New saga in Finland: The rise of Diplodia sapinea in Scots pine

The intensity of fungal virulence is likely to increase in northern forests as climate change alters environmental conditions, favoring pathogen proliferation in existing ecosystems while also facilitating their expansion into new geographic areas. In Finland, Diplodia sapinea, the causal agent of disease called "Diplodia tip blight", has emerged as a new pathogen within the past few years. To reveal the current distribution of the novel fungal pathogen, and the effect of temperature and rainfall on its distribution, we utilized citizen science for the detection and collection of symptomatic Scots pine (Pinus sylvestris) shoots. The Finnish culture collection of D. sapinea was initiated using in vitro cultured symptomatic samples, and selected strains were studied for their virulence and disease cycle. Furthermore, the mycobiome of selected symptomatic and asymptomatic Scots pine shoots was studied using amplicon sequencing and the presence of D. sapinea was confirmed with culturing, qPCR, and species-specific PCR. Based on over 500 Scots pine shoots testing positive for D. sapinea, the distribution of this fungal pathogen is concentrated along the coastal areas of Finland, extending up to 200 km inland from the coastline. The observed presence of D. sapinea followed the period of highest average temperatures recorded in Finland in 2023 and was also found to be related to less precipitation. The amplicon sequencing showed that abundance of D. sapinea was higher in the healthy tissues of symptomatic shoots compared to visually healthy shoots. Similarly, the abundance was higher in samples collected from coastal areas in Southwestern Finland, which are the most heavily impacted by this disease. Here, we show that the presence of D. sapinea is more extensive than previously assumed, and lastly illustrate the hypothesized disease cycle of the fungal pathogen in Finland based on observations made in the field from 2021 to 2024 and in vivo and in vitro studies.