Molecular and biochemical characterization of Sclerotinia sclerotiorum laboratory mutants resistant to dicarboximide and phenylpyrrole fungicides

Sclerotinia sclerotiorum is a necrotrophic, phytopathogenic, filamentous ascomycete with a broad host range and a worldwide distribution. Application of fungicides is the principal tool for controlling Sclerotinia diseases on most crops. Unfortunately, the extensive use of a single fungicide selects for resistant populations and leads to control failures. The phenylpyrrole fungicide fludioxonil has been reported to have high activity against S. sclerotiorum and to control Sclerotinia stem rot in rapeseed. In this study, biochemical characteristics of laboratory-induced mutants of S. sclerotiorum were determined. The results indicated that the fludioxonil-resistant mutants were sensitive to osmotic stress (high sugar). Compared to the wild-type strains, the fludioxonil-resistant mutants had a significant increase in cell membrane permeability, glycerol and oxalate content, and phenylalanine ammonia-lyase and peroxidase activity, but did not differ in exopolysaccharide content. Sequencing indicated that three wild-type strains were identical, and the mutants SZ45R and HA61R had a single point mutation while NT18R had both a single point mutation and a frameshift in the amino acid sequence coded by the two-component histidine kinase gene (Shk1, SS1G_12694). Therefore, we concluded that the biological differences between the resistant mutants and the wild-type strains may be related to mutation in Shk1. The information will increase our understanding of the resistance mechanism of S. sclerotiorum to fludioxonil and could provide new reference data for the management of Sclerotinia stem rot caused by S. sclerotiorum.