Scots pine: Questions need answered over Curreya pithyophila spread

EVERY once in a while an alien insect pest or fungal pathogen arrives on our shores showing all the hallmarks of impending devastation for native trees, woodland and commercial forestry. The most recent example was Chalara ash dieback in 2012 – a fungal pathogen spread by airborne spores onto one of the most common native broadleaf trees, causing terminal disease, with no worthwhile genetic resistance displayed within the population. Common ash was essentially finished as soon as Chalara ash dieback found its way into the wider UK environment.

Now our one and only native pine (Pinus sylvestris) faces something more complex, an alien fungal pathogen conspiring with a native insect pest (an adelgid called Pineus pini) to initially damage Scots pine, and pave the way for Crumenulopsis sororia, a native, wound-infecting, secondary-invading pathogen to aggravate the existing condition. 

It read like a conundrum when first announced by Forest Research, which described it as “a new ‘old’ threat” to Scots pine. That’s because the alien Curreya pithyophila fungal pathogen, in cahoots with the native adelgid (an aphid-like insect) and the native wound-infecting fungal pathogen, was occasionally described in the UK (and elsewhere) on various conifers since the 1800s, although information is virtually absent from the archives.

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This raises an interesting and very pertinent first question. If the alien fungal pathogen has been in the UK environment for several centuries, why has this complex condition now exploded into what sounds like the beginnings of an economically-damaging and future-limiting problem for Scots pine?

Changing climatic conditions come to mind, for which there are precedents. Ink disease of sweet chestnut (Castanea sativa), caused by Phytophthora cambivora and Phytophthora cinnamomi, was initially reported as a minor disease in the UK during the 1930s. Plant pathologists of that period predicted it could increase in frequency and seriousness should the UK climate warm up. It is doing that right now, which is presumably why ink disease is becoming a ‘blot’ on the landscape. 

Curreya pithyophila in plain sight. (Image: Supplied)

Next-best option is the arrival or evolution of a new and more pathogenic genetic lineage of Curreya pithyophila. Forest Research, in its latest pronouncement, surmised that, on the basis of information gained from past literature and morphological (what the fungus looks like) and genetic analyses of the fungal isolates collected from Scotland and Devon, the current population of Curreya pithyophila in the UK comprises two distinct but co-occurring species. 

It’s not written in black and white, but we can infer current problems are caused by a different species of Curreya than the one which caused previous outbreaks on plantation Scots pine in Perthshire in the 1900s and north-east Scotland during the 1960s.

If so, it clearly begs the question: Where did the new species come from and how did it get past our defences? This is perhaps answered, in part, by a ‘Management Recommendation’ given at the end of the same Forest Research document: “Source stock that has been propagated and grown in the UK by a reputable nursery and preferably one that is ‘PLANT HEALTHY CERTIFIED’.” As an aside, does this mean UK forest nurseries will henceforth be inspected and scrutinised for the condition, and the components which cause it, in the same way that nursery pines are inspected for Dothistroma needle blight, and torched if just a trace of the disease is found? 

The second question concerns the future of Scots pine as a commercial timber tree.

Apart from a single report from Devon, the affected trees are what Forest Research describes as ‘Caledonian Pine’, the name awarded to endemic Scots pine which colonised Britain following the last ice age. These populations are found in the Scottish Highlands and, despite their fragmented nature, have huge ecological, conservation and cultural value due to the host of native flora and fauna they support. 

But what about Scots pine in its wider timber context, with stands grown throughout the UK, and that are much more valuable since Dothistroma curtailed future large-scale planting of the higher-yielding, exotic Corsican pine? 

The cankers caused are clearly disfiguring, but even more concerning are the documented effects on Scots pine growth and development. According to Forest Research: “Some Scots pine trees show high levels of infestation and weak or suppressed trees can exhibit severe dieback, but the most common symptom is crown thinning and dieback of shoots and branches in the lower crown.”

These sound like serious timber-yield-limiting effects, likely to strike fear into the hearts of foresters and land-owners hoping to harvest sawlogs 20–30 years down the line. So it is fair to ask what the long-term prospects are for Scots pine if the condition starts to spread around the UK. Are we talking about something on a par with what Dothistroma did for the future prospects of Corsican pine?

Symptoms are described in graphic detail. The Curreya fungal pathogen “forms a black stroma which encircles the shoots, branches and sometimes the main stems of Scots pine, typically at branch junctions. The fungus itself remains superficial, but encased beneath the black fungal stroma are thriving nymph (juvenile) colonies of the native Scots pine woolly adelgid, Pineus pini, an aphid-like insect which feeds on the tree, initiating the wounds.”

