Commercial conifers: How are species standing up to threat of pests?

UK commercial forestry doesn’t match European industries in size, but it is still significant, given the relatively small area devoted to forestry here. Hardwoods are historically important, but today’s industry is based on softwood-yielding conifers, almost entirely exotic in origin.

The UK’s mild maritime climate allows a wide range of exotic conifers to thrive, but they are now at real risk from alien insect pests and pathogens thriving in the same conditions.

THE PHYTOPHTHORA CONUNDRUM

Last rites were delivered to Japanese larch in 2014, soon after Phytophthora ramorum was found in south-west England in 2009. Nothing would stop ‘sudden larch death’ destroying this premium softwood tree, but UK plant health authorities could learn and prevent another Phytophthora catastrophe. But learn they did not. In September 2021 Phytophthora pluvialis, also causing foliar blight and terminal bark necrosis, was found in Cornwall on Western hemlock and Douglas fir. Widespread findings have been made since then. 

The disease pattern mirrors P. ramorum and is typical of Phytophthora pathogens that are moisture dependent. Demarcated areas now form a distinct pattern down the wetter, western flank of the British Isles. Wales is worst hit with around half of the country a demarcated area.

P. pluvialis was confined to the Pacific Northwest of North America and New Zealand, so how it arrived here is a mystery because importation of conifers from these two regions is banned. No such mystery surrounds the entry of P. ramorum. The pathogen reared its head in 1993 in Dutch and German nurseries on rhododendron and viburnum as the EU1 genetic lineage. The first wider-environment infection was reported two years later in Northern California on native North American and tan oaks, but as the NA1 genetic lineage. 

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UK authorities banned the import of potential host plant material from North America. NA1 and NA2 genetic lineages remain excluded to this day, but not EU1 and EU2. They strolled in via quarantine-less trade between EU member states.

When questioned by a House of Lords Select Committee in March 2014, Stuart Goodall, chief executive at Confor, said: “It requires a bold and confident government to save the day when faced with entry of potentially epidemic and fatal tree diseases.” Those in charge of biosecurity and quarantine in the mid 1990s appeared more concerned about not rocking the EU free-trade boat than preventing the destruction of precious UK timber resources.

The first finding of P. ramorum was on viburnum at a Sussex garden centre in 2002.

The first infected tree, a North American red oak, was also found in Sussex in 2004.

Rhododendron ponticum as an evergreen understorey shrub thriving in moist, maritime conditions was subsequently found to be a perfect foliar template for spore production. A similar scenario had occurred in the US where the evergreen California bay laurel or Oregon myrtle (Umbellularia californica) provided the foliar template for spore production, infection and terminal disease on thousands of native and tan oaks. Americans called the disease ‘sudden oak death’ or ‘SOD’. 

English oak looked high risk, but P. ramorum caused SLD (sudden larch death) with Japanese larch as the hapless host. The genetic versatility of P. ramorum means hybrid larch and European larch were clearly at risk with some infections subsequently found. That apart, all larch has essentially been tarnished as potential planting material by the surge of P. ramorum on Japanese larch.

Spirally arranged needles on the long shoots of Japanese larch infected by Phytophthora ramorum, showing the classic foliar blight symptoms of this disease called sudden larch death. (Image: FJ)

P. ramorum also infected other commercial conifers including a small number of Sitka spruce. Work was underway in Scotland to study stands of Sitka spruce mixed with ‘alternative’ conifers to bolster resilience of Sitka spruce. One alternative was Lawson’s cypress, which brings us to the next tree and disease.

A LOOSE-CANNON PATHOGEN

The Forestry Commission identified Phytophthora lateralis in November 2011. Response was muted despite genetic closeness to P. ramorum, probably because tree hosts were ‘arb’ conifers. Lawson’s cypress (Chamaecyparis lawsoniana) was the primary host, with a few Northern white cedar trees (Thuja occidentalis) and a Sawara cypress (Chamaecyparis pisifera) also infected.  

Attitudes changed in 2014 when P. lateralis was identified on western red cedar (Thuja plicata) at Bridge of Allan in Stirling. The FC’s website displayed a distribution map of mainland Britain using red dots to identify outbreaks, similar to the format already being used for Chalara ash dieback. P. lateralis was concentrated in west central Scotland (14 red dots with 21 separate Statutory Plant Health Notices (SPHNs) issued for the destruction of trees). There were two in Devon and one each in Yorkshire, Leicestershire, South Wales, and East Sussex.  

Ten years on, the latest map still shows the main concentration in central Scotland, with nine cases in England (Cumbria, Lancashire, Yorkshire, Leicestershire, Bedfordshire, Surrey, Sussex and Cornwall) and one in South Wales. There is a concentration of cases in Northern Ireland, though I don’t recall whether this cluster of cases pre-dates 2014. 

The latest available map is from 2016, which suggests the FC doesn’t regard this disease as a priority, perhaps because Lawson’s cypress is essentially an arb conifer.

However, extension of Lawson’s cypress into commercial forestry has been considered.

Dothistroma needle blight is now endemic in the wider environment throughout the UK, but forest nurseries are still forced to lift and torch hundreds of thousands of Corsican, lodepole and Scots pine at the first whiff of disease in the nursery. Corsican pine seen here in East Anglia is riddled with Dothistroma needle blight disease. (Image: FJ)

Back in 2014, the integrity of Sitka spruce in the face of P. ramorum (to which it is susceptible) was considered critical for the long-term future of UK softwood production as other conifers were picked off one by one. A project had been established by FC Scotland (as it was then called) to look at stands of Sitka spruce mixed with alternative conifer species, with Lawson’s cypress as a candidate, but as I remarked at the time, there was a wider plant pathology picture to consider. 
P. lateralis and P. ramorum are in the same subgroup of the Phytophthora genus and are genetically close to each other. P. lateralis is spread by air-borne spores called sporangia just like P. ramorum, with documented evidence for this in Brittany.

Mixing Sitka spruce with some other conifers could well bolster stand resilience to P. ramorum, but mixed plantings with Lawson’s cypress may not be a good idea. Inter-specific hybridisation (ramorum x lateralis) could create a new subspecies. Infection of Sitka spruce and Lawson’s cypress by P. ramorum was already confirmed by the FC. Lawson’s cypress could turn out to be a ‘loose-cannon’ conifer carrying a ‘loose-cannon’ pathogen, requiring caution before ‘firing’ into commercial forestry.

However, the potential of Lawson’s cypress as a timber-yielding species in the UK is worth consideration. I recall visiting the Surrey Hills around 2015 with a group of ‘feet-on-the-ground’ FC foresters. We came across a small stand of fine-looking Lawson’s cypress and I was told the FC had considered the species as an insurance replacement should something nasty happen to Sitka spruce. Ten years on that ‘nasty something’ could be upon us. It is called Ips typographus, the eight-toothed spruce bark beetle, recently found in Scotland. Apparently the unsuitability of sawmill machinery used in the UK to cope with Lawson’s cypress bark was one of the only things that held the conifer back from integration into commercial timber production.

Last but not least, Lawson’s cypress had already been recorded as a host of yet another Phytophthora, originally called Phytophthora austrocedrae but changed in 2014 to Phytophthora austrocedri. Another exotic pathogen also in Clade 8 of the Phytophthora genus, it is starting to decimate the last remaining redoubts of common juniper, one of only three conifers native to the British Isles.

A SMALL CONIFER WITