Hideo Ishii,1,2 Takuya Miyamoto3 and Kota Katayose1

1Kibi International University, Minami-awaji, Hyogo, Japan; 2University of Tsukuba, Tsukuba, Ibaraki, Japan; 3Horticultural Research Institute, Ibaraki Agricultural Center, Kasama, Ibaraki, Japan


Differential Pattern of Cross Resistance to SDHI Fungicides and Association with sdh Gene Mutations in Corynespora cassiicola

In recent years, Corynespora cassiicola has become an important pathogen on various crops in many countries. This fungus causes Corynespora leaf spot on cucumber and strains resistant to boscalid, a succinate dehydrogenase inhibitor (SDHI), have developed quickly in populations.1 Lack of cross resistance to fluopyram has been found in very highly and highly boscalid-resistant isolates of this fungus strongly indicating that the pattern of cross resistance is not uniform among SDHI fungicides.2 In this study, some newer SDHI fungicides were included and differential pattern of cross resistance was examined. Ten isolates of C. cassiicola, their sequences of sdhB, sdhC, and sdhD genes were analysed previously,3 were chosen and sensitivity to eight fungicides (boscalid, penthiopyrad, fluopyram, isopyrazam, isofetamid, fluxapyroxad, pyraziflumid, and benzovindiflupyl) was tested based on mycelial growth inhibition on fungicide-amended YBA agar medium. As reported before, fluopyram as well as isofetamid showed high activity to inhibit the growth of isolates with very high or high boscalid resistance, carrying sdhB-H278Y and sdhB-H278R mutations, respectively.2,4 Cross resistance was clearly observed to all of the other SDHI fungicides used. However, benzovindiflupyl exhibited higher inhibitory activity than these fungicides. In inoculation tests conducted on potted cucumber plants, benzovindiflupyl showed superior suppressive activity to boscalid against Corynespora leaf spot disease irrespective of the level of boscalid resistance of pathogen isolates.

1Miyamoto, T. et al. (2009) Occurrence of Corynespora cassiicola isolates resistant to boscalid on cucumber in Ibaraki Prefecture, Japan. Plant Pathol. 58: 1144-51.
2Ishii, H. et al. (2011) Lack of cross-resistance to a novel succinate dehydrogenase inhibitor, fluopyram, in highly boscalid-resistant isolates of Corynespora cassiicola and Podosphaera xanthii.
Pest Manag. Sci. 67: 474-482.
3Miyamoto, T. et al. (2010) Distribution and molecular characterization of Corynespora        cassiicola isolates resistant to boscalid. Plant Pathol. 59: 873-881.
4Tsukuda, S. (2015) Fungicidal properties and sensitivity study of a novel fungicide isofetamid (KENJA®). Abstracts of the 25th Symposium of Research Committee on Fungicide Resistance: 11-21.