mulch. I also used a summer oil spray on all the plants. Within two years I had it licked. In the Spring I raked up what was left of the old mulch and replaced it with fresh mulch. I tossed the old mulch just to be on the safe side.

That was no small thing as our neighbor's entire back yard was one huge mess of rust and we were downwind of it.

Thanks, Jenna

You've made me realize. I think the reason why that batch is infected and the others are not, is because it stands in a "wind corredor" between houses.

This weekend I did everything you said. Chopped everything down. I actually had about 80 canna in that area. Now there are 40 or so little babies, of about 6-12 inches high.

The area is now well ventilated, and I baught an anti-rust spray from the local nursery.

The only thing I haven't done is replace the soil and add mulch. I will be giving this some thought.

I have some red Ti plants which were also getting rust stains. For those I just ripped the stained leaves off as they don't reproduce so quickly.