Monitoring in Corynespora cassiicola resistance to boscalid, trifloxystrobin, and carbendazim in China

Abstract

Corynespora leaf spot (CLS), caused by Corynespora cassiicola, is a serious disease of greenhouse cucumbers. With high use of fungicides, C. cassiicola has developed resistance to various fungicides. However, with a lack of new fungicides to be selected, it is still necessary to use existing fungicides for effective control. Therefore, this study monitored the resistance of C. cassiicola to three commonly used and effective fungicides, boscalid, trifloxystrobin, and carbendazim, from 2017 to 2021. The resistance frequency to boscalid showed an increasing trend, and the highest frequency was 85.85% in 2020. The resistance frequency to trifloxystrobin was above 85%, and resistance to carbendazim was maintained at 100%. Among them, multiple-resistant strains were found that showed resistance to both boscalid, trifloxystrobin, and carbendazim, accounting for 32.00%, 25.25%, 33.33%, 43.06%, and 37.24%, respectively. 87% of the strains that were resistant to boscalid had CcSdh mutations, including seven genotypes: B-H278L/Y, B-I280V, C-N75S, C-S73P, D-D95E, and D-G109V. And six mutation patterns of the Ccβ-tubulin gene were detected: E198A, F167Y, E198A&M163I, E198A&F167Y, M163I&F167Y, and E198A&F200C. Detection of mutations of the CcCytb gene in resistant strains showed that 98.8% of the strains were found to have only the G143A mutation. A total of 27 mutation combinations were found and the development of genotypes showed a complex trend. The resistance levels were different according to the genotype and a gradual increase in multiple fungicide resistance. Therefore, it is necessary to understand the types of mutations and the trend of resistance to guide the use of fungicides.