Plasmopara halstedii virus causes hypovirulence in Plasmopara halstedii, the downy mildew pathogen of the sunflower

While no significant difference was found in the pathogenicity of P. halstedii strains with and without virus towards sunflower seedlings of different resistance (pathotype differentials), the aggressiveness of the oomycete was diminished by PhV. Compared to the virus-free strains, the time required for the first sporulation (latent period) increased by about 1 day post inoculation. Progression of the pathogen from the hypocotyl into the epicotyl of sunflower (systemic infection) was reduced by about one third in the presence of virus. In the virus containing strains, the average density of sporangia produced per cm2 cotyledon reached only 75% of the virus-free controls. In summary, the presence of PhV leads to hypovirulence effects by weakening the aggressiveness of P. halstedii.