Sumo / Dekopon / Shiranui cultivation insights
The history on the Dekopon is that after the cross was made, initial trials reported poor results and the industry shelved it. One grower, though, 'liberated' some budwood and went about trying to make it work. Over years, he figured out how to prune, when to pick, how to store, etc., and turned it into a winner. The rest is history (in Japan at least).
I ran across this article where some folks took a tour at a grower and the grower seemed generous with cultivation info. I've posted the link below.
Here is a link that might be useful: Sumo growers' tips