Susceptibility of Douglas-fir provenances used in European forestry to the emerging pine pathogen Diplodia sapinea

The North American conifer Pseudotsuga menziesii (Mirb.) Franco (Douglas-fir) is important for European forestry due to its rapid growth and drought tolerance. Recently, the emerging fungal pathogen Diplodia sapinea (Fr.) Fuckel) (syn. Sphaeropsis sapinea (Fr.) Dyko & B. Sutton) has been associated with the mortality of mature Douglas-fir trees in Austria. This study investigated the susceptibility of ten Ps. menziesii provenances commonly used in Central Europe to this pathogen. A wound inoculation experiment was conducted on 300 two-year-old seedlings (30 seedlings/provenance) with a virulent D. sapinea isolate. Disease progression was monitored over 60 days, evaluating external symptoms (needle discoloration and necrosis, hanging/wilting of shoots), extension of phloem necrosis and blue-stain in the sapwood, and seedling mortality. All provenances were susceptible to D. sapinea , but varied in symptom severity and mortality. Phloem necrosis differed significantly among provenances, with South West Germany 1 ( SW Germany 1 ) exhibiting significantly shorter extension of phloem necrosis than South West Germany 2 ( SW Germany 2 ) and Randle . We did, however, not find significant differences in the other tested traits (external symptom development, blue-staining in the sapwood, and mortality) due to high within-provenance variation in disease response. Nevertheless, the provenances SW Germany 1 and La Luzette consistently exhibited the lowest disease incidence or intensity across all recorded symptoms. Our results emphasize the importance of screening for variation in disease resistance during provenance selection. Identifying more tolerant seed sources can contribute to maintaining Douglas-fir as a viable option for European forestry under changing climatic conditions. • Seedlings of Douglas-fir provenances show high susceptibility to Diplodia sapinea . • High within and among provenance variation in susceptibility was observed. • Needle necrosis, shoot wilting and dieback, bark necrosis, and blue-stain occurred. • Rapid disease progression leads to high seedling mortality within a few weeks. • Provenance selection should consider disease resistance and drought tolerance.