I'm fighting a delaying action against downy mildew on my onions for the second consecutive year, pruning infected foliage and spraying with Actinovate and Serenade. I'll have some onions to harvest, but they're small. What can I do to prevent downy mildew next year?I've destroyed my perennial onions; I am dumping onion debris under black rubber, in the sun, far from, the garden; I will make sure of a 5-year rotation. What else? Does it help to heat the soil with black plastic? To leave the onion beds unmulched over the winter and hope they'll freeze? To drench the soil with an organic fungicide? (Non-OG fungicides aren't an option). Any advice greatly appreciated.

Joanna