Use of mutants to demonstrate the role of oxalic acid in pathogenicity of Sclerotinia sclerotiorum on Phaseolus vulgaris

The role of oxalic acid in pathogenicity of the bean white mould fungus, Sclerotinia sclerotiorum, was investigated. Prototrophic mutants of the fungus deficient in oxalic acid production were obtained by UV irradiation of ascospores. Deficiency in oxalic acid production was screened by colour change on potato dextrose agar containing the indicator dye bromophenol blue. An enzymatic method, gas chromotography and high performance liquid chromatography indicated that the selected mutants did not produce oxalic acid in carbohydrate-rich media or bean blossoms, a natural substrate. In growth chamber experiments using whole plants, stems and leaves and in laboratory tests using pods, these acid minus mutants were nonpathogenic while the oxalic acid-producing wild type and revertant were pathogenic. Production of oxalic acid was induced when mutants were grown on nutrient media containing sodium succinate. Mutants grown on this medium and inoculated on bean leaves penetrated and incited small lesions. No association between pathogenicity and pectolytic enzymes was found. These studies present confirming evidence that oxalic acid is a pathogenicity determinant.