Disease development of Dothistroma needle blight in seedlings of Pinus sylvestris and Pinus contorta under Nordic conditions

Dothistroma needle blight (DNB), caused by Dothistroma septosporum, was observed for the first time in the Nordic countries during the 21st century, and the dynamics of the disease under Nordic conditions are still poorly explored. In this study, we followed the development of DNB on seedlings of Pinus sylvestris and Pinus contorta, planted at two forest sites in central Sweden. PCR with species-specific primers was used to detect infections of D. septosporum in needle samples collected over a two-year period. The seedlings were also examined for typical red bands and fruit bodies (conidiomata). One-year-old needles that were present on the seedlings at the time of planting became infected during the first summer. The first conidiomata appeared on P. sylvestris in autumn the same year and on P. contorta in spring the following year. The first infections of the current-year needles of both host species occurred in summer, as they were starting to elongate, and the following spring the first conidiomata appeared. On one of the sites, many seedlings carried latent infections without any symptom development. At some time points, infections of D. septosporum were detected in more than 50% of the seedlings, but red bands and conidiomata were only observed on a small number of the seedlings throughout the study period. No clear difference was observed in the susceptibility of infection between the two host species; nonetheless, at the same site, the mortality of P. sylvestris was higher than that of P. contorta, and this is likely primarily due to other stresses and the presence of another needle pathogen, Lophodermium seditiosum.