Resistance of Sclerotinia homoeocarpa Field Isolates to Succinate Dehydrogenase Inhibitor Fungicides

Sclerotinia homoeocarpa isolates were collected from golf courses in Japan and the United States (2016-2017). Japan isolates were collected during a monitoring study and the U.S. isolates were collected due to field failure. Five succinate dehydrogenase inhibitor (SDHI) active ingredients (boscalid, fluopyram, fluxapyroxad, isofetamid, and penthiopyrad) were examined using in vitro sensitivity assays to determine cross-resistance. Sequence analysis revealed a point mutation leading to an amino acid substitution (H267Y) and a silent mutation (CTT to CTC) at codon 181 in the SdhB subunit gene. Isolates with the B-H267Y (n = 10) mutation were resistant to boscalid and penthiopyrad and had increased sensitivity to fluopyram. SdhB silent mutation 181C>T isolates (n = 2) were resistant to boscalid, isofetamid, and penthiopyrad. Sequence analysis revealed 3 mutations leading to an amino acid substitution (G91R, n = 5; G150R, n = I; G159W, n = 1) in the SdhC subunit gene. Isolates harboring the SdhC (G91R or G150R) mutations were resistant to boscalid, fluxapyroxad, isofetamid, and penthiopyrad. A genetic transformation system was used to generate mutants from a SDHI sensitive isolate to confirm the B-H267Y and C-G91R mutations were direct determinants of SDHI resistance and associated with in vitro sensitivity assay results.