Emerging virulence arising from hybridisation facilitated by multiple introductions of the sunflower downy mildew pathogen Plasmopara halstedii

The sunflower downy mildew pathogen Plasmopara halstedii is an invasive plant pathogen in Europe of American origin. Despite efforts to produce resistant host varieties, nationwide monitoring in France has revealed the rapid emergence of new virulent races increasing the number from one founder identified in 1966 to as many as 14 today. We have genotyped 146 samples (including all 14 races) using 13 nuclear and one mtDNA marker. Samples of the same race were found to share alleles/mtDNA haplotype and the two most common races had individuals with multiple matching genotypes. Cluster analyses confirmed that the samples form three groups to which races strongly adhere. Clusters were highly differentiated (F(ST) 0.65) and characterised by high inbreeding coefficients. Despite this, samples of recently emergent races, including six that are unique to France had mixed ancestry between the groups suggesting they have arisen in situ due to hybridisation. Five such samples also had conflicting mtDNA and nuclear DNA profiles. This demonstrates that multiple introductions have aided the establishment of this pathogen in France, and suggests recombination facilitated by these introductions is driving the emergence of new and endemic races in response to host resistance.