Downy mildew of sunflower, caused by Plasmopara halstedii (Farl.) Berl. et de Toni, is a relevant disease of this crop. High virulent pathotypes have been identified in several countries, while there are few data on the spread of P. halstedii pathotypes in some important sunflower-growing areas of Europe. The goal of this study was to give up-to-date information on the pathotype structure of P. halstedii in Hungary and provide some actual data on the virulence phenotype of the pathogen for six European countries. Infected leaves of different sunflower hybrids and volunteers were collected in seven countries (Hungary, Bulgaria, Serbia, Turkey, Greece, Romania, and Italy) between 2012 and 2019. A universally accepted nomenclature was used with a standardized set of sunflower differential lines for pathotype characterization of isolates. The virulence pattern of the isolates was determined by a three-digit code (coded virulence formula, CVF). A total of 109 P. halstedii isolates were characterized. As a result of our survey, 18 new P. halstedii pathotypes were identified in Europe. Two out of the eighteen pathotypes were detected from the Asian part of Turkey. The detailed distribution of pathotypes in Hungary is also discussed.

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.