Isolation, characterization and pathogenicity of Sclerotinia trifoliorum arabinofuranosidase-deficient mutants

A procedure for the isolation of mutants of Sclerotinia trifoliorum affected in extracellular hydrolases is described. The procedure involves visually screening large numbers of single ascospore isolates for the presence of extracellular enzymatic activities. Three arabinofuranosidase-deficient mutants were obtained in a screening of 4000 single ascospore isolates from a mutagenized ascospore population. Two of the mutants had a small amount of enzymatic activity remaining. This remaining arabinofuranosidase activity was thermolabile and, in one case, had an altered Michaelis constant (K_m). The pathogenicity of the arabinofuranosidase-deficient mutants was evaluated by inoculating alfalfa and winter pea stems and measuring the extent of necrosis induced by the pathogen. Under the conditions of the tests, the arabinofuranosidase-deficient mutants were significantly virulent on the winter pea stems but not on alfalfa stems. We conclude that under some conditions, arabinofuranosidase may play a role in pathogenicity.