Phenotypic groups including pathogenic, morphological and genetic characteristics for 50 Plasmopara halstedii (downy mildew) isolates belonging to seven races based on interactions with sunflower plants were revealed. Pathogenicity for pathogen isolates were analysed in sunflower plants showing different levels of resistance. Based on the reaction for the P. halstedii isolates to sunflower hybrids varying only in Pl resistance genes, there were three identified groups based on virulence reaction. Index of aggressiveness was calculated for pathogen isolates and revealed the presence of significant differences between isolates of 100 and 3xx races (more aggressive) and isolates of 710 and 7xx races (less aggressive). Morphological analyses were carried out on zoosporangia and sporangiophores for P. halstedii isolates produced on the surface of cotyledons in sunflower plants infected thought roots. There were no groups based on the morphology of zoosporangia and sporangiophores for pathogen isolates. Genetic relationships were detected between pathogen isolates using 12 EST-derived markers. There was no intra-race genetic variation, but five genetically-identified groups were detected among pathogen isolates of all races. Combining data of pathogen’s variation with variability in sunflower to arrive at durable resistance against P. halstedii was discussed.
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.
The fast evolution of Plasmopara halstedii (downy mildew) remains a major risk for sunflower crop, as new races of the pathogen are bypassing the resistance of sunflower hybrids. In order to understand the processes which led a new virulence to appear in a local P. halstedii population, the genetic relationships were studied using 12 EST (Expressed Sequence Tag)-derived markers between five progeny isolates of races 300, 304, 314, 704 and 714 and two parental ones of races 100 and 710. All genetic analyses were carried out using five single zoosporangium isolates per P. halstedii isolate. There was no intra-isolate genetic variation among the seven pathogen isolates and five multilocus genotypes (MLG) were identified among the 35 P. halstedii single zoosporangium isolates. The single zoosporangium isolates of races 314, 704 and 714 had an intermediary genetic position between the single zoosporangium isolates of two parental isolates. The single zoosporangium isolates of three isolates of races 100, 300 and 304 were localized in the same genetic clade. Two genetic mechanisms could explain the emergence of new virulence in P. halstedii as a recombination between races and mutations in a clonal lineage.
Pathogenicity including virulence and aggressiveness characteristics was studied on three Plasmopara halstedii (the causal agent of downy mildew) isolates of races 710, 714 and 704 using five single zoosporangium isolates per pathogen isolate. Based on the reaction for the P. halstedii isolates to four sunflower hybrids H1 to H4 varying only in their downy mildew resistance genes, there were differences in virulence spectrum in pathogen isolates. Analysis of five single zoosporangium isolates for P. halstedii isolates showed significant variability within pathogen isolate for all aggressiveness criteria. There were no significant differences among pathogen isolates for all aggressiveness criteria. There were significant differences in the morphology of zoosporangia and sporangiophores for pathogen isolates. Genetic relationships were detected between pathogen isolates using 12 EST-derived markers. There was no intra-race genetic variation, but three genetically-identified groups were detected among pathogen isolates.