There's a lot of scientific information available about daylilies that have been tested for performance with Rust, and also some for Spring Sickness. Keep in mind that a rating of "resistant" does not mean a cultivar will not get these diseases. "Resistant" only means that they were observed to get 50% or less of their foliage affected with those diseases, and thus performed well enough to be considered acceptable garden and helpful breeding specimens. Resistant cultivars were seen to self-recover without any treatment and were found to be presentable for garden display with foliage trimmed to remove (and discard) affected leaves.
Averaged Rust rankings can be found for individual cultivars in the ATP database here.
Or, you can search for all ranked cultivars in the Search by Characteristics page here.
More information about the sources of the scores is here.
Scores for cultivars that were evaluated for Spring Sickness are linked at the bottom of the post shown: here.
Additionally, listed below are some of my own daylily recommendations for cultivars that have been tested, or have lineages that show testing, for both rust and spring sickness. Recommendations come from a non-commercial, home hobby-hybridizing garden in a coastal state with temps ranging from 25-105 degrees, marine and desert/sandy soil, and low rainfall.
I'll update recommendations over time, so you are welcome to check back to see more listings, and feel free to send a tree-mail if you have questions or feedback about any of the daylilies listed. I'd love to hear from you about any that are growing and doing well in your own gardens!
Just a few examples of Daylilies that are, or have lineages that include cultivars that on average tested resistant (50% or less foliage affected) for rust and spring sickness:
Daylily (Hemerocallis 'Dil-Robin Nichols')
Daylily (Hemerocallis 'Handwriting on the Wall')
Daylily (Hemerocallis 'Little Liza Jane')
Daylily (Hemerocallis 'Open Secret')
Daylily (Hemerocallis 'Regal Giant')
Daylily (Hemerocallis 'Tremor')
Daylily (Hemerocallis 'Woodside Ruby')
Updated June 2019, and to be continued as seasons go by ... !