... the stereotype that African Americans are excessively fond of watermelon emerged for a specific historical reason and served a specific political purpose. The trope came into full force when slaves won their emancipation during the Civil War. Free black people grew, ate, and sold watermelons, and in doing so made the fruit a symbol of their freedom. Southern whites, threatened by blacks’ newfound freedom, responded by making the fruit a symbol of black people’s perceived uncleanliness, laziness, childishness, and unwanted public presence. This racist trope then exploded in American popular culture, becoming so pervasive that its historical origin became obscure. Few Americans in 1900 would’ve guessed the stereotype was less than half a century old.
He traces the use of watermelon as a symbol of contempt to "the early modern European imagination" in which "the typical watermelon-eater was an Italian or Arab peasant." when transplanted to North America, this hostile image took on a more particular racial aspect:
These tropes made their way to America, but the watermelon did not yet have a racial meaning. Americans were just as likely to associate the watermelon with white Kentucky hillbillies or New Hampshire yokels as with black South Carolina slaves. ...Black closes his piece with this useful observation:
This may be surprising given how prominent watermelons were in enslaved African Americans’ lives. Slave owners often let their slaves grow and sell their own watermelons, or even let them take a day off during the summer to eat the first watermelon harvest. ...
But Southern whites saw their slaves’ enjoyment of watermelon as a sign of their own supposed benevolence. Slaves were usually careful to enjoy watermelon according to the code of behavior established by whites. When an Alabama overseer cut open watermelons for the slaves under his watch, he expected the children to run to get their slice. One boy, Henry Barnes, refused to run, and once he did get his piece he would run off to the slave quarters to eat out of the white people’s sight. His mother would then whip him, he remembered, “fo’ being so stubborn.” The whites wanted Barnes to play the part of the watermelon-craving, juice-dribbling pickaninny. His refusal undermined the tenuous relationship between master and slave.
It may seem silly to attribute so much meaning to a fruit. And the truth is that there is nothing inherently racist about watermelons. But cultural symbols have the power to shape how we see our world and the people in it, such as when police officer Darren Wilson saw Michael Brown as a superhuman “demon.” These symbols have roots in real historical struggles—specifically, in the case of the watermelon, white people’s fear of the emancipated black body. Whites used the stereotype to denigrate black people—to take something they were using to further their own freedom, and make it an object of ridicule. It ultimately does not matter if someone means to offend when they tap into the racist watermelon stereotype, because the stereotype has a life of its own.A much longer version of Black's article appears in How Watermelons Became Black: Emancipation and the Origins of a Racist Trope Journal of the Civil War Era (March 2018)