I wouldn't say it's exactly like the flu, where you get different strains circulating in a particular year and then different ones in another year but that's an interesting question. I would assume as each strain evolves in one garden, it could be shipped to another or spread on the wind locally. Unless it dies out it could still be in circulation or it could remain local. But yes, new strains can keep on emerging but I don't think at a high rate when you consider that there is a new generation of spores every couple of weeks under the most favourable rust environmental conditions, and each rust spot can produce 1-2,000 spores in 24 hours.
It's relatively new in the United States, first reported around the year 2000. It's been known since the 1800s in Asia.
There have been formal scientific tests (the numerical rust ratings in the NGA database are based on those). The database also has another system of shows resistance or shows susceptibility. Since it only reflects what the last person to edit it entered, I wouldn't personally find a "shows resistance" rating very useful simply because susceptibility varies depending on the environmental conditions. Scientific research can only do so much because there are just way too many daylily cultivars for more than a small proportion of them to be tested.
Yes, a daylily may be more susceptible or resistant to different strains. We had hoped this rust would be slow to develop new strains because it doesn't appear to be completing its full life cycle in North America as it does in Asia (which involves patrinia plants) but it has managed to do it anyway.
Hope I answered all the questions adequately.
