Tomato leaf mold fungus ( Passalora fulva ) sensitivity to succinate dehydrogenase inhibitor fungicide in Mie Prefecture, Japan

From October 2016 to March 2019, 83 tomato leaf mold fungal strains (Passalora fulva) were isolated from 15 greenhouses in Mie prefecture, Japan. Their susceptibility to succinate dehydrogenase inhibitor (SDHI) fungicides was investigated. The occurrence of boscalidand penthiopyrad-resistant strains was confirmed, and all penthiopyrad-resistant strains had acquired cross-resistance with boscalid. We also confirmed that several fungal strains had developed resistance to the SDHI fungicidal agents isopyrazam and pyraziflumid, neither of which had been used in the field from which the leaf mold fungus was isolated. Consequently, it was suggested that cross-resistance might have developed due to previous boscalid and penthiopyrad treatment. The frequency of boscalidand penthiopyrad-resistant strains increased significantly as the frequency of SDHI fungicide spraying increased. Significant differences were also observed between the fungicides in terms of changes in resistance rate; boscalid treatment resulted in more resistance than penthiopyrad treatment.