Hi Uli, I have had problems with a dry rot during summer storage of winter growers. Last year was very bad. Unlike your description, my infection turns the bulbs hard and if broken, they are powdery and tan-colored. It seems to be an infection that I suspect is spread by fungus gnats in the spring. It doesn't seem to matter how the bulbs are stored or if they get supplemental water during the summer. I've learned that even if some bulbs in the same pot with infected bulbs look ok, they rarely are and don't wake in the fall. All pots are treated for gnats, but they breed in the ground here so they still investigate the pots. My best defense is keeping multiple pots of each species in case a pot succumbs. If anyone knows a way to treat a pot so that the gnats won't run all over it, I'd love to hear it. monica