Linda,
This looks to be a fungal issue (could be bacterial, but I doubt it). No visible signs of virus here. It's a rot of some sort.
I am unsure why, but I notice that cut leaves of my Section
Mastigopetalum species (which includes
Paph. gardineri,
Paph. praestans,
Paph. rothschildianum, etc.) will be infested with fungus just days later (leaves discarded in the garden). After about a week, I notice that they look like old, shriveled, black banana peels...
Make sure you keep that leaf FAR away from your plants, just to be safe. I would assume that it is a leaf of either a previously bloomed growth, or a mature growth that hasn't bloomed after 2 seasons?
...and perhaps a lower leaf?
-Pat