Evaluation on Synergistic Activity and Resistance Development of the Mixture of Iprodione and Fluopyram Against Botrytis Cinerea

Gray mold caused by Botrytis cinerea seriously threatens the quality and yield of vegetables or fruits in China. B. cinerea has developed different degrees of resistance to various fungicides because of the extensive and long-term application of fungicides. In this study, the mixture of iprodione and fluopyram was used to control resistant isolates and delay the evolution of resistance. A total of 28 isolates that include sensitive and resistant isolates were selected to determine the combined effect of binary mixtures of iprodione and fluopyram at different proportions, and a binary mixture of iprodione and fluopyram at a ratio of 4:1 (Ipr: Flu [4:1]) presented synergistic effects by 14 isolates of B. cinerea . The optimal proportion of the mixture determined by a variety of isolates showed a significant difference. Next, the mean EC 50 value of 106 isolates collected from different regions in 2021 was 1.84 mg/L for Ipr: Flu (4:1), lower than iprodione (2.41 mg/L) and fluopyram (20.53 mg/L), which indicated the adaptability of the mixture against B. cinerea . Furthermore, The disease suppression of the mixture at 200 mg/L was similar to that of fluopyram at 300 mg/L, which provided the possibility for the application of a mixture with a lower dosage. Finally, UV mutagenesis combined with fungicide-selection experiments was performed to evaluate the risk of resistance development. Compared with iprodione and fluopyram, the binary mixture of iprodione and fluopyram at 4:1 delayed the resistance development of B. cinerea. This study suggested Ipr: Flu (4:1) could enhance disease suppression against sensitive and resistant isolates and delay the resistance development of isolates, which greatly facilitated the application of the mixture of fungicides against B. cinerea and resistance management.