Variation in aggressiveness of Plasmopara halstedii (sunflower downy mildew)

Abstract

Variability in aggressiveness was studied in seven Plasmopara halstedii (sunflower downy mildew) parental isolates of races 100, 300, 304, 314, 704, 710 and 714 using five single zoosporangium isolates per parental isolate. Aggressiveness criteria, including latent period and sporulation density, were analysed in one sunflower inbred line showing a high level of quantitative resistance. Analysis of five single zoosporangium isolates of each parental isolate showed variability within parental isolate for the two aggressiveness criteria, but not for all parental isolates. The parental isolates of races 100, 300, 304 and 314 had a shorter latent period and greater sporulation density than the parental isolates of races 710, 704 and 714. Consequently, parental isolates of races 100, 300, 304 and 314 were significantly more aggressive than the parental isolates of races 710, 704 and 714