Relationship between virulence and aggressiveness in Plasmopara halstedii (sunflower downy mildew)

Relationship between virulence and aggressiveness was studied in seven Plasmopara halstedii (sunflower downy mildew) pathotypes including five progeny pathotypes of races 300, 304, 314, 704 and 714 arising from two parental pathotypes of races 100 and 710. Aggressiveness criteria including percentage infection, latent period, sporulation density and reduction of hypocotyl length were analysed in one sunflower inbred line showing a high level of quantitative resistance. There were significant differences between P. halstedii pathotypes for all aggressiveness criteria. Pathogenicity of progeny pathotypes as compared with parental ones (relationship between virulence and aggressiveness) seems to be positive, negative or uncorrelated. Hypothesis explaining these cases are discussed.