Diplodia sapinea as a contributing factor in the crown dieback of Scots pine (Pinus sylvestris) after a severe drought

•Droughts can facilitate new pathogens in the North.•In 2018, Diplodia sapinea was discovered on drought-stressed Scots pine.•This study confirmed D. sapinea’s contribution to observed damages in Scots pine.•Severely affected pines experienced increased mortality.•Recovery of the surviving trees was independent of previous dieback levels. The frequency and impact of drought on global ecosystems have increased within the last century, while drought has affected tree health in many regions. Diplodia sapinea is a widespread, opportunistic pathogen infecting most conifers, causing Diplodia tip blight, thriving on hosts impacted by stress such as drought, heat, or mechanical wounding. In summer of 2018, a large-scale drought was recorded all over Europe. In late summer, pine trees all over Gotland showed crown dieback, where necrotic twigs and needles were found, especially in the upper part of the crowns. Symptoms were consistent with a potential outbreak of D. sapinea. Effects of the combination of drought and Diplodia tip blight on mortality or recovery of Scots pine in Nordic conditions are unknown. This study confirmed the presence and potential contribution of D. sapinea in the observed damages of Scots pine. Shoot blight and drought led to crown defoliation which was observed one year post-drought, while trees showed a clear recovery of newly grown shoots within the second year. Severely affected pines (>70% of the upper third of the crown with shoot blight) showed increased mortality. Recovery of the surviving trees was independent of previous dieback levels. Diplodia sapinea was most abundant in twigs with shoot blight of the symptomatic trees compared to healthy-looking twigs from the same trees and asymptomatic trees in affected and healthy pine stands. Sampling on affected and healthy sites showed possible endophytic infections with low abundance within healthy-looking twigs. Spore deposition of D. sapinea was monitored on healthy and affected sites for two consecutive years after crown damages occurred to confirm the presence of the opportunistic pathogen in the affected region. Spore deposition was observed during all seasons and correlated with high precipitation during sampling. Our observations provide insights into the emergence of Diplodia tip blight in the Northern countries and underline the potential impact of D. sapinea on tree health in the course of a changing climate.