Resistance to succinate dehydrogenase inhibitors in field isolates of Podosphaera xanthii on cucumber: Monitoring, cross-resistance patterns and molecular characterization

New succinate dehydrogenase inhibitor fungicides (SDHIs), isopyrazam, pyraziflumid and isofetamid were introduced in the Japanese market in 2017-2018 to control powdery mildew on cucumber. SDHI resistance of the disease fungus (Podosphaera xanthii) was first reported during 2008-2009 against boscalid. Then, penthiopyrad which belongs to SDHIs was introduced in 2010, but subsequent monitoring study was not performed. We investigated the sensitivity of P. xanthii field isolates from Ibaraki Prefecture, Japan, to SDHIs and SdhB, SdhC and SdhD gene mutations, using a leaf disc assay and SDH gene analysis. A total of 19 out of the 22 selected isolates showed resistance to SDHIs. The 19 isolates were phenotypically categorized into three types: Resistant I as moderately and Resistant II as highly resistant to penthiopyrad, isopyrazam and pyraziflumid but sensitive to isofetamid and Resistant III as highly resistant to isofetamid but sensitive to the other three SDHIs. SDH gene analysis revealed that Resistant I and III isolates carried a substitution in PxD-S121P and PxC-A86V, respectively. Resistant II carried three different substitutions: PxC-G151R, PxC-G172D, and PxD-H137R. Among 127 isolates sampled from 16 cucumber greenhouses, 54 exhibited Resistant I phenotype and carried only PxD-S121P. Fifty-six isolates exhibited Resistant II and carried PxC-G151R (four isolates), PxC-G172D (24), and PxD-H137R (28). Only two isolates expressed the Resistant III phenotype carrying PxC-A86V. To the best of our knowledge, this is the first report demonstrating cross-resistance patterns and the molecular characterization of SDHIs in P. xanthii.