Section 17. The said director, either personally or through his assistants, shall inspect at least once each year each nursery or place where nursery stock is grown, and if no dangerous insects or fungous pests are found therein, he shall give a certificate to that effect. If such pests are found therein the owner of the stock shall take such measures to suppress the same as the director shall prescribe, and no certificate shall be given until he has satisfied himself by a subsequent inspection that all such pests have been suppressed. He shall determine the season for inspecting nurseries and the form of certificates to be given but in no case shall he issue a certificate to continue in force after July first next following the date of inspection.