If the director finds that part of a nursery is infested or infected with insect pests or plant diseases and that the remainder of it is not so infested or infected, or if he has reason to believe that a nursery is liable, by reason of its proximity to infested or infected premises, to become so infested or infected before the next inspection, he may prescribe in writing such measures of precaution, or may make in writing such conditions as to the use of his certificate as may in his judgment be necessary, and he may withhold a certificate until such conditions have been accepted in writing by the owner of the nursery; and the use of such certificate without taking such measures of precaution or observing such conditions shall subject the owner of said nursery to the penalties prescribed for violation of this act. In any case coming to the attention of the director in which a nurseryman, dealer, plant grower or agent furnishing or selling nursery stock appears to be guilty of fraudulent practice, the director shall have authority to make such investigation as he may deem proper and proceed with such prosecution as may be necessary for the protection of the interests of the buying public, and, in addition, the director may revoke his license.