Cal. Food & Agr. Code § 58451
If any grower or producer of any farm product within this state voluntarily executes and causes to be delivered to a dealer or processor of farm products, either as a clause in a sales contract or other instrument in writing, a notice of assignment of dues to a nonprofit agricultural commodity organization directly representing the specific product involved, by which the processor or dealer is directed to deduct a sum from the price to be paid for such product and to pay the same over to such association as dues for the grower or producer, the processor or dealer shall deduct from the price to be paid for any farm product being sold by any such grower or producer to any such processor or dealer, the amount authorized and pay it over to the organization as assignee.
Ca. Food and Agric. Code § 58451