Scots pine: What threat does obscure fungus pose to conifer?

This piece is an extract from our A View from the Forest (previously Forestry Features) newsletter, which is emailed out at 4PM every Wednesday with a round-up of the week's top stories. 

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THEY tower over large swathes of the Scottish Highlands and have been called a symbol of "durability". But, Scots pine trees face a new threat – a previously unnoticed infection. 

Curreya pithyophila, a distinctive and rare fungus, has been confirmed in Scotland and, at the time of writing, at one site in Devon, England. Forming a black stroma which encircles shoots, branches and sometimes the main stems, C. pithyophila is superficial but encases "thriving colonies" of adelgid, Pineus pini, an aphid-like insect which feeds on the tree, initiating wounds. 

C. pithyophila grows between the inner and outer bark of the pine, pushing up the outer bark. (Image: Ewan Purser, Scottish Forestry)

Described as "obscure" by Forest Research, the fungus has only occasionally been mentioned in academic papers, and it reportedly did not produce any match on a global database when scientists tried to get to the bottom of cankering on Caledonian Scots pine in October 2022. 

Understandably, the threat is causing some concern among foresters, as is the mystery surrounding its resurfacing. News of its presence was first confirmed towards the end of 2023, with one newspaper report going so far as to quote Forest Research's Dr Sarah Green as saying: "It’s here to stay. You cannot stop something spreading that is already so widespread." 

WANT MORE ON SCOTS PINE? 

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Forestry Journal was later told by Forest Research that Dr Green's comments were taken out of context, but it did not provide further details.

In January, Dr Green and  Jo Taylor, of the Royal Botanical Garden, Edinburgh, penned a blog post, in which they provided some more information on the fungus' spread. 

"Historical reports suggest that previous C. pithyophila outbreaks occurred on plantation Scots pine in Perthshire in the 1900s and in north-east Scotland during the 1960s, with sporadic findings on individual trees from the 1970s onwards," they wrote. "The relatedness of current specimens of C. pithyophila to the fungal specimens which caused the previous outbreaks is unknown. 

"Further investigations are required to understand the drivers behind this new, widespread outbreak of C. pithyophila on Scots pine in Scotland. These investigations will focus on determining whether one or both forms of C. pithyophila in Scotland represent new introductions into the UK or whether climatic factors have triggered the outbreak." 

If it emerges that a different species of Curreya is involved, it then raises the question over its origins. As FJ's Dr Terry Mabbett put it: "If true [that it was imported], they need to explain where it came from and how it got into the UK."

Scots pine was only last week hailed by MSPs as a symbol of 'durability' (Image: Getty/Stock)

What does this mean for the future of Scots pine in Scotland (just last week described in glowing terms by MSPs on the anniversary of it being named the country's national tree)?

According to Dr Green and Jo, it is a little too soon to say. 

The wrote: "Since our research into this disease is still at an early stage it is hard to give an informed prognosis for the future health of Scots pine in Scotland."

Watch this space.