Biocidal Activity of Mixtures of Methionine and Riboflavin against Plant Pathogenic Fungi and Bacteria and Possible Modes of Action

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Abstract
A mixture of methionine + riboflavin (MR) was biocidal to various plant pathogenic fungi and bacteria under continuous illumination for as brief as 2 h depending on pH of the mixture (approximately 1 W m-2 light intensity). Among various species of fungi and bacteria tested, Verticillium dahliae, Peronophythora litchii, Botrytis cinerea, Agrobacterium tumefaciens, and Pseudomonas syringae were most sensitive to the MR mixture. Agrobacterium tumefaciens showed exceptional sensitivity in that the cells were killed even in total darkness after 4 h. Blue-rich Sylvania Gro-lux was superior to cool white fluorescent lights at the same light intensity in enhancing the biocidal activity. At pH 3-8, activity of MR mixture increased with a decrease of pH. Above pH 7, activity was greatly reduced. In the presence of 1 mM DL- or L-methionine, riboflavin appeared to be the best photosensitizer for the biocidal reaction. Among various amino acids, methionine or its derivatives were the most effective substances for biocidal activity. The most effective test solution contained 1 mM methionine and 26.6 .mu.M (10 mg/L) riboflavin at pH 4.0; increase of methionine or decrease of riboflavin reduced activity. Mode of action and possible application of MR as a biocide for use in plant disease control are discussed.