Diplodia tip blight (Diplodia sapinea) and site conditions shape Scots pine (Pinus sylvestris) endophytic mycobiome

Diplodia sapinea (Fr.) Fuckelis is an opportunistic pathogen of Scots pine ( Pinus sylvestris (L.) that causes Diplodia tip blight following host stress. The factors driving its shift from endophyte to pathogen are not well understood, particularly in relation to the surrounding fungal community. The objective of the current study was to determine the association of D. sapinea and the composition of the general endophyte community with symptomatic and asymptomatic sites, trees, twigs and tissues in an ongoing outbreak of Diplodia tip blight. The endophytic fungal community was characterized using metabarcoding of the ribosomal ITS2 region. We found that variation in fungal community composition was most influenced by differences between sites, highlighting the importance of site-specific environmental conditions such as previous drought impact and associated crown dieback. However, the fungal communities also varied between symptomatic and asymptomatic trees indicating the significance of tree health. The fungal communities of symptomatic trees, especially in twigs with tip blight symptoms, included D. sapinea , Therrya pini , and Lophodermium arboricola . These results are consistent with the balanced antagonism hypothesis, suggesting that shifts in community composition under stress may facilitate the transition of D. sapinea from a latent endophyte to a pathogen. D. sapinea was found in both healthy and symptomatic twigs, with a ∼60-fold increase in symptomatic tissues. Site differences accounted for 42.6 % of fungal community variation. In contrast, taxa from the order Phaothecales were more abundant in asymptomatic twigs and in healthy tissues of of symptomatic twigs - suggesting potential antagonism. Our findings provide insights into early disease detection and underscore the importance of monitoring endophyte community shifts to support Scots pine forest resilience under climate stress. • Distinct fungal communities were found in healthy and necrotic tissues within symptomatic twigs . • Diplodia sapinea was abundant in both necrotic and healthy tissues of symptomatic twigs. • Endophytic fungal communities varied significantly by site, tree health, and tissue type. • Asymptomatic Scots pines host fungi, like Phaeothecales , that may suppress D. sapinea . • D. sapinea co-occurs with specific fungi like Lophium arboricola but dominates symptomatic tissues.