It shall be unlawful to sell or offer for sale by variety name, seed not certified by an official seed certifying agency, when the variety is one for which an application for a certificate has been filed, or for which a certificate of plant variety protection has been issued under authority of the Plant Variety Protection Act (7 U.S.C. § 2321 et seq.) requiring that the variety is to be sold as ‘Certified Seed’ only.