That was for Diana since she'd asked about it.
It started in 1937 but became a specific law, that includes other crops as well, in 1949.
http://en.wikipedia.org/wiki/National_Raisin_Reserve
The Supreme Court heard the case of Horne vs Dept of Ag today....hopefully they will put a stop to this practice now that someone has finally decided to fight back.
http://www.washingtonpost.com/opinions/shriveled-grapes-shriveled-liberty/2015/04/17/2cb75458-e468-11e4-b510-962fcfabc310_story.html

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic