| Bacteria? But you didn't say anything about the rotted callas smelling really bad. And they should. And I mean *really* bad! If no icky smell, then likely an environmental glitch -- such as soil too wet. Although I must add that the large white callas grow streamside in their native habitat (but they do have good drainage rather than standing water) whereas the callas with colored blooms tend to be more sensitive to wet soils. So, which do you have? The large whites or the smaller colored forms? Also, because I'm not familiar with your temperatures, I have to ask if they survive your winters outdoors. Or are you trying to grow them as annuals, then take indoors for the winter? | 
