The late blight we are seeing here in the Northern US and Canada can NOT overwinter (except on potato tubers.) See usablight.org for more information. It is only one mating type and two are required for the disease to become soil borne, and even if both were present in Canada IMO it likely wouldn't survive the winter.
If LB overwintered in the North, growing tomatoes or potatoes here wouldn't be possible, after years like 2009.
Drenching your soil with fungicide will only serve to kill benficial fungus in your soil and weaken future plantings by weaking the soil food web, IMHO.
I strongly recommend anyone growing potatoes or tomatoes read up on usablight.org, it is an invaluable resource.
The advice to rotate your plantings, while not a bad idea, generally applies to farmers and not small backyard gardeners who don't sterilize tools and don't have the space and only serves to discourage people from planting at all, IMO.