Sunflower downy mildew, caused by Plasmopara halstedii, remains one of the most destructive diseases worldwide. The genetic diversity of P. halstedii populations continues to challenge resistance breeding efforts. This study evaluates the effectiveness of key resistance genes against P. halstedii isolates collected in Hungary. Eight isolates were tested using the sunflower differential lines HA-335, RHA-419, and RHA-340, with the resistance genes Pl6, Pl_Arg, and Pl8, respectively. Disease development was assessed by observing sporulation and symptoms including stunting, chlorosis, damping-off, and abnormal development at three time points after inoculation. Plant height was also measured to evaluate growth responses. The Pl6 resistance gene (HA-335) did not provide effective protection against any of the tested isolates, indicating that Pl6 does not confer reliable resistance against the Hungarian isolates examined in this study. The resistance conferred by Pl8 was not uniformly effective against the Hungarian isolates tested. This study provides the first report of Pl8-virulent P. halstedii isolates identified in both Hungary and Central Europe. The resistance gene Pl_Arg (RHA-419) conferred resistance to all tested P. halstedii isolates. These findings highlight the changing virulence profiles of P. halstedii populations in Hungary, emphasizing the need for ongoing pathogen monitoring and strategic use of resistance genes.