Sensitivity of tomato leaf mould-causing Fulvia fulva to seven succinate dehydrogenase inhibitor (SDHI) fungicides in Nara Prefecture, Japan and high efficacy of isofetamid in controlling SDHI-resistant isolates

Leaf mould caused by Fulvia fulva is a major disease of tomato in Japan. Resistance to several types of fungicides, including succinate dehydrogenase inhibitors (SDHIs), has been reported in isolates from Japan. In this study, we evaluated the sensitivity of F. fulva to SDHI fungicides, SDHI fungicide efficacy in controlling sensitive and resistant isolates, and the presence of mutations in the Sdh B gene, the target site of SDHI fungicides. The minimum inhibitory concentration (MIC) of six SDHI fungicides, excluding isofetamid, ranged from 0.1 to >100 mu g/mL against mycelial growth of 45 F. fulva isolates collected in Nara prefecture, Japan, whereas that of isofetamid ranged from 0.1 to only 10 mu g/mL. The peak MIC distribution of fluopyram and isofetamid was 10 mu g/mL, that of inpyrfluxam was 100 mu g/mL and was >100 mu g/mL for the remaining SDHI fungicides. In a greenhouse inoculation experiment, only isofetamid controlled the highly resistant isolates with an MIC of 10 mu g/mL and the remaining fungicides had MIC >100 mu g/mL. In addition, moderately and highly resistant isolates were controlled by the application of isofetamid 6 h or 7 days before inoculation or 3 days after inoculation. Mutations in the Sdh B gene were not observed among sensitive, moderately resistant and highly resistant isolates, suggesting that mutations in other Sdh genes and/or changes in efflux pump expression were involved. Our results indicate the presence of cross-resistance and different levels of sensitivity reduction among SDHI fungicides. Isofetamid showed the lowest resistance development among the tested SDHI fungicides and is considered effective in controlling tomato leaf mould.