Efficacy of CAA fungicides on CAA resistant Italian populations of Plasmopara viticola

Dimethomorph was the first carboxylic acid amide (CAA) used to control downy mildews in Italy (since 1994) while other compounds belonging to the same class such as iprovalicarb, benthiavalicarb, valifenalate and mandipropamid were authorized from 2002 to 2009. The mode of action of CAA compounds is linked to the inhibition of cellulose synthesis in the Oomycete plant pathogen. The mutations conferring CAA resistance in Plasmopara viticola’s CesA3 gene are G1105S/V. The aim of this work was to monitor the sensitivity of Italian P. viticola populations to mandipropamid and dimethomorph. Bioassays on leaf discs, seedlings and grafted plants were carried out. Samples sensitive to CAAs showed EC50 values < 1 and around 5 mg/l for mandipropamid and dimethomorph respectively, while CAA resistant strains had EC50>100 mg/l for mandipropamid and 15-30 mg/l ........................