Subsistance farming for a limited number of people can be done by burying potatoes in the ground and digging them back up in the autumn. Well, it's a bit more to get a good yield, but not that much more, you can (and some places do) teach that to literal kindergartners in a single day. We can't sustain 7 billion of people this way, but if some catastrophe kills 99% of humanity, the availability of better cultivars makes subsistence farming for the remainder much easier than five hundred years ago.