It shall be unlawful for any person to disseminate, or cause to be disseminated, any false advertisement concerning seed, by the United States mails, or in interstate or foreign commerce, in any manner or by any means, including radio broadcasts: Provided, however, That no person, advertising agency, or medium for the dissemination of advertising, except the person who transported, delivered for transportation, sold, or offered for sale seed to which the false advertisement relates, shall be liable under this section by reason of disseminating or causing to be disseminated any false advertisement, unless he or it has refused, on the request of the Secretary of Agriculture, to furnish the Secretary the name and post-office address of the person, or advertising agency, residing in the United States, who caused, directly or indirectly, the dissemination of such advertisement.