We describe methods and materials for inhibiting the growth of a microbial pathogen of a plant. The method comprises the steps of applying to the plant a combination of a dye-bearing compound which is a reactive oxygen species photosensitizer; and a copper compound, which increases the microbial lethality of the dye-bearing compound towards the microbial pathogen; and exposing the plant to light to activate the combination of the dye-bearing compound and the copper compound and inhibiting growth of a microbial pathogen of a plant. The combination of the dye-bearing compound and the copper compound may result in a synergistic ability to reduce the growth of a plant pathogen.