The role of selection pressure in shaping zoxamide resistance in Plasmopara viticola populations

BACKGROUNDZ: oxamide, a beta-tubulin inhibitor, is widely used in vineyards to control downy mildew caused by the high-risk pathogen Plasmopara viticola. This study aimed to investigate the selection of zoxamide resistance and its characterization, providing practical insights for resistance management, through a twofold approach: a quantitative assessment of selection pressure effects on oospore populations and the molecular characterization of resistance-associated mutations in P. viticola strains. RESULTS: A total of 126 populations sampled from 57 vineyards mainly located in North-eastern Italy, were analyzed over a 6-year period (2017-2022). Based on toxicological parameters, 90% of the samples were fully sensitive to zoxamide (EC50 < 0.2 mg/L; EC95 and MIC<10 mg/L). Resistant individuals, able to germinate at 100 mg/L zoxamide, were detected in low frequency (<12%) within 13 samples. Only two samples showed a high frequency of resistant individuals (24-33%). Resistance was primarily found in vineyards treated more than four times per season with zoxamide. Partial sequencing of beta-tubulin gene revealed different polymorphisms at codon 239 associated with resistant isolates: the known C239S/G mutations, with the SG genotype being predominant, and a potential novel C239T mutation, not previously reported. CONCLUSION: This study highlighted a low risk of resistance under moderate fungicide application frequencies, indicating the importance of limiting fungicide applications to preserve sensitivity. The genetic diversity of resistance mechanisms, reflected in the various mutations in the beta-tubulin gene, underscores the need for a deeper investigation into the fitness of the different genotypes to evaluate resistance spread in P. viticola populations. (c) 2025 The Author(s). Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.