I have never had much success rooting Diffenbachia in water.My usual way of handling these is to cut them in sections,each with an eye or visible node and then placing these on top of pots of moistened vermiculite, with the node in contact with the moist medium. This method seems to provide the ideal amount of water without over doing it so as rot sets in. Roots are sometimes slow in developing as in several weeks or more. Hope this is helpful. Ross.

I would use the method illustrated at propagating dieffenbachia

My dumb cane rotted at the soil line this year, so cutting the top cane into 3-4 inch chunks [with visible nodes showing] and got six of them. One I tried to root in water ... the others I placed in a pot. The placed-in-a-pot ones took off after about two months. They each are enjoying a new pot now.