Observatree volunteers are helping to unravel complicated relationships between Curreya pithyophila and Scotland’s national tree

The fungus Curreya pithyophila was first described in Germany in 1823 on Scots pine and the suspected first UK record was made in Wales in 1876 on a ‘spruce fir’. The first Scottish record was made in 1907 on planted Scots pine in Perthshire. Occasional reports continued to be made in the 1960s and ‘70s suggesting infections were sporadic and interesting but not particularly associated with damage to host trees  (Green et al., 2023).

The first indication that this may be changing was a report sent to TreeAlert in 2022. On further investigation, Forest Research found caviar-like fungal stroma at branch junctions, blackened cankers and lower branch dieback were widespread in Scots pine growing in the Cairngorms area. The only recent report they could otherwise find of similar symptoms was in an Assynt field club article from 2014: further evidence (not that any is needed) of the crucial observational role played by citizen scientists (Haines, 2023).

When Scottish Forestry were made aware of the findings, our tree health officers helped locate new areas of infection. It soon became clear we needed to learn much more about the impact Curreya pithyophila is now having on Scots pine.

Risk assessment is a key first step towards developing policy for tree pests and diseases. It is just as important for organisms with the potential to spread within the country as those yet to be introduced. To assess risks posed by Curreya pithyophila, we need information on its distribution, hosts, consequences of infection, conditions needed by the fungus, how it spreads naturally and if it can be transported to new areas by other means. This information may then be used by the UK Plant Health Service to decide if the fungus should be assessed for the UK plant health risk register.

Researchers at Forest Research and the Royal Botanic Garden Edinburgh are continuing to gather information on these factors, with funding from Scottish Forestry and the Plant Health Centre. They determined Curreya pithyophila colonises the outer bark of Scots pine, forming root-like structures called stroma that encase populations of the commonly occurring, native, pine woolly adelgid (Pineus pini) on branches. This affords the adelgid nymphs protection from predators as they feed on Scots pine phloem and cambium while the fungus presumably benefits from the sugary exudate they release (Green and Taylor, 2024).

While the adelgids are not reliant on the fungus, the fungus is never found without the adelgids and dies when the adelgids die or escape from beneath the stroma. Wound sites from the feeding adelgids are then colonised by a second pathogen called Crumenulopsis sororia causing characteristic blackened cankers, with a ‘burst open’ appearance. This pathogen is also native to Scotland but caused relatively little damage before Curreya pithyophila enabled it widespread and easy access to newly wounded Scots pine branches.

Infected trees are rarely killed and only then where other stressors such as waterlogging are also found. Infected young trees can grow away from infection, but the worst infections show dieback in up to 50% of a tree crown.

What triggered the switch from rare encounters with Curreya pithyophila to widespread infections weakening Scots pine? Canker dating conducted by Forest Research suggests an increase in infections occurred in the Cairngorms from 2005 onwards. But the key to answering this question may lie in a forthcoming publication (based on Plant Health Centre funded research) identifying two different spore forms of Curreya pithyophila as almost identical, but different, species. The plot of this symbiotic, fungal-adelgid complex thickens once more! Watch out for name changes too, as it will not be called Curreya in future.

We still need help from citizen scientists (especially in Scotland) to collect more observations of Curreya pithyophila symptoms. See the Observatree factsheet and winter webinar for ID tips. We suspect Curreya pithyophila is more widespread throughout both native and planted populations of Scots pine than shown by the current distribution map. And what of that report of the symptoms on ‘spruce fir’? Are infections on spruces, firs, exotic pines and larch going unnoticed as they had until recently on Scots pine? Please continue sending in TreeAlerts for suspected symptoms on any host species.