Diversity in Plasmopara halstedii, the causal agent of sunflower downy mildew

Diversity of the level of morphological, pathogenic and genetic characteristics was studied in seven Plasmopara halstedii (sunflower downy mildew) isolates of seven races namely 100, 300, 304, 314, 710, 704 and 714. All analyses were carried out by using five single zoosporangium isolates per pathogen isolate. Morphological analyses were performed on zoosporangia for P. halstedii single zoosporangium isolates. Aggressiveness criteria were analysed in one sunflower inbred line showing a high level of quantitative resistance. Genetic relationships were detected between the single zoosporangium isolates using 12 EST-derived as SNPs markers. Based on the aggressiveness reaction for the P. halstedii single zoosporangium isolates, there were significant intra and inter-race differences for all criteria studied. Isolates of races 100 and 3xx were less virulent and more aggressiveness than isolates of races 7xx. There was no relation between morphology of zoosporangia and pathogenic characteristics for 35 single zoosporangium isolates. There was no intra-race genetic variation, but five genetically-identified groups were detected among pathogen isolates of all races. No correlation was detected between EST genotypes on the one hand and both pathogenic traits and morphological characteristics on the other.