Third Q and A down on this page, but basically to be safe you need for it to get cold enough for all the foliage on all the daylilies to be killed down to the ground all at the same time for an as yet undetermined period of time. Cold alone does not kill daylily rust spores, and the fungal body inside the leaves may be able to survive the same temperature that the leaves can survive. That's going to vary by cultivar and probably local climate.
http://web.ncf.ca/ah748/FAQ.ht...
