I am more concerned with introducing root-knot nematodes into my field with vendor tubers than I am with introducing mildew spores with vendor cuttings. I almost never plant tubers from vendors directly in my field, instead I take cuttings and plant the cuttings. Since vendors cut off all the feeder root before storing/shipping tubers, it is very difficult to diagnose a root-knot nematode infection until the tubers are dug in the Fall.
Most, if not all, of the organic fungicides are preventative for powdery mildew but do not do much for an established infection. Of the organics, probably the horticultural oils are the most effective. Sulfur (micronized) is probably equally effective but do not use when spraying any oils. My father used to swear by Compost Tea but it is tricky to make and can be dangerous if not done properly. He would buy compost made specifically for compost tea and aerate the mixture while brewing. It was quite a process but worked for him.
