Pathogenic groups identified among Plasmopara halstedii isolates belonging to several races

Pathogenic groups among 50 Plasmopara halstedii (sunflower downy mildew) isolates belonging to races 100, 300, 304, 314, 710, 704 and 714 were identified. Based on the reaction for the P. halstedii isolates to sunflower differential line D3, these isolates were divided into two groups as more virulent isolates of the 7xx races and the less virulent isolates of 100 and 3xx races. Index of aggressiveness (sporulation density/latent period) was calculated for each isolate and the presence of significant differences between isolates of 100 and 3xx races (more aggressive) and isolates of 7xx races (less aggressive) was revealed. Consequently, it seems that P. halstedii isolates may be divided into two pathogenic groups as more virulent and less aggressive isolates of 7xx races and less virulent and more aggressive isolates of the 100 and 3xx races.