Certified GKH Gardening Expert
I believe this to be a fungal issue. I have a method that I use to cure, and prevent further disease. It is alright if you don't have every ingredient, but the soil priming you should not skip. the recipe is as follows:
First, you will want to prime the soil with dolomitic lime, then spread wettable sulfur to Kill off any infection in the soil. I Also like to spray the plant itself. Mix a few drops of one or more of these ingredients into warm water with a spoonful of coconut oil: Rosemary oil, lavender oil, neem oil, garlic oil, cinnamon extract/oil and for really bad cases, also add clove oil.
Here is an article with more information on the plants:https://www.gardeningknowhow.com/ornamental/bulbs/canna/canna-lily-care.htm