New Mechanisms of Resistance to <scp>CAA</scp> and <scp>OSBPI</scp> Fungicides in <i>Phytophthora infestans</i>

ABSTRACT Field observations from Denmark in 2021 suggested a reduction in the efficacy of mandipropamid (MPD), a carboxylic acid amide (CAA) fungicide used against potato late blight ( Phytophthora infestans ). Three P. infestans isolates with reported low sensitivity to MPD were molecularly characterised revealing the presence of a G to S mutation in the cellulose synthase A catalytic subunit 3 (cesA3) protein in position 1105. Fungicide resistance monitoring, based on innovative next‐generation sequencing techniques, and a resistance management programme were therefore put in place, with the aim of monitoring resistance evolution to CAAs and to those additional fungicide classes that could have contributed to mitigating the newly emerged resistance. The monitoring results in 2022 and 2023 showed that the mutation cesA3_1105S appeared to have spread beyond Denmark to Benelux, the Nordics and Germany among others; in addition, mutations in position 770 (G to V) and 837 (I to F) in the oxysterol binding protein (osbp) of P. infestans , target of the oxathiapiprolin fungicide (OXTP, belonging to the OSBPI class), arose in 2023. The mutations conferring resistance to OXTP and CAA fungicides in P. infestans were consequently monitored in European fields and their origins discussed. This study presents the first report of the mechanism behind MPD resistance in P. infestans , the newly discovered presence of combined resistance to CAAs and OXTP in potato fields and recommendations for revised fungicide management strategies for potato late blight control.