At one time I had the notion that it became the apple becauase the OE word ęppel was a generic word for any kind of fruit. I also have a little note stuck to that entry in my Junk Drawer Memory® that says that's not correct. Not the bit about ęppel being the word for any kind of fruit, but the bit about that being the reason for the Fruit of the Tree of the Knowledge of Good and Evil being associated with the apple. Details are a little hazy but Nuncle z might shed some light on it.