According to George Pullen Jackson's
White Spirituals in the Southern Uplands (as referenced in
John Shelton Reed's
Whistling Dixie: Dispatches from the South) in the
19th century to "Southernize" was a phrase used in American
music publishing.
Gospel songs originally written and published in
the North were given different
arrangements when re-published in
the South -- the non-
melody parts were made more complex and given their own tunes to sing instead of having just a few notes serving as
harmony to the main melody. "Southernized by" credits appeared on
sheet music along with the original composer's name.
"Jackson points out that this often worked to the detriment of the ensemble. It was less harmonious, more chaotic than the original, un-Southernized version. But what the hell -- these people weren't singing for an audience. They were singing for themselves and for the glory of God. If someone wanted to sing with them, instead of standing aside and finding fault with the harmony, he'd soon realize that the Southern way was more fun for the singers, whatever impression it might make on observers." (Reed uses this as a metaphor for Southern individualism.)