Sclerotinia sclerotiorum is a destructive plant pathogen causing white mold and other crop diseases. This study identified and deleted the SsMet1 gene, which is essential for methionine production in this fungus. Fungi lacking this gene could not grow properly, form survival structures called sclerotia, or infect plants. These findings suggest that blocking methionine biosynthesis could be a new way to develop fungicides against this important crop pathogen.