Occurrence, Distribution, and Characteristics of Boscalid-Resistant Corynespora cassiicola in China

Corynespora blight, caused by Corynespora cassiicola (Berk. & M.A. Curtis) C.T. Wei, has become an important disease affecting cucumber in China. Its management mainly depends on fungicides; however, no research has been conducted to assess the sensitivity of C. cassiicola in China to boscalid, a succinate dehydrogenase inhibitor (SDHI). To facilitate boscalid resistance monitoring, baseline sensitivity was established. The EC₅₀ value (i.e., the concentration that results in 50% mycelial growth inhibition) frequency distribution was unimodal with a right-hand tail; with the means 0.95 ± 0.51 μg/ml and the range 0.03 to 2.85 μg/ml. We then assessed the sensitivity of C. cassiicola to boscalid using discriminatory doses and EC₅₀ values. In total, 27.8% of the 798 isolates were resistant, distributed in five provinces and two municipalities. Thirty-seven isolates with different resistance levels to boscalid were also evaluated for their sensitivity to carboxin, fluopyram, and penthiopyrad. Seven SDHI resistance patterns were observed (i.e., I: Bos^MRFluo^MRPen^LRCar^SS; II: Bos^VHRFluo^SSPen^MRCar^R; III: Bos^LRFluo^MRPen^LRCar^R; IV: Bos^MRFluo^MRPen^MRCar^R; V: Bos^HRFluo^MRPen^HRCar^R; VI: Bos^HRFluo^HRPen^HRCar^R; and VII: Bos^HRFluo^SSPen^LR Car^R, VHR: very highly resistant; HR: highly resistant; MR: moderately resistant; LR: low resistant; R: resistant; SS: supersensitive), corresponding to seven mutations in sdhB/C/D genes, respectively.