Lalambchop1 said:Can rust kill a DL?
Typically rusts don't kill plants - they need living tissue to feed and reproduce so if the plant dies the rust dies too except for any spores it has managed to produce - but those spores themselves can only infect living tissue so that can be a "dead end" anyway depending on the life cycle of the particular rust. As Larry said, a plant that is weakened by rust (the fungal body inside the plant is "stealing" food from the plant and interfering with photosynthesis) may possibly be more susceptible to other problems. A few rust diseases are systemic (such as orange bramble rust) and therefore the plant is never rid of them, but fortunately that doesn't apply to daylilies.
In practical terms, though, in areas where rust is endemic and winters not cold enough to kill the foliage and/or the daylilies are evergreen year round, plants can presumably be infected and re-infected continually as long as environmental conditions favour the rust, which could be pretty hard on them if they are not rust tolerant (different from resistant).
Some cultivars have a reaction to the rust that results in the foliage dying back, or at least forming dead brown areas around the pustules (called a hypersensitivity reaction) - this was thought to be a form of resistance, i.e. kill the leaf or surrounding area and you kill/starve the rust. I think I read something recently that cast some doubt on that sequence of events but I'd have to try and find it again. There's also the "green island" effect where tissue remains green around each pustule, which would enable the fungus to keep feeding even while the rest of the leaf is dying. (There's a picture of that on the FAQ link I gave above - BTW I'm glad the other pictures on there were useful to some of you). So what with all that and the multiple life stages, rusts are quite complex diseases.
Regarding Becky's question about whether the non-survival of dormants could be related to being infected - dormants, or at least some dormants, have had a reputation for not doing well in hot climates since well before the advent of rust in North America, so it's hard to say what influence the rust might have had in an individual case. The species daylilies still survive in Asia, are often badly infected by daylily rust, and the majority of those are deciduous. The full life cycle in daylily rust that requires patrinia as an alternate host is based on daylilies being deciduous. I guess when the rust fungus arrived in North America and found all these evergreens in warm climates, meaning it didn't even need an alternate host to persist from year to year, it went
