Host resistance modulates in planta efficacy of seed‐applied fluxapyroxad against spot form net blotch in barley

BACKGROUND Seed‐applied fungicides are used to protect barley seedlings from spot form net blotch, caused by Pyrenophora teres f. maculata , among other diseases. In vitro assays evaluate fungicide efficacy but often overlook host resistance and in planta dynamics, limiting predictions of fungicide performance. We determined whether: (i) in vitro fungicide concentrations align with glasshouse in planta measurements; (ii) barley genotype affects fungicide distribution and dissipation patterns; and (iii) host resistance traits influence the fungicide concentration threshold required for effective disease control. RESULTS Glasshouse experiments integrating fungicide quantification and disease assessments showed that protection concentration thresholds differed between in vitro and in planta systems. The minimum inhibitory concentration in in vitro assays was 27.9 μg L −1 , whereas minimal protective concentrations (MPCs) varied among cultivars. Fathom, a moderately resistant variety, required an MPC of 10.3 μg kg −1 , whereas Spartacus, a susceptible to very susceptible variety, required 29.2 μg kg −1 . However, Maximus CL (moderately susceptible to susceptible) exhibited a higher MPC (18.9 μg kg −1 ) than Zena CL (susceptible to very susceptible; 14.8 μg kg −1 ), indicating a host genetic effect on MPCs. Liquid chromatography–mass spectrometry revealed variation among genotypes in fungicide distribution and decline over time. Principal component analysis indicated that cultivar biomass contributed to these differences, with high‐vigour plants diluting fungicide concentrations more rapidly than low‐vigour plants. CONCLUSION Linking fungicide dynamics with plant genotype, this study defines variety‐specific efficacy thresholds and supports tailoring the timing of second foliar fungicide applications by barley variety to minimise yield loss and maximise grower profitability.