Kevin, does the dandruff look like the bottom picture on this page (shed aphid skins?):
http://www.daylilies.org/ahs_d...
And does the yellowing look like either of these?:
http://www.daylilies.org/ahs_d...
http://www.daylilies.org/ahs_d...
It's not likely to be rust in your area unless you've acquired new daylilies from an infected garden this year. Daylilies like moisture so it would have to be very wet for them to turn yellow from lots of rain.
If you could post a picture of what the problem looks like it would help figure this out.
