Where did the idea come from that the forbidden fruit in the Garden of Eden was an apple? Genesis just says "fruit." Does Jewish tradition have it as an apple, or is it strictly a Christian thing? Come to think of it, the fruit of discord of the Greek goddess Eris was also an apple. Why are apples considered to be the troublemakers of the produce world?
—Sluggo, via e-mail
The apple's many admirers like to portray it as a symbol of wholesomeness—apple cheeks, an apple for teacher, and so forth. Don't be deceived. This is a fruit with a history.
Let's review the story. Genesis depicts Adam and Eve leading the plush life in Eden. They 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. As you suggest, the original Hebrew says only "fruit," but in latter-day Western art ranging from serious religious painting to about a million cartoons, the item in question is invariably depicted as an apple.
But it wasn't always. 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.
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, which gives you an idea how things are starting to go, metaphorwise.
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 one you cite, 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 fact, you can read Christian iconography as a long, twilight struggle between figs and apples over which is the alpha temptation symbol. The apple has a lot to recommend it: red (blood) or golden (greed), round (fertility) and sweet-tasting (desire). The fig, on the other hand, has a certain phallic look, noted as far back as the ancient Greeks, who, admittedly, thought everything looked phallic. By the Renaissance, almost simultaneously we have Albrecht Dürer depicting Adam and Eve and the serpent with an apple (1504, 1507), and Michelangelo equipping the same cast with figs on the Sistine Chapel ceiling (circa 1510).
Ultimately the apple prevailed. In Areopagitica (1644), Milton explicitly described the fruit of the knowledge of good and evil as an apple, and that was pretty much the ball game. Islamic tradition, however, commonly represents the forbidden fruit as the fig or olive.
A related question: what's meant by the "knowledge of good and evil"? Take your pick:
Ethical discrimination, knowing right from wrong. One problem with this interpretation: if Adam and Eve had no knowledge of right and wrong before eating the fruit, how would they know disobedience was wrong?
Knowledge of sex. The first thing Adam and Eve do after their snack is realize they're naked.
Knowledge, period. In this view, "good and evil" is an encompassing bookend phrase, like "A to Z." Having tasted of the tree, mankind wants to know everything.
In any event, the gist is clear: knowledge = the loss of innocence; ignorance = bliss.