Baseline Sensitivity and Resistance Risk Assessment of the Fusarium graminearum Species Complex to Pyrifenox in China

Pyrifenox is a pyridine fungicide belonging to the pyridine subgroup of demethylation inhibitor (DMI) fungicides (FRAC group 3), and the resistance risk of the Fusarium graminearum species complex (FGSC) to this fungicide remains poorly understood. In this study, 122 FGSC isolates were collected from five provinces across China during 2024–2025. The baseline sensitivity was characterized using a mycelial growth assay, yielding a mean EC50 value of 3.300 ± 0.227 mg/L. Four phenotypically resistant variants were generated via fungicide adaptation combined with UV mutagenesis, with resistance factors ranging from 5.53 to 450.26, covering low, moderate, and high resistance phenotypes. After ten serial transfers on fungicide-free medium, the resistance of these variants decreased significantly. These variants also exhibited reduced sporulation and pathogenicity compared with their parental isolates. However, because these assays were performed on tenth-generation variants that had already lost most of their resistance, these changes cannot be directly attributed to resistance. Cross-resistance assays revealed positive cross-resistance only with tebuconazole, whereas no cross-resistance was found with prothioconazole, pyraclostrobin, and three other fungicides. Target gene sequencing identified no mutations in CYP51A, CYP51B, or CYP51C, implying non-target-site resistance. Overall, the FGSC presents a low resistance risk to pyrifenox. However, species identity within the FGSC was not resolved in this study, and this conclusion should be interpreted with caution. This study established, to our knowledge, the first sensitivity baseline for pyrifenox against the FGSC in China, providing fundamental data for rational field application to control Fusarium head blight.