Monitoring and tracking changes in sensitivity to zoxamide fungicide in Plasmopara viticola in Italy

Plasmopara viticola is controlled by fungicides with different modes of action, which include zoxamide. Zoxamide was developed in 1998 by Dow AgroSciences LIC (Indianapolis, IN) and has been commercialized since 2001 by Rohm and Haas Company. This fungicide is highly effective against oomycetes, used for foliar application in vine crops, potato, and other vegetables to control oomycete-induced diseases. Zoxamide acts by causing mitotic arrest by binding to β-tubulin, inhibiting tubulin polymerization and cell division of the pathogen. In the past three decades, the management of fungicide resistance has emerged as a significant concern for both growers and regulatory authorities; this is primarily due to the substantial increase in resistance, in terms of occurrence and spread, towards most fungicide groups. The monitoring of the effectiveness of fungicides is the only way to accurately identify as soon as possible the onset of resistance. In this study, we were interested in tracking the changes in the sensitivity of zoxamide to P. viticola populations collected during a 6-year monitoring (2017–2022) in two Italian locations: the autonomous province of Trento and the autonomous region of Friuli-Venezia Giulia. Bioassays on leaf discs were carried out, and EC50 and MIC values were elaborated. From our results, zoxamide showed for a long period of time high sensitivity, but over the last years we observed a change, as in 2022 most of the samples tested in these two regions showed MIC > 100 mg L−1.