But the real coup de grace appears to come from the native Crumenulopsis sororia fungus taking advantage of these wounds as a secondary invader to cause the blackened, perennating cankers and expanding to kill the branches. And can we expect reports of this condition to be found on other conifers, and will the clear complexity of the problem make the condition that much harder to decipher and manage? 

There must be many like me who fear Scots pine will be the next tree to succumb to the hordes of alien pests and diseases increasingly plaguing this country over the last two decades. But this is not the first time the tree has been under the spotlight.

The current crisis calls to mind another situation from 12 years ago.

Pinus sylvestris clearly has a close association with Scotland. But the tree grows well in a wide variety of locations across the UK. Scots pine along the boundary of the Wrotham Park estate where Greater London meets the county boundar (Image: FJ)

This was, of course, when Chalara ash dieback came on the scene, effectively calling time on common ash. I recall how the Forestry Commission had almost every last man and woman, including cooks and bottle washers, stalking the highways and byways looking for diseased ash trees and urging the rest of us to ‘play our part’. I am afraid my attitude was ‘it’s your mess, you clear it up’. By this time the disease had already spread into the wider environment, starting in East Anglia, and the game was already up for common ash, even though they did not want to admit it.

It was in this febrile atmosphere that experts started to cast around for more high-risk alien pests and pathogens waiting in the wings, poised to penetrate the UK’s defences, such as they were and are, to demolish more of our native trees. Against this background, statements were made about the potential predicament of Scots pine, aptly led by an expert from one of Scotland’s premier universities. Dr Steve Woodward, reader in tree pathology at the University of Aberdeen, gave his considered opinion on the potential risk of Scotland’s national tree becoming the next casualty. He was speaking at the Science Media Centre in late October 2012, where leading researchers outlined the risks. Neither Dothistroma nor the ‘new’ fungal pathogen insect pest association were on the agenda. The susceptibility of Scots pine to Dothistroma had only just been established and today’s ‘new’ fungus insect association was clearly not on anyone’s radar. 

The thoughts of Dr Steve Woodward and others received national coverage in The Herald, The Times and The Guardian, among other national dailies. Dr Woodward saw the threat to Scots pine coming from the Fusarium circinatum fungal pathogen, the cause of a disease called ‘pitch canker of pine’, a parasitic nematode (microscopic roundworm) called the pine wood nematode (Bursaphelenchus xylophilus). The nematode is vectored (carried and spread) by pine sawyer beetles which do not occur naturally in the UK. Woodward and other experts claimed that the fungus and the nematode could be poised to pulverise Pinus sylvestris after establishing in western Europe. A simultaneous twin-pronged attack would be devastating, they said. 

“I am extremely worried about the Scots pine,” said Dr Woodward. “It is an iconic tree to these islands and it is particularly vulnerable to these two tree pathogens, both of which have established themselves in France, Spain and Portugal and are causing tremendous damage there. We know that pitch canker is now established in northern Spain where it affects native pines, and we know that the Scots pine is susceptible to it. We also know that thousands of hectares of pines have had to be felled in Portugal to try to contain the pine wood nematode there. Now it has spread to Spain. It could arrive here at any time.”

ast Scots pine standing? Fears were expressed for the future of Scots pine back in November 2012 in the wake of the unfolding Chalara ash dieback disaster. (Image: FJ)

At that time, the biggest overall threat was from ‘free trade in goods and services’ (and pests and diseases) within the European Union, of which UK was still a member, and what I dubbed the ‘Alexandre Dumas Doctrine’ (All for one and one for all). Billions of plants and trees were traded within the EU annually, often with large amounts of soil attached to each plant. Experts like Dr Woodward feared such quantities of soil material made it impossible to control pathogens. “Some plants have 1,000 litres of soil with them,” said Dr Joan Webber, principal pathologist of Forest Research, the research agency of the Forestry Commission. “That soil is a black box full of billions of bacteria. It is very difficult to inspect and intercept.” 

The same volume of trade still exists but with more paperwork, set to get worse as the UK government’s new border policy, the ‘Border Target Operating Model’ (BTOM) for plant material, started to ‘kick in’ on 31 January 2024 – and still with large volumes of soil that is virtually impossible to effectively check for contamination.

Indeed, there certainly doesn’t seem to be any let up in the arrival and establishment of pests and pathogens since Brexit. 

The threats from pitch canker and pine wood nematode to Scots pine and perceived by Steve Woodward in 2012 still exist today as additional risks on top of Dothistroma needle blight and the new fungal pathogen–insect pest association already affecting and causing damage to native Scots pine.

Incidentally, members of the Scottish Parliament recently hailed Scots pine as a ‘symbol of durability’, exactly 10 years on from it being declared Scotland’s national tree. Let’s hope their praise does not prove mistimed and misplaced.