It was a bewitching shade of purple and green back in May (you can see a leaf that's more like what it should look like in the background). And it was in a gold medal winning garden. Just goes to show that even RHS judges can be wrong sometimes.
Now I'm not a canna expert but I'd say this was a virus. I was talking to the nice man at Hart Cannas earlier this year for an article, and he tells me virus is rife in the canna industry - in fact it's got so bad that it's far more likely that a canna you buy will have a virus than not. Apparently it's all because a shipment of cannas to the UK from the Netherlands was riddled with it, and it's spread like wildfire ever since. At Hart Cannas they're actually starting again from scratch, breeding their own virus-free stock: they don't have a lot yet, but it's building up and I think may soon be the only refuge for those who like their cannas without horrible streaks, mottlings and leaf puckerings.
The only option for me with this one is to dig it up and burn it. Rats.