How should one treat a Shmita etrog? Can it ever be discarded, and how?
After it has dried out and is no longer edible, it can be discarded like any other esrog. The minhag is that since it is a mitzva item, we don’t throw it out in an embarrassing way, and many people save their lulav etc. for using for wood for the Pesach fire. The idea being that an item that we already did one mitzva, we will use it for another mitzva.