Fungicide Resistance Profile of Monilinia fructicola and Monilinia laxa Isolates From Commercial Peach and Nectarine Orchards in the Ebro Valley, Spain

Brown rot management in Spain relies on chemical control, making knowledge of fungicide resistance essential for designing effective programmes. In 2021, peach‐growing areas in Spain were surveyed, and only Monilinia laxa and M. fructicola were recovered, at frequencies of 53% and 47%, respectively. A total of 77 M. fructicola and 51 M. laxa single‐spore isolates were tested for sensitivity to five fungicides (cyprodinil, fenpyrazamine, fludioxonil, fluopyram and pyraclostrobin). Half maximal effective concentration (EC 50 ) values were determined using dose–response assays on conidial germination. Seventy‐five percent of M. fructicola and 67% of M. laxa isolates were sensitive to fungicides. The remaining isolates (> 25%) represented 10 phenotypes with varying tolerance levels, including two M. fructicola phenotypes moderately resistant to fluopyram and/or pyraclostrobin. Over 15% of M. fructicola and 6% of M. laxa isolates showed low resistance to fluopyram, while 4% of M. fructicola exhibited moderate resistance. Three M. fructicola and eight M. laxa isolates showed low resistance to pyraclostrobin, and one M. fructicola isolate showed moderate resistance. No significant tolerance to fenpyrazamine or fludioxonil was observed in M. fructicola , nor to cyprodinil in M. laxa . To investigate resistance mechanisms, genomic analyses targeted succinate dehydrogenase inhibitor (SDHI) genes. No differences were found in succinate dehydrogenase subunits B (SDHB) or C (SDHC) among fluopyram‐resistant isolates. Resistant strains displayed amino acid substitutions in oxidative phosphorylation proteins, including V‐type H + ‐ATPase subunits and COX10, COX15, and ubiquinol cytochrome c reductase components, suggesting adaptive modifications in respiratory pathways.