Prevalence of Quinone Outside Inhibitor Fungicide Resistance and Dynamics of Cytochrome b Genotypes Under Different Fungicide Selection Pressure in Botrytis cinerea Isolates from Korean Strawberry Farms

Botrytis cinerea, the causal agent of gray mold, is a major pathogen in strawberry production. Quinone outside inhibitor (QoI) fungicides have been widely used for disease control, but resistance driven by the G143A mutation in cytochrome b (cytb) has limited their field efficacy. This study evaluated the sensitivity of 229 B. cinerea isolates collected from farms in major Korean strawberry-growing regions during 2022-2024 to the QoI fungicide pyraclostrobin. The EC50 values for pyraclostrobin ranged from 0.10 to 329.21 μg mL-1. In total, 86.46% of the population showed resistance to pyraclostrobin, harboring the G143A mutation in cytb. In planta and greenhouse assays confirmed that pyraclostrobin failed to control the QoI-resistant isolates. Furthermore, strong cross-resistance to pyraclostrobin, mandestrobin, and pyribencarb indicates that QoI-resistant populations substantially limit the field efficacy of QoI fungicides. To assess how the QoI resistance mutation affects selection under non-QoI fungicide pressure, mixed QoI-sensitive and -resistant isolates were treated with alternative fungicides. Allele-specific qPCR revealed that pydiflumetofen, fludioxonil, and fluxapyroxad maintained a higher proportion of the QoI-sensitive genotype at all concentrations, whereas pyraclostrobin consistently favored the resistant genotype. These findings indicated that QoI fungicides impose strong selection pressure promoting resistant B. cinerea populations, while non-QoI fungicides help maintain sensitive individuals. Our study suggests the need for fungicide class rotation and the targeted use of non-QoI fungicides to delay QoI resistance development and preserve the efficacy of gray mold management in strawberry production.