Okay, so this is not about Oca. I am growing yacon again this year to try as a bi-crop with Oca, and as I was watering yesterday, I noticed one yacon plant that was different:
The plant has grown from a propagule consisting of about three little tubers, and only one of these has produced the non-standard leaf pattern, so it looks like the parent plant was normal, and the mutation has occurred in the replant tuber.
The plant is the most vigorous of all that I have, and the stem in question is the most vigorous on the plant.
The root system is strong and healthy. Perhaps this will be a yacon with 50% extra.
Update 4/9/10. The stem is flowering now.
Update 28/3/11. Saved tubers killed by heavy frost during the winter.