Characterisation of the level and type of resistance of potato varieties to late blight (Phytophthora infestans).

Understanding the level and type of resistance in potato varieties is relevant for integrating varietal resistance into the management of potato late blight. Accordingly, fifty-four potato varieties were tested for their level and type of resistance to late blight in 2019 and 2020 in Denmark. Spreader rows were artificially inoculated to ensure an even inoculum distribution in the trial. Disease severity was assessed once or twice per week. Cluster analysis (CA) was done based on the weighted mean absolute rate (WMAR), the relative area under the disease curve, the onset of disease (Xo), the severity of disease in the middle of the season, the time to reach 1% disease severity, the time to reach final disease severity, and the maximum disease severity. The resistance types were determined by comparing the tested varieties to Bintje (susceptible reference) for Xo and WMAR. The CA ranked the varieties as susceptible, moderately resistant, resistant, and very resistant based on their level of resistance. Except for a few varieties, the expressed resistance levels varied between the years. Several varieties that were susceptible in 2019 were moderately resistant in 2020. Also, the types of resistance that the varieties exhibited varied from year to year. In 2020, most varieties exhibited race-specific resistance, while in 2019 they mostly showed susceptible characteristics. The variation between years for the level and types of resistance of the varieties highlights the importance of regularly monitoring varietal resistance across time and space.