In planta fluxapyroxad gradients from seed treatment differentially select fungicide-sensitive and reduced-sensitive genotypes of Pyrenophora teres f. maculata.

BACKGROUND: Fungicides are essential for controlling crop diseases and sustaining agricultural productivity, but the escalating emergence of resistance threatens their long-term efficacy and global food security. Recent advances in liquid chromatography coupled with high-resolution mass spectrometry enable accurate quantification of fungicide distribution. When combined with concurrent assessment of spatial variation of pathogen performance during infection, this enables a clear understanding of both fungicide efficacy and factors influencing the selection pressure leading to fungicide resistance. This study used the succinate dehydrogenase inhibitor fungicide fluxapyroxad and the barley pathogen Pyrenophora teres f. maculata (Ptm; the causal agent of spot form net blotch) as a model system to investigate how within-plant fungicide concentrations influence pathogen genotype-specific responses. RESULTS: Fluxapyroxad, applied as a seed treatment, was later quantified across multiple leaves along the main stem of barley across growth stages. Fluxapyroxad concentrations were correlated with disease severity following inoculation with fungicide-sensitive and fungicide-reduced-sensitive genotypes of Ptm. The fungal load and pathogen genotype proportion were quantified using digital polymerase chain reactions (PCR) to understand shifts in the genotype-dependent response in the presence of fluxapyroxad. Our results showed distinct fungicide concentration gradients between barley leaves, which correlated with variations in disease severity and pathogen genotype proportions. CONCLUSION: These results establish a quantitative link between spatial fungicide heterogeneity within plants and pathogen genotype responses, demonstrating that local fungicide concentration drives differential selection within plant tissues. The findings provide an empirical foundation for refining fungicide application strategies to sustain efficacy, reduce unnecessary inputs, and delay the evolution of fungicide resistance. © 2026 The Author(s). Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.