The inventors provide the use of a chelating agent and an antimicrobial agent that is effective against a plant pathogenic microorganism to inhibit the growth of and/or kill a plant pathogenic microorganism on a plant; the use of a chelating agent to increase the activity of an antimicrobial that is effective against a plant pathogenic microorganism; a method of inhibiting the growth of and/or killing a plant pathogenic microorganism comprising administering to a plant in need thereof a chelating agent and an antimicrobial agent that is effective against a plant pathogenic microorganism; and a method of increasing the activity of an antimicrobial that is effective against a plant pathogenic microorganism comprising using said antimicrobial with a chelating agent. Also provided is a composition comprising a chelating agent and an antimicrobial agent that is effective against a plant pathogenic microorganism, use of said composition to inhibit the growth of and/or kill a plant pathogenic microorganism on a plant, and a method of inhibiting the growth of and/or killing a plant pathogenic microorganism comprising administering to a plant in need thereof said composition. Further provided is the use of a composition comprising an antimicrobial polypeptide comprising Blad or an active variant thereof to kill, or inhibit the growth of a plant pathogenic bacterium on a plant, and a method of killing, or inhibiting the growth of a plant pathogenic bacterium on a plant, said method comprising administering to said plant a composition comprising an effective amount of an antimicrobial polypeptide comprising Blad or an active variant thereof.