On August 2, 1937, President Roosevelt sign into law The Marijuana Tax Act, requiring all growers, sellers, manufacturers, importers and distributors to register with the Treasury Department. It also placed an occupational tax on growers and placed prohibitive taxes on most Cannabis transfers.
The extremely high tax on cannabis made it financially impossible to grow hemp. Some people believe that the act was a corporate-led conspiracy put into place after seeing the hemp industry as a threat to the king industries (oil/gas, cotton, timber, chemical/pharmaceutical.)
Tax Stamp for producing cannabis (auvergnefarm.wordpress.com)