I've got a nice dieffenbachia, seems perfectly healthy and is growing well. But I noticed a spot of mushy rot on the stem. Stem is 2 1/2" thick and the rot part is about 1" into this. This spot is well above the soil and seems to have come from a leaf that's previously been there. It has good air circulation, so this is sorta odd for it to be here. Hmmm. Question...is this reasonable to excise the rotted part and hope for the best or is this a lost cause? Is there anything I can apply to the cut part that will allow it to heal? Is this fungus or bacteria? Hate to lose it...I know they're plentiful at big box stores, but well...you know how we plant people are.