Where a denomination of a variety or its propagating material or essentially derived variety or its propagating material registered under this Act has been applied to the variety or its propagating material or essentially derived variety or its propagating material, as the case may be, on sale or in the contract for sale of such variety or its propagating material or essentially derived variety or its propagating material, as the case may be, the seller shall be deemed to warrant that the denomination is a genuine denomination and not falsely applied, unless the contrary is expressed in writing signed by or on behalf of the seller and delivered at the time of the sale of the variety or its propagating material or essentially derived variety or its propagating material, as the case may be, on contract to and accepted by the buyer.