I recently bought an infected canna. I knew it was rust when I bought it, but I really liked the variety, all the others were infected, and I didn't own any other Cannas... so I took the risk and bought it diseased.

I used the following regimen for about two weeks before I finally eliminated it. I say that with my fingers crossed, I haven't seen it in a couple of weeks.

What I did is soak a lot of cornmeal in water, I allowed it to begin brewing, and I dumped this into the soil.

I then took Kefir, a fermented milk beverage (composed of 20-40 different symbiotic bacterial and yeast strains), and painted it onto the 'bad' sections of the leaf.

The next day most of the canna spots had turned brown, and seemed to be inactive, so I made up a spray diluting the kefir down, and began spraying the leaves with it every night as the sun went down.

The spray knocks out the existing rust spores, but the problem is that new ones will come back up. I guess this is from the spores they had originally dropped in all the soil around the plant, so I started spraying about a foot or so around the canna as well, on the wall behind it, the ground, the grass, etc. Just to make sure the kefir had come in contact with everything.

After the first few days, I only noticed a few spots on a single leaf each day, then every other day, and then it basically stopped. I had to be very proactive about it making sure to spray after it had rained, etc. but eventually it seemed to suppress it.

Why I think the kefir spray works. A few random guesses.

1) very acidic, it might mess with the surface chemistry of the leaf, bumping it outside of the ph range for the rust. the chemicals in kefir also are known to suppress a lot of other bacteria and fungi.

2) stimulation of plant defenses. kefir is composed of trillions of colony forming units of bacteria and yeast. none of the bacteria hurt the plant, you can spray it on a plant all day and not really hurt it, but i'm guessing the bacteria (there are 20-40 strains in kefir) or some of their by products in the kefir activate their plant defense systems.

3) the kefir bacteria might actually eat the canna fungus. kefir is a strange substance... all the bacteria have adapted to live with one another peacefully but to kill off other types of bacteria and yeast. supposedly when exposed to foreign pathogens like e coli or salmonella the bacteria and yeast in kefir will begin engaging in "chemical warfare" with one another. some of the bacteria also serve as the food source for the yeasts I believe.

also, unlike other yogurt products, kefir contains lactobacillus plantarum (http://en.wikipedia.org/wiki/Lactobacillus_plantarum) , which i believe colonizes not only the gut, but the soil. its possible that the kefir bacteria and yeasts are living on and positively affecting the soil that the canna rust is colonizing on.