The use of single-site fungicides to control plant pathogens in the agroecosystem can be associated with an increased selection of resistance. The evolution of resistance represents one of the biggest challenges in disease control. In vineyards, frequent applications of fungicides are carried out every season for multiple years. The agronomic risk of developing fungicide resistance is, therefore, high. Plasmopara viticola, the causal agent of grapevine downy mildew, is a high risk pathogen associated with the development of fungicide resistance. P. viticola has developed resistance to most of the fungicide classes used and constitutes one of the most important threats for grapevine production. The goals of this review are to describe fungicide resistance evolution in P. viticola populations and how to conduct proper monitoring activities. Different methods have been developed for phenotyping and genotyping P. viticola for fungicide resistance and the different phases of resistance evolution and life cycles of the pathogen are discussed, to provide a full monitoring toolkit to limit the spread of resistance. A detailed revision of the available tools will help in shaping and harmonizing the monitoring activities between countries and organizations.

BACKGROUND: Resistance to fungicides is one of the aspects that must be considered when planning treatments to achieve an optimal control of grey mold, caused by Botrytis cinerea, in vineyards. In this study, extensive fungicide resistance monitoring was carried out in Northern Italy (Lombardy region) to evaluate several aspects of fungicide resistance (frequency of resistance, effect of field treatments, mechanism of resistance and fitness) on 720 B. cinerea strains isolated from 36 vineyards.RESULTS: Of the characterized strains, 12% were resistant to a single fungicide class (3% to the succinate dehydrogenase inhibitor boscalid, 4% to the anilinopirimidine cyprodinil; 5% to the phenylpirrole fludioxonil; 0.1% to the ketoreductase inhibitor fenhexamid) and 0.8% to two fungicide classes contemporaneously. Resistance was associated with mutations reported in the literature for boscalid (H272Y/R) and fenhexamid (P238S or I232M). Two new mutations in sdhC (A187F) and in sdhD (I189L) could be related to boscalid resistance. Strains resistant to fludioxonil did not show any known mutations. No significant differences were found in the fitness of sensitive and resistant strains. CONCLUSION: Overall, field populations of B. cinerea showed a relatively low frequency of resistance, but the geographical distribution of resistance, genetic mechanisms of resistance and fitness of resistant strains suggest that management of resistance should be implemented, at local and regional levels. Particular attention should be given to the fungicide sprays planned before veraison, since they seem to be associated with a higher frequency of resistant strains in vineyards.

P. viticola secondary infections contribute significantly to the epidemiology of this important plant pathogen. However, the infection dynamics of asexual spores produced by this organism are still poorly investigated.

Background: Oxathiapiprolin is a novel fungicide and the first of the piperidinyl-thiazole-isoxazoline class to be discovered. This fungicide has been reported to have high activity against Plasmopara viticola, the grapevine downy mildew agent, and other plant-pathogenic oomycetes. In this study, the baseline sensitivity of Italian P. viticola populations towards oxathiapiprolin was established on 29 samples collected in 10 different viticultural areas. Two insensitive strains were characterized for their mechanism of resistance.Results: Oxathiapiprolin exhibited substantial inhibitory activity against 27 of the 29 populations tested, with EC 50 values ranging from a minimum of under 4 × 10 −5 mg L −1 to over 4 × 10 −1 mg L −1 , with an average value of 3.2 × 10 −2 mg L −1 . Two stable suspected oxathiapiprolin-resistant mutants were isolated from population exhibiting reduced sensitivity, and sequenced for the oxathiapiprolin target gene PvORP1. The comparison with wild-type isolates revealed that the resistant isolates possessed a heterozygous mutation causing the amino acid substitution N837I, recently reported in the literature. Conclusion:The results obtained indicate a risk for Italian P. viticola populations to develop resistance to oxathiapiprolin in association with the N837I mutation at PvORP1. Anti-resistance strategies should be carefully implemented and the sensitivity levels to this molecule should be monitored accurately in future to preserve its effectiveness.

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