Field resistance to boscalid and molecular mechanisms of strawberry Botrytis cinerea in Chongqing, China

Gray mold is a common and severe disease that affects strawberry production and storage. To clarify the occurrence and mechanism of Botrytis cinerea in strawberry resistance to boscalid [2-Chloro-N-(4'-chlorobiphenyl-2-yl) nicotinamide], a widely used fungicide, 137 isolates of Botrytis cinerea in strawberry were collected from Beibei, Xiema, and Changshou of Chongqing by random sampling method in 2021–2022 and monitored for resistance in the field by the discriminatory dosage method. The succinate dehydrogenase (SDH) related gene of 55 resistant strains was sequenced, and molecular docking was used to predict the binding mode of boscalid to wild-type and mutant SDH of B. cinerea. The results showed that the resistance frequency of B. cinerea to boscalid in Chongqing was 93.43%. Mutation sites on the B subunit of SDH were found in 45 of the 55 resistant strains, with three primary mutation types, P225F, N230I, and H272R, responsible for 43.6%, 16.4%, and 21.8% of mutations, respectively. Out of the 55 resistant strains, five were found to have mutation sites on the C subunit of SDH. Only one mutation type, G85A+I93V+M158V+V168I, was observed in these strains, where four amino acids were mutated simultaneously. However, no mutation sites were detected in the remaining five resistant strains. Molecular docking results predicted that the P225F mutation prevented boscalid from entering the binding pocket, which affects the formation of hydrogen bonds and other interaction forces and ultimately hinders the ability of boscalid to bind to SDH. The H272R mutation narrows the cavity bottom of the binding pocket and rotates part of the boscalid chemical bond, while the N230I mutation has no effect on boscalid binding. This study clarified the occurrence and mechanism of boscalid resistance in B. cinerea in the Chongqing region, and the results provide an experimental data for formulating strawberry gray mold management strategies and also provide some insights for the development of succinic dehydrogenase inhibitors (SDHIs) fungicides.