A New Epidemic of Woolly Adelgid‐Associated Cucurbidothis spp. Is Damaging the Health of Scots Pine in Scotland

Scots pine ( Pinus sylvestris ) is a keystone native conifer species within the Caledonian forest of Scotland and an economically important timber species. It is host to a new and widespread outbreak of two stroma‐forming fungi, Cucurbidothis pithyophila and Cucurbidothis parmeliarum , which only occur in association with the Scots pine woolly adelgid, Pineus pini . Affected trees exhibit blackened cankers and shoot dieback. This study investigated the impact of the current outbreak of Cucurbidothis spp. on the health of Scots pine at six sites in the west of Scotland, confirmed the role of a secondary pathogen in causing the cankers and estimated when the outbreak started. More than 60% of surveyed trees had stromata of Cucurbidothis spp. at four of the six sites and four sites had over 90% incidence of cankers. Severity of crown dieback was greater in trees with Cucurbidothis stromata and in trees with cankers. Crown dieback also increased with the severity of ‘visible disease’ (stromata and/or cankers). The wound pathogen Crumenulopsis sororia was detected in cankers at three sites and is an additional factor contributing to crown dieback. Dendrochronological analyses of branch cankers revealed that the current epidemic of Cucurbidothis spp. probably started during the 2000s. This study demonstrated the ubiquity of Cucurbidothis in association with P. pini on both planted and naturally regenerating Scots pine in western Scotland and the detrimental effects on pine health. The results are discussed in relation to possible factors driving the outbreak and steps outlined for mitigating its effects on Scots pine.