Evolution of the Resistance of to Carbendazim and the Current Efficacy of Carbendazim Against Gray Mold After Long-Term Discontinuation.

Gray mold caused by is a fungal disease that critically threatens agricultural production, and carbendazim was the first fungicide used to control . However, developed serious resistance to carbendazim, and this fungicide has thus rarely been used in the past decade in China. Due to the extended discontinuation of carbendazim use, the evolution of the resistance of to carbendazim in recent years is unclear, and whether carbendazim can effectively control gray mold is largely unknown. Therefore, this study determined the sensitivity of 407 isolates collected from 2014 to 2018 to carbendazim and the ability of carbendazim to control gray mold in the field. The results showed that the frequency of isolates resistant to carbendazim remained above 95%. Three different mutation types responsible for the resistance of to carbendazim were identified at codon 198 in the gene sequence: E198V (changed from GAG to GTG), E198A (changed from GAG to GCG), and E198K (changed from GAG to AAG). Over the last 5 years, E198V was the major mutation. However, an analysis of its evolution revealed that the percentage of the E198V mutation declined after 2017 to 56.5% in 2018. In addition, the proportion of isolates with the E198K mutation decreased over time, and no isolates with this mutation were found in either 2017 or 2018. The proportion of the E198A mutation increased over the 5-year test period to reach 43.5% in 2018. Furthermore, three greenhouse experiments demonstrated that carbendazim has lost its ability to control gray mold. We attribute the above findings to our results showing that the carbendazim-resistant isolates had no fitness penalties compared with the carbendazim-sensitive isolates for sporulation and mycelial growth. In particular, the E198A mutant isolates exhibited a strong ability to sporulate, suggesting that the E198A mutation might become dominant in the future. Interestingly, the results showed that carbendazim-sensitive isolates could be easily controlled by four conventional fungicides, namely boscalid, procymidone, iprodione, and pyrimethanil, with mean EC values of 0.71 ± 0.2 mg liter, 1.33 ± 0.39 mg liter, 0.59 ± 0.33 mg liter, and 6.02 ± 3.02 mg liter, respectively. In conclusion, carbendazim has lost its application value and is ineffective for the control of gray mold.