EFFICACY OF CARBOXYLIC ACID AMIDES (CAA) FUNGICIDES TOWARDS CAA SENSITIVE and CAA RESISTANT PLASMOPARA VITICOLA POPULATIONS: IN VIVO TESTS AND MOLECULAR STUDIES ON PVCESA3 GENE

Plasmopara viticola is controlled by fungicides with different modes of action, including carboxylic acid amides (CAAs). Dimethomorph was the first CAA introduced in 1988, followed by iprovalicarb, flumorph, benthiavalicarb, mandipropamid, valifenalate and latest pyrimorph in 2010 (Gisi et al. 2012). The mode of action of CAA compounds is linked to the inhibition of cellulose synthesis in the Oomycete plant pathogens. The mutations conferring CAA resistance in P. viticola located on the CesA3 gene are G1105S and G1105V.The aim of this work was to evaluate the activity of CAAs on CAA sensitive and CAA resistant isolates of Plasmopara viticola. Bioassays on leaf discs, on detached leaf and on grape plants trials were carried out. In leaf discs assay, CAAs tested on Italian strains showed different level of activity and in particular dimethomorph showed lower levels of EC95. The detached leaf tests showed under preventative conditions that the CAA-sensitive strains were fully controlled by all CAAs, while the CAA-resistant strains were best controlled by dimethomorph (Nanni et al. 2015). Under curative conditions all CAAs showed good activity on the sensitive strains, while the resistant strains were not controlled by any CAA. This finding confirms the cross resistance between CAAs. The good activity of dimethomorph on CAA resistant isolates under preventive conditions was also confirmed in greenhouse tests where whole plants were inoculated. All CAA resistant strains carried the G1105S/V mutations, which were detected by molecular techniques such as pyrosequencing, CAPS-PCR or qPCR. In order to gain a better understanding of the different behaviour among the CAA fungicides, molecular modelling and docking studies are still ongoing.