Brad, the iris people around here said that rot was Botrytis. I don't know if that is really what it was, but that was the general consensus
. Not sure what causes it or how to prevent it. It was widespread in this area last spring, but varied greatly in severity from garden to garden. I lost a few seedlings to it, but just one named plant, Ghio's Boy Genius. It did reduce my bloom though, especially on newly planted irises as it affected blooming sized rhizomes more severely than the smaller ones.