Question 3 on this FAQ addresses the temperature question:
http://web.ncf.ca/ah748/FAQ.ht...
Unless you have rust already, I don't think there's any reason to remove foliage. If you do get rust at some point it may help because you're close to the borderline area for rust winter survival, which is roughly considered to be Zone 7 (if you winter mulch that would possibly increase the risk by preventing more foliage from dying back). If you had patrinia plants in the vicinity then it could overwinter much colder than zone 7 as it does in Asia, but so far it doesn't seem to be doing that in North America. Usually when it re-surfaces in colder zones it is because it came back on new plant arrivals that year.
In theory, yes it could get carried on seeds from an infected garden. But the spores would have to remain viable until there was green growth from the seed otherwise they'll eventually just die. The spores would also have to get onto the green growth otherwise they can't infect (more likely where people germinate seeds all together in a baggie or seed starter or something and not in soil, so that they touch each other).
