Well, eggplant (aubergine on my side of the pond) is a member of the nightshade family, so I wonder if that's got something to do with it.
Back when the duct tape study came out, one theory was that anything irritating the area enough to get the immune system to kick in would work.
For people who haven't ready the article, duct tape worked twice as well as cryothérapie.
We were told to cut the potato in half, rub it on the wart, put the potato back together and bury it.