Becky, I am in zone 6b. We had such a drought in 2012 that my husband watered everything to keep them alive. I believe (maybe wrongly) that by watering from the top, that the rust that was already there was allowed to spread through all the daylilies even faster. It may be that it was going to do that anyway since I did have some evidence of rust before. I just didn't have it bad enough to worry about spraying.
