A Nonsynonymous Substitution in the Iron-Sulfur Protein Subunit of Succinate Dehydrogenase (SdhB) Confers Sdhi Fungicide Resistance to Sclerotinia Homoeocarpa Field Isolates.

Succinate dehydrogenase inhibitor (SDHI) fungicides have been widely applied to control dollar spot (caused by Sclerotinia homoeocarpa) on turfgrass. Despite the frequent use of SDHI fungicides, resistance to SDHI fungicides in S. homoeocarpa has not been reported. Two S. homoeocarpa field isolates from one of golf courses in Japan exhibited resistance to boscalid (SDHI fungicide) in a fungicide resistance monitoring survey. Four SDHI active ingredients (boscalid, flutolanil, fluxapyroxad, and penthiopyrad) were examined by in vitro sensitivity assays to determine cross resistance among two SDHI resistant and six SDHI sensitive field S. homoeocarpa isolates. The SDHI target gene, iron-sulfur protein subunit of succinate dehydrogenase (SdhB) was sequenced to investigate the genetic factors involved in resistance. The two resistant isolates displayed cross resistance to boscalid, penthiopyrad, and fluxapyroxad, but not flutolanil. These isolates harbor an amino acid substitution (H267Y) in SdhB gene, but the six sensitive isolates did not harbor the mutation. SdhB^H267Y mutants were generated from a SDHI sensitive isolate through a genetic transformation system and confirmed the mutation is the direct determinant of SDHI resistance. This is the first report of SDHI field resistance and elucidation of the molecular mechanisms responsible for SDHI resistance in S. homoeocarpa.