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.

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http://dx.doi.org/10.1002/ps.2092DOI Listing

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Article Synopsis
  • Corynespora blight, caused by Corynespora cassiicola, is a major disease impacting cucumbers in China, primarily controlled through fungicides.
  • Recent research established a baseline sensitivity to boscalid, revealing that 27.8% of tested isolates were resistant, with a distribution across five provinces and two municipalities.
  • Seven different SDHI resistance patterns were identified, linked to mutations in specific genes, highlighting the need for ongoing resistance monitoring and management strategies.
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