No, we don't know. Genesis simply says 'fruit of the tree of knowledge'. Almost every fruit under the sun has been suggested, buy there's simply no way of knowing.
If you look closely into myths and stories you see the apple portrayed a lot as an important fruit. Like in snow white where she goes into a coma from eating an apple. Or perhaps when the bibles were written apples were just a rare fruit considered to be "forbidden". We can't know.
The Bible was originaly writen in Greek. And in greek they used the word fruit. When it was translated the word apple was put in it's place. Because at the time the word apple was used to describe all fruit. it was only later on that other terms were used.
Thus, that is why "fruit" is somtimes confused with "apple"