Sensitivity, Resistance Risk Assessment of Bipolaris oryzae to Three DMI Fungicides and Their Control Efficacy Against Banana Eyespot Disease in Hainan Island, China

Bipolaris oryzae was recently identified as a new species causing eyespot disease of banana in China. Propiconazole, flusilazole and tebuconazole, belonging to demethylation inhibitor (DMI) fungicides, are commonly used for controlling this disease in China. In this study, the propiconazole, flusilazole and tebuconazole sensitivity of 108 B. oryzae isolates collected from banana leaf samples in different locations of Hainan Island, China, in 2021–2023 was determined by mycelial growth inhibition, and the control efficacy of the three DMI fungicides against B. oryzae was assayed in pot experiments. The sensitivity frequency distributions of 108 B. oryzae isolates to propiconazole, flusilazole and tebuconazole indicated the emergence of subpopulations with reduced sensitivity, with EC50 values ranging from 0.029 to 3.495 μg/mL, 0.008 to 3.665 μg/mL, and 0.022 to 8.288 μg/mL, with means and standard errors of 0.826 ± 0.059 μg/mL, 0.204 ± 0.036 μg/mL and 0.952 ± 0.113 μg/mL, respectively. However, the sensitivity distributions of 82.41%, 82.41%, and 80.56% of B. oryzae isolates at the main peak, respectively, to the three fungicides fitted to a normal distribution. Thus, the mean EC50 values of 0.594 ± 0.032 μg/mL, 0.118 ± 0.007 μg/mL and 0.574 ± 0.033 μg/mL of these isolates could be used as the baseline for testing the sensitivity of B. oryzae to propiconazole, flusilazole and tebuconazole, respectively. No significant sensitivity differences to each fungicide among different geographical populations were observed, but a statistically significant and positive association in sensitivity between all three fungicides existed. Seven unstably inherited DMI-resistant isolates all exhibited lower fitness levels than those of the sensitive isolates, suggesting a low risk of resistance to the three DMI fungicides in B. oryzae. Three fungicides applied separately exhibited 100% protective control efficacy against B. oryzae and less than 40% curative control efficacy in pot experiments. Therefore, propiconazole, flusilazole and tebuconazole should be used as protective rather than curative fungicides. These results provide essential information on the evolution of DMI resistance in B. oryzae in Hainan Island, China, and may serve as a reference point in the management of B. oryzae and future DMI resistance monitoring programs.