There has to be something more than moisture in the ground for rot to form I think. I grow most of my daylilies in a very boggy area and have had almost no root rot. Some plants just seem to be more prone to it. I think there is some type of fungus in the ground that has to be present also for root rot to be a problem. We are not even sure that is what the problem is, so no need to panic for sure. Maybe contact you county extension agent and see if they can give some insight on the problem.
