SENSITIVITY OF PLASMOPARA VITICOLA POPULATIONS AND PRESENCE OF SPECIFIC AND NON-SPECIFIC RESISTANCE MECHANISMS

The Oomycete Plasmopara viticola is controlled by fungicides with different modes of action, which include cyazofamid and ametoctradin. Both fungicides are inhibitors of the mitochondrial complex III, with different binding behaviour. Cyazofamid1 is classified in the QiI group and ametoctradin2 in the QoSI group. Compounds with a site-specific mode of action, are more prone to provoke resistance due to the occurrence of target site mutations. However, the first identified resistance mechanism for these two modes of action was a non-specific adaptation phenomenon, which affected all the inhibitors of the complex III. This mechanism is the overexpression of the alternative oxidase (AOX), which forms a bypass of the complex III. The aim of this work was to test the sensitivities of P. viticola populations from different North–Italian regions towards cyazofamid and ametoctradin and the underlying resistance mechanisms. Bioassays on leaf discs were carried out on sixty-four populations, and performed with both fungicides and with and without adding propylgallate (PPG), an inhibitor of AOX. From this trial approach it could be concluded if a reduced sensitivity was caused by an AOX overxpression and/or by a specific resistance. Samples with a specific resistance phenotype were analysed for mutations in the cytochrome b gene. In our samples the mutation L201S or an insertion of two amino acids (E203 VE V204) were detected as mechanisms for specific target site resistance to cyazofamid. Overexpression of AOX has also been been found in our samples, which affected the sensitivity to all complex III inhibitors. If the described resistance types lead to fitness penalties needs further investigation.