Hi! I have a set of large dieffenbachias that I propagated from a plant too big to move. The ones I rooted up in water are doing fine, but the ones I rooted up in soil have some kind of recurring infection at the cut. Every few months, the cut gets mushy and dies back a node or two, but then the infection stabilizes and scabs over. (The one in the photo has just scabbed back over.) Any ideas what I could be doing to keep the infection from recurring?

More information: The soil was new and sterile when I planted the cane, and I cleaned the knife with alcohol before cutting the cane. The plants spent some time outside last summer, inside in a slightly shaded south window, and inside in a bright west window. The inside air has been pretty dry this winter. I tend to let them dry out a bit between waterings, but not let them go bone-dry, but since the infection restarted I've been watering them less.

At first I suspected that the infection was erwinia rot, but now I'm not totally sure-- I recently lost one of these dieffenbachia cuttings to an infection that was DEFINITELY erwinia, and that infection had a distinctive smell and spread rapidly throughout the stem from the soil line. Not only am I worried about salvaging these cuttings, I'm worried about the prospect of transferring my water-rooted cuttings to soil. Are they just going to pick up the same infection?

Thanks for any thoughts!