Larry, your link, like Maurice's, is from the American Phytopathological Society and written by two specialists in daylily rust, so therefore should be credible concerning plant diseases. In fact one of the authors, Dr. Ono, years ago supplied me with the images of infected patrinia and the life cycle images of daylily rust for my daylily rust info site. I co-authored two articles with him for the AHS's Daylily Journal on daylily rust and hosta rust. He did tell me the same thing but apparently they don't much grow daylily cultivars in gardens in Japan anyway. I don't know if the cultivars they do grow were developed there, in which case maybe they selected against the rust, which has been there for at least a hundred years as far as I recall. As you say, it can't be the climate since the wild species there get rust. Could it also perhaps be that in urban residential gardens there is some protection from wind blown spores from the wild areas, perhaps?
Edit: I should add that most wild daylilies in Japan are deciduous so the rust needs patrinia in order to return the following year - I don't know if that also applies to the cultivated daylilies but perhaps could also be a factor.
