Pathogenic variability in Plasmopara halstedii (sunflower downy mildew)

Abstract

Pathogenic variation was studied in 19 Plasmopara halstedii isolates belonging to races 100, 300, 710 and 714. Based on the reaction for the P. halstedii isolates to four sunflower hybrids H1 to H4 differing only in their downy mildew resistance genes, there were differences in virulence spectrum in pathogen isolates. All sunflower hybrids were resistant to isolates of races 100 and 300 and sensible to isolate of the race 714, H1 was only sensible to isolates of race 710. Regarding aggressiveness criteria on two sunflower inbred lines showing different levels of quantitative resistance, there were significant differences in aggressiveness for P. halstedii isolates. The index of aggressiveness (sporulation density/latent period) varied between 8.1 and 23.6. There was a significant correlation (r = −0.456) between the two aggressiveness criteria at p < 0.05.