Isofetamid efficacy against SDHI-resistant fungal pathogens

In this study, we report the results of sensitivity tests to succinate dehydrogenase inhibitor (SDHI) fungicides performed on isolates of cucumber Corynespora leaf spot, powdery mildew, and tomato leaf mold pathogens collected in Japan. Isofetamid and fluopyram showed strong activity against boscalid-resistant isolates of <i>Corynespora cassiicola</i> from Tokushima Prefecture in <i>in vitro</i> antifungal assays and <i>in vivo</i> pot experiments. Isofetamid and fluopyram also showed comparable efficacy against boscalid- and penthiopyrad-resistant isolate of <i>Podosphaera xanthii</i> collected in Saga Prefecture using a leaf disc assay. No change was detected in the amino acid sequence of sdhB in the causal pathogen. In addition, during <i>in vitro</i> antifungal tests, isofetamid demonstrated high activity against <i>Fulvia fulva</i> isolates resistant to boscalid, penthiopyrad, and fluopyram obtained in Gifu Prefecture. Collectively, these results indicated that isofetamid maintained similar efficacy against isolates resistant to other SDHIs as against sensitive isolates of these pathogens.