According to Wikipedia, which is always right, "Dixie" has its origins with an Ohio composer, though the specific Ohioan is subject to debate. Its popularity comes from minstrel shows.
U.S. Grant, the General/ President, wrote that he only knew two melodies. One was the Star Spangled Banner, the other one wasn't. When I first read that I thought it was a kind of 'meta' joke, that Grant could recognize only one tune, and the other tune simply was something different. But I realized later that more likely it was his way of saying the other one was "Dixie".