Wow! for a New Yorker hate to say I had no idea why it was called the "Big Apple" but I found some information

In the early 1920s, "apple" was used in reference to the many racing courses in and around New York City. Apple referred to the prizes being awarded for the races -- as these were important races, the rewards were substantial.
John Fitzgerald, referred to New York City's races "Around the Big Apple." It is rumored that Fitzgerald got the term from jockeys and trainers in New Orleans who aspired to race on New York City tracks, referring to the "Big Apple."
http://gonyc.about.com/cs/atozinde1/a/bigapple.htm