CAA, phenylamide and QoI resistance assessment in Plasmopara viticola oospores

The development of appropriate anti-resistance strategies should be based on reliable data on the sensitivity to fungicides used for the control of P. viticola populations which were collected early in the season before the start of grapevine development. Sensitivity tests were carried out by assessing the germination of oospores, the overwintering structures of the pathogen, incubated at increasing concentrations of metalaxyl-M and mandipropamid or at a single discriminatory concentration of azoxystrobin. EC50 values (for metalaxyl-M and mandipropamid) and the percentage of resistant oospores (for QoI) were determined to estimate resistance in field populations. No resistance was detected for metalaxyl-M and mandipropamid, but different levels of resistance were found for QoIs. The use of PA, CAA and QoI fungicides in mixture resulted in reduced EC50 and percentages of resistant oospore, whereas higher rates of resistant oospores were associated with the application of QoIs as solo formulations.