An environmentally-safe method for protecting plants is provided for reducing the use of chemically-synthesized pesticide. Specifically, an anti-microbial and/or anti-viral composition comprising a laccase, a cell of a laccase-producing microorganism or a treated product thereof, and one or more kinds of phenol derivatives each represented by the general formula (I) shown below or a salt of the phenol derivative is provided. Furthermore, a plant disease-preventing agent, seed-sterilizing agent, post-harvest agent, and nutrient solution-sterilizing agent, each containing the composition; and plant disease-preventing method, seed-sterilizing method, post-harvest-treatment method, and nutrient solution-sterilizing method in hydroponics and solid-medium hydroponics using each f the agents are provided.
(in this formula, each of R1 to R5 independently represents hydrogen, an alkyl group having 1 to 6 carbon atoms, substituted or unsubstituted alkenyl group having 2 to 6 carbon atoms, alkoxy group having 1 to 6 carbon atoms, substituted or unsubstituted aryl group having 6 to 20 core carbon atoms, carboxyl group having 1 to 10 carbon atoms, carbonyl group having 1 to 10 carbon atoms, or halogen atom, and at least one of R1 to R5 is an alkenyl group having 1 to 10 carbon atoms; provided that at least one of R1 and R5 is not an alkoxy group).