AbstractDothistroma needle blight (DNB) is a severe needle disease of pines worldwide, caused by two closely related species, Dothistroma septosporum and D. pini. The two fungal species are similar not only in their morphological characteristics, but also cause very similar symptoms in their hosts, and have a similar ecology. The aim of this study was to compare the virulence of the two Dothistroma species in natural infection experiments on 2-year-old seedlings of two DNB susceptible pine species, Pinus nigra and P. mugo, in two seedling stands for each pathogen species. The virulence of the pathogens and presence of symptoms (symptomatic needles, red bands and acervuli) were assessed after 2 years of exposure to inoculum. The incidence of seedlings with DNB symptoms was 65% and 76% for P. nigra and P. mugo, respectively. No difference was found between D. septosporum and D. pini in any of the three DNB symptoms evaluated on seedlings of P. mugo. However, symptoms of disease differed between the two Dothistroma species on P. nigra. Variables that reflect the intensity of disease development, the number of red bands and acervuli per needle, showed a difference in virulence between D. septosporum and D. pini, but only in the case of the host species P. nigra. The results suggest that the virulence of the two Dothistroma species could be affected by host pine species and that there are differences in the susceptibility of individual pine species to D. septosporum and D. pini. Further factors could affect the virulence of these pathogens, including isolate origin, climatic or environmental factors.