Variation in two components of pathogenicity and its relationship with genetic diversity in Plasmopara halstedii (sunflower downy mildew)

Abstract

The specificity of the two components of pathogenicity: virulence and aggressiveness and its relationship with genetic variability were analysed in a local Plasmopara halstedii (sunflower downy mildew) population. Pathogenic and molecular analyses were carried out on seven isolates including five progeny isolates of five races arising from two parental races 100 and 710. P. halstedii isolates showed significant differences for all aggressiveness criteria and important genetic variations. Three cases of relationship between virulence and aggressiveness for progeny isolates as compared with parental ones were found as positive, negative or uncorrelated. For solving the specificity of these cases, relationship between the two components of pathogenicity among the isolates of three different races localised in the same genetic clade was positive. The hypothesis explaining these cases is discussed.

Keywords:

• aggressiveness
• EST-derived markers
• obligate parasite
• qualitative resistance
• virulence

Acknowledgements

This study was done at INRA – Clermont-Ferrand; we thank all persons who participated in the molecular and biological tests. We gratefully thank Jalal Al-Attar for statistical helping.