Daniel - I do agree with you on the points you made. I get most everything you can imagine including Spring Sickness, but leafminer has been a very minimal issue so far here. My humid and hot climate seems to attract the bad. I started growing daylilies because I had also heard they had very little disease and insect issues. So not true!!! Maybe years ago it was, but not anymore. But something that I have discovered ... some of the new daylilies I have received and transplanted into my garden seemed to be prone to problems. A couple of them I did in fact lose, but most just took a beating from the transplant stress of heat, disease, and pests. I have developed the attitude that if it is a strong plant that can adapt to my climate and environment and overcome health issues, then it is a good one to keep in my yard. Surprisingly, most of my newly acquired daylilies (acquired within a year) usually surprise me and step it up the 2nd year. I have learned to get them situated and then leave them alone except for cleaning up debris around them, fertilizing, and watering. I grow organically, so I am not big on chemicals at all. I do try organic or less chemical treatments if I am trying to save plants, but usually I let them fight it out with whatever is ailing them.
The only plants that I have that seem to be disease and pest free are native plants. If it originated here, then it is very hardy and healthy and typically remains so.
I can not grow ditch lilies here. I had tons of them (30+) that I got in a trade years ago and they slowly died one by one. No disease was apparent. I think it was the hot and dry late summer months that did them in. After 3 years, they are pretty much gone. I have 2 or 3 that I still see little leaf blades coming up, but they are struggling and never get very big and never bloom anymore. Maybe I should pot them up and see what happens, but honestly it really isn't worth my time. If I plant them back into the ground, they are goners most likely.
