The Extent of <em>Lecanosticta acicola</em> Spread Along the Polish Baltic Coastline

This paper describes the study conducted to investigate the spread of Lecanosticta acicola, the cause of Brown Spot Needle Blight (BSNB), along the Polish coast of the Baltic Sea, with the main focus on Pinus mugo stands originally established for dune stabilization. Between 2023 and 2025, 22 sites were surveyed, including coastal forests and ornamental plantings. Characteristic BSNB symptoms were recorded in 21 of the 22 locations, and the pathogen’s presence was confirmed through culture isolation and species-specific PCRs. The disease was most severe in compact, monospecific P. mugo thickets, where defoliation exceeded 30%, while mixed stands with P. sylvestris or occasionally P. nigra exhibited lower infection rates. A degree of infection of P. sylvestris was observed in proximity to heavily infected P. mugo stands, confirming interspecific transmission under high inoculum pressure. We observed nearly ubiquitous occurrence of L. acicola along the coast suggesting an advanced stage of establishment. However, the very recent detection of the pathogen at the westernmost sites indicate ongoing westward spread. Human activity, particularly tourism, likely facilitates the pathogen’s dispersal between dune forests. These findings highlight the vulnerability of coastal P. mugo populations and underline the need for genetic diversity data that would allow to trace the origins and pathways of L. acicola spread in the south-eastern Baltic region. The potential adaptation of the pathogen to P. sylvestris poses a serious future risk for Polish forestry.