The department is authorized to inspect all nurseries in the state and all nursery stock grown within the state. If the nursery stock is found to be free of plant pests, an inspection certificate shall be issued certifying that the nursery stock has been inspected and is believed to be free from plant pests. The certificate shall be valid for one license year. If, at any subsequent inspection, the nursery is found to be infested with plant pests, the certificate may be canceled until the conditions are corrected.