Do all daylilies catch rust?
Apparently, yes. Matthew Kaskell reports that all daylilies in his garden show rust, but in differing amounts. Others report that species catch rust too. A daylily that could not catch rust would be properly termed a non-host or insusceptible. Immunity in a scientific sense means insusceptibility due to training of an animal immune system (plants don’t have immune systems), but since no daylilies seem insusceptible, the term probably shouldn’t be used as a casually either. All daylilies seem to be susceptible to rust, as opposed to resistant to rust. Resistance means relative difficulty for infection to occur. No resistance has been reliably reported yet in daylilies. The variation that Matthew and others report is properly described as tolerant versus sensitive. A tolerant plant will have the disease, but not be much affected by it. A sensitive plant will be severely affected by the same disease.
