Determining the fitness cost associated with virulence in Plasmopara halstedii (sunflower downy mildew)

The fitness cost associated with virulence was analyzed in a local Plasmopara halstedii (sunflower downy mildew) population. Pathogenic and molecular analyses were carried out on seven pathogen isolates including five progeny isolates of five P. halstedii races arising from two parental ones. P. halstedii isolates showed significant differences for all aggressiveness criteria and important genetic variations. Two cases of relationship (positive and negative) between virulence and aggressiveness for progeny isolates as compared with parental ones were found. Mean virulence cost values varied between 19.9% for positive relationship between the two components of pathogenicity and 50.8% for negative one. For solving the presence of two cases in pathogenicity, the relationship between virulence and aggressiveness among the isolates of three different races localized in the same genetic clade was positive. The hypothesis explaining these cases are discussed