But it did get me thinking and like any good lecturer I went away and came back with all the info and will share it here!
The Book of Genesis depicts Man and Woman (Adam and Eve) leading the good life in Eden. God decrees that the may eat fruit from any tree except one, "the tree of the knowledge of good and evil." Unsurprisingly, they eat the forbidden fruit and are expelled from paradise. The original Hebrew says only "fruit," (gah I can't get the Hebrew font in!) but in latter-day Western art ranging from serious religious painting, to cartoons, to hair care products, to perfume, the item in question is invariably depicted as an apple.
Early rabbis suggested the fruit was:
The Fig because the next verse mentions sewing together fig leaves to make loincloths;
grapes, which later cause trouble for Noah, not to mention many other vino lovers;
The Citron, a lemonlike fruit which in Hebrew is etrog, a pun on ragag, "desire";
Wheat, khitah in Hebrew and thus a pun on khet, "sin" - a stretch, considering wheat isn't a fruit and doesn't grow on trees; or
The Carob, because in Hebrew its name puns on the word for "destruction."
Many modern scholars think the author(s) of the text had the pomegranate in mind.
The Book of Genesis doesn't mention apples, but Proverbs 25:11 says a timely word is like apples of gold in a setting of silver. More significantly, in the Song of Solomon the apple is an erotic symbol indicating sweetness, desire, and the female breast.
Early Christian scholars often took the forbidden fruit to be an apple, possibly because of the irresistible pun suggested by the Latin malum, which means both "apple" and "evil." At least one early Latin translation of the bible uses "apple" instead of "fruit." A contributing factor no doubt was that apples were a lot more popular in Europe than in the Middle East, where it's generally too hot for them to thrive.
It wasn't just Christians who picked up on the apple's racy side. The most famous apple of Greek myth is the gold apple labeled "To the fairest" that Eris, goddess of discord, throws among the guests at a wedding party, leading to the judgment of Paris (he has to choose whether Hera, Aphrodite, or Athena is the most beautiful) and ultimately to the Trojan War. You get the picture: apples may look good, but they're trouble. Christian scholars knew the Greek myths and adapted many to their new religion.
Still, the apple wasn't the unanimous choice for forbidden fruit. Carved depictions of Adam and Eve with apples are found in early Christian catacombs and on sarcophagi. The apple was the favored representation of the forbidden fruit in Christian art in France and Germany beginning around the 12th century. But Byzantine and Italian artists tended to go with the fig.
In Areopagitica (1644), Milton explicitly described the fruit of the knowledge of good and evil as an apple, and that was pretty much it for the apple!
Some Advertising reasons I might have got confused: