I managed to get some slips started from a store bought (treated) sweet potato in May this year. Took a very long time to get going but eventually got 3 slips started. One of the three has two leaves affected by what looks like a powder, but it is not loose powder. I tried to rub some off of the leaf at left but nothing detached, I only managed to crush the surface of the leaf. The underside of the leaf looks completely normal.