Sensitivity Studies of Plasmopara Viticola to Carboxylic Acid Amide Fungicides: in Vivo Test and Molecular Studies of PvCesA3 Gene

Many oomycetes species are pathogens of plants, animals and humans; some of them are well studied because they cause significant economic losses in agriculture and acquaculture. The cell wall of oomycetes consists mainly of cellulose, β- (1,3) and β- (1,6)- glucan and in some species a small amount of chitin is also present. The biosynthesis of cellulose in oomycetes is still poorly studied and therefore misunderstood, although it’s the target of some fungicides that inhibit this process, such as carboxylic acid amides. Since 2010, single amino acid exchanges in CesA3 protein conferring CAA resistance in Plasmopara viticola have been identified. In this work, we initially provide the first evidence of the presence of mandipropamid resistant populations of Plasmopara viticola in commercial vineyards in Italy (paper I). We continue by studying the different activity of four CAAs fungicides (benthiavalicarb, dimethomorph, iprovalicarb, mandipropamid) toward P. viticola resistant strains (paper II). The results show that the G1105S mutation affects all four CAAs, but its impact is varied. These results confirm that they are cross resistant, although many gaps in the mode of action are still present. In order to confirm our previous findings, we performed a microscopical base method to assess the sensitivity of four CAAs, and preliminary microscopical data confirmed the different activity toward the CAA-resistant and CAA-sensitive populations (chapter ten). Furthermore, we present a study (chapter eleven) in which we randomly selected samples from paper I, in order to test them using a different approach, simulating a scenario more close to the field. With this test, we were able to confirm the data presented in the paper I.