Molecular and Biological methods to evaluate the sensitivity to Carboxylic Acide Amide (CAA) fungicides of Plasmopara viticola populations

Carboxylic Acid Amides (CAA) represent compounds from three different chemical groups. Dimethomorph was the first carboxylic acid amide fungicide used to control downy mildews [1]; in Italy it has been authorized on grape since 1994 while the other compounds belonging to the same class have been in use since the early 2000s: iprovalicarb (2002), benthiavalicarb (2007), valifenalate and mandipropamid (2009). The mode of action of CAA compounds is linked to the inhibition of cellulose synthesis in the Oomycete plant pathogen [2]. At the end of the 2000s the single point mutation leading to an amino acid change from glycine to serine at codon 1105 (G1105S) in the CesA3 protein was discovered, conferring CAA resistance in P. viticola [3]. The occurrence of CAA fungicide-resistant populations of Plasmopara viticola was described for the first time in France in 1994 against dimethomorph, but it did not create problems in the field [4]. During the following years, resistance has increased gradually in Europe although field performance has remained good. The recessive nature of CAA resistance led the FRAC group to classify the resistance risk as moderate, and cross resistance among its members was demonstrated [5]. ...................................................