8. A growing crop certificate must not be issued containing a statement that seed potatoes are capable of being marketed as basic seed potatoes of any class unless, in the opinion of the authorised officer, the immediately succeeding crop produced from the seed potatoes is likely to contain—
(a)not more than 0.25 per cent by number of growing plants which do not breed true to variety and not more than 0.1 per cent by number of plants which are of a different variety; and
(b)not more than 4 per cent by number of growing plants which show symptoms, other than slight symptoms, of either mild or severe virus diseases prevalent in Europe where those symptoms are attributable to infection in the mother crop.