In October 2022, disease symptoms in the form of multiple blackened cankers and dieback of shoots and branches were noted in Scots pine ( Pinus sylvestris ) in Scotland. Trees were affected across a range of site types, including natural Caledonian pine forests, planted native woodland and commercial plantations. We investigated the geographical extent of symptoms in the UK, which occurred across most regions of Scotland, and identified one of the primary causal agents to be a previously obscure stroma-forming fungal taxon ‘ Curreya pithyophila ’. The fungus harbours immature colonies of the native Scots pine woolly adelgid, Pineus pini , which live beneath the stroma and feed on the tree, initiating wounds. These wounds can then be invaded by secondary agents, such as a fungal pathogen of pine, Crumenulopsis sororia , which is thought to cause the blackened cankers. Historical reports suggest that previous outbreaks of ‘ Curreya pithyophila ’ occurred on plantation Scots pine in Perthshire in the 1900s and in north-east Scotland in the 1960s. A literature review of the taxonomy and ecology of ‘ Curreya pithyophila ’ is presented. The perplexing ability of this fungus to produce two ascospore forms (phragmospores and dictyospores) from apparently morphologically identical stromata, at the same site and often on the same tree or branch, was investigated. After morphological, culture and genetic analyses of single spore isolates, we conclude that the current population of ‘ C. pithyophila ’ in the UK comprises two distinct but co-occurring species. Multigene analyses show that the closely related but separate species occur in the family Leptosphaeriaceae in a clade with Alloleptosphaeria and sister to Leptosphaeria . Morphological comparisons with historical syntypes confirmed that the dictyospore-producing species corresponds to Sphaeria pithyophila (as ‘ pityophila ’), while the phragmospore-producing species matches a syntype of Sphaeria parmeliarum . We retain Cucurbidothis as the correct generic name, designate a lectotype for the type species Cucurbidothis pithyophila , and recombine the second species as Cucurbidothis parmeliarum comb. nov. , also designating a lectotype. These species differ not only in spore morphology but also in culture appearance, with C. pithyophila producing grey and C. parmeliarum producing apricot coloured cultures. Both species can also produce a coelomycetous asexual morph comprising brown, ellipsoidal conidia, with pycnidial conidiomata often occurring adjacent to the ascomata in the same stroma. Histological studies showed that both taxa only penetrate between the periderm layers with no evidence of fungal growth in the living phloem and cambium below, supporting the hypothesis that these species are dependent on the adelgids for nutrition. A third species, Cucurbidothis shangrilana comb. nov. , is also discussed.
Taylor et al. (Thu,) studied this question.