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

Abstract

Relationship between aggressiveness and zoosporangia viability was studied in seven Plasmopara halstedii (sunflower downy mildew) isolates of races 100, 300, 304, 314, 710, 704 and 714. Aggressiveness criteria including latent period and sporulation density were analysed on sunflower inbred line showing a high level of quantitative resistance. There were significant differences between pathogen isolates for the two aggressiveness criteria. Viability analyses were performed on oval and spheric zoosporangia. The number of zoospores released from oval zoosporangia was significantly higher than those released from spheric ones. The oval zoosporangia for more aggressive isolates of races 100 and 3xx produced more zoospores than the oval ones for less aggressive isolates of races 7xx. There was a significant correlation between aggressiveness criteria and the number of zoospores released from oval zoosporangia and vice versa for zoospores released from spheric ones. It is concluded that the relationship between aggressiveness and oval zoosporangia viability may be established in P. halstedii.