The last couple of years, I've been treating my remaining rusty daylilies with a biologic fungicide called Actinovate. In spring, I douse the plants with it, then once the weather is warmer I spray every two weeks. I also use it on my plumeria, cannas and regularly on my orchids.
I'm having fairly good results with this, and it's more or less affordable. You buy Actinovate as a powder, and mix with water. It's not cheap, but you don't use very much either. There is a caution that you shouldn't store it too long, as it loses potency the longer it is kept. So I buy the small envelopes of it off amazon, or Gardener's Supply.
To be honest, though I've reduced my daylilies down to the few that are so far rust resistant - Siloam Double Classic, Licorice Twister and Buttered Popcorn do very well and stay clean with no spraying. The rest of my daylilies have either petered out or I've taken them to Utah, where I make a garden at my daughter's house.
Now, I'm only planting new cultivars that are rust resistant.
