Pathogenicity and its alternation with both morphological and genetic variation in Plasmopara halstedii (sunflower downy mildew)

Morphological, pathogenic and genetic variation was studied in seven Plasmopara halstedii (sunflower downy mildew) isolates of several races using five singlezoosporangium isolates per pathogen isolate. 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 expressed sequence tags (EST)-derived markers. Analysis of the five single zoosporangium isolates for P. halstedii isolates showed variability within pathogen isolates for all aggressiveness criteria, but not for all pathogen isolates. Isolates of races 100 and 3xx were characterised with shorter latent period and higher sporulation density than the isolate of races 7xx. All pathogen isolates showed high percentage infection values and caused a large reduction in seedling size except for one isolate involved in dwarfing. There was no relation between zoosporangia form or size and race virulence profiles or aggressiveness criteria. There was no intra-genetic variation for all pathogen isolates, but it was observed an important genetic variation between single zoosporangium isolates of all races. No correlation was detected between pathogenicity traits and EST genotypes.