<human>:  When did tobacco become a cash crop?
<bot>: Tobacco became a cash crop in the late 1700s, when it was cultivated in the southern United States. Tobacco was a major crop in the American South in the 18th and 19th centuries, and was one of the most important crops in the U.S. economy. The U.S. was the largest exporter of tobacco in the world, and the American South was the largest producer of tobacco.