Lack of cross-resistance to a novel succinate dehydrogenase inhibitor, fluopyram, in highly boscalid-resistant isolates of Corynespora cassiicola and Podosphaera xanthii.

BACKGROUND: Recently in Japan, isolates resistant to boscalid, a succinate dehydrogenase inhibitor (SDHI), have been detected in Corynespora cassiicola (Burk. & Curt.) Wei and Podosphaera xanthii (Castaggne) Braun & Shishkoff, the pathogens causing Corynespora leaf spot and powdery mildew disease on cucumber, respectively. Resistant isolates of C. cassiicola are widely distributed and represent a serious problem in disease control at present. Novel SDHI fungicides, including fluopyram, are now under development.
RESULTS: The growth of very highly boscalid-resistant, highly resistant and sensitive isolates of C. cassiicola was strongly suppressed on fluopyram-amended YBA agar medium. Although boscalid and another SDHI, penthiopyrad, hardly controlled Corynespora leaf spot and powdery mildew on cucumber plants when very highly or highly boscalid-resistant isolates were employed for inoculation, fluopyram still exhibited excellent control efficacy against these resistant isolates as well as sensitive isolates of C. cassiicola and P. xanthii.
CONCLUSION: Differential sensitivity to boscalid, penthiopyrad and fluopyram, clearly found in these two important pathogens of cucumber, may indicate involvement of a slightly distinct site of action for fluopyram from the two other SDHIs. This finding may lead to the discovery of unique SDHIs in the future.