Section 26. When the commissioner has heard an appeal and has rendered a decision that the action of the director from which the appeal was taken is sustained, the director shall notify in writing the owner, occupant or person in charge of the trees, shrubs or other plants concerned, of the decision, and shall direct him to treat or destroy the trees, shrubs or other plants within a given time in accordance with a method prescribed in the notice. If the person so notified refuses or neglects so to treat or destroy such trees, shrubs or other plants within the time prescribed, the director may cause such property to be so treated or destroyed, and the cost thereof to be recovered as provided in section twenty-four.