Mutations in the Membrane-Anchored SdhC Subunit Affect Fitness and Sensitivity to Succinate Dehydrogenase Inhibitors in Botrytis cinerea Populations from Multiple Hosts.

Succinate dehydrogenase inhibitors (SDHIs) are an essential group of fungicides for managing gray mold, caused by Botrytis cinerea, in numerous crops. Resistance to boscalid, an early-generation SDHI, is widespread worldwide and was linked to mutations in the iron-sulfur protein encoding the SdhB subunit of the SDH complex. Herein, we report on four simultaneous dependent mutations at codons 85 (G85A), 93 (I93V), 158 (M158V), and 168 (V168I) of the membrane-anchored SdhC subunit of B. cinerea. Isolates without and with mutations in SdhC were referred to as C- and C+ genotypes, respectively. The C+ genotype was found in all the five surveyed hosts from different U.S. regions but its frequency was higher, 25 to 40%, in the tree fruit isolates compared with 12 to 25% in the small fruit populations. The four SdhC mutations were found in isolates without mutations in SdhB or with mutations known to confer resistance to the SDHIs in SdhB. However, the frequency of C+ isolates was significantly higher in the SdhB wild-type isolates, which suggests that SDHI sprays may have played a role in selecting for the C- over the C+ genotype. Field C+ isolates exhibited reduced sensitivity to fluopyram and increased sensitivity to boscalid and penthiopyrad in vitro and on detached fruit. Homology modeling confirmed the positioning of the four mutations in the ubiquinone-binding pocket. The SdhCG85A is found in the proximal ubiquinone binding site and SdhCM158V is positioned in the iron sulfur protein interface next to the [3Fe-4S] cluster, whereas SdhCI93V is positioned next to the heme b with vital functions in the SDH enzyme. Beside the differential sensitivity to the SDHIs, these mutations caused a significant fitness cost in the C+ isolates including sporulation and increased sensitivity to reactive oxygen species. The presence of Botrytis populations differentially sensitive to the SDHIs suggests increased risks for resistance development but also opens up new perspective for future gray mold management using different SDHI fungicides.