The Deputy Administrator is authorized to issue the administrative instructions for which provision is made in the regulations in this part, for the purpose of preventing dissemination of plant pests into the United States or interstate. In addition, whenever the Deputy Administrator shall find that existing conditions as to pest risk involved in the movement of plant pests, means of conveyance, or other products or articles to which the regulations in this part apply, make it safe to modify by making less stringent the restrictions contained in any of such regulations, he shall publish such findings in administrative instructions, specifying the manner in which the regulations shall be made less stringent whereupon such modification shall become effective.