Occurrence, distribution, and characteristics of boscalid-resistant corynespora cassiicola in China

Abstract

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 EC 50 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 mg/ml and the range 0.03 to 2.85 mg/ml. We then assessed the sensitivity of C. cassiicola to boscalid using discriminatory doses and EC 50 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 MR Fluo MR Pen LR Car SS ; II: Bos VHR Fluo SS Pen MR Car R ;III:Bos LR Fluo MR Pen LR Car R ; IV: Bos MR Fluo MR Pen MR Car R ;V:Bos HR Fluo MR Pen HR Car R ; VI: Bos HR Fluo HR Pen HR Car R ; and VII: Bos HR Fluo SS Pen 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.