The management of grapevine downy mildew: from anti-resistance strategies to innovative approaches for fungicide resistance monitoring

Efficient grapevine downy mildew control necessitates the implementation of anti-resistance strategies to ensure the ongoing efficacy of available substances and optimal disease control. With the gradual disappearance of multi-site fungicides from the market, reliance on single-site fungicides poses a long-term risk of selecting strains resistant to multiple modes of action. Challenges in disease management encompass selecting optimal spray programs and monitoring field population sensitivity. This study evaluated the efficacy of anti-resistance strategies, including two single-site fungicides (mandipropamid and oxathiapiprolin), on disease control and fungicide sensitivity through a combination of field trials and laboratory tests for the biological and molecular characterization of the pathogen populations over a three-year period (2019-2021). Mandipropamid, a cellulose synthase inhibitor, is used since a long time for downy mildew control, while oxathiapiprolin, an OxySterol Binding Protein homologue Inhibitor, was introduced recently. Field trials demonstrated effective disease control, even in the presence of mandipropamid-resistant strains (with G1105S/V mutations in PvCesA3) and revealed a pronounced selection and spread of resistance to both fungicides in the vineyard where disease pressure was higher. Characterizing pathogen strains remained a significant obstacle in sensitivity monitoring, hindering precise determination of resistance frequencies related to fungicide programs. Traditional techniques, in fact, lack the resolution required for high-throughput isolation and characterization of resistant individuals. To address this challenge, we propose utilizing flow cytometry and fluorescence-activated cell sorting on field sporangia populations, a method able to determine both the number of resistant isolates and isolate pathogen strains in a single assay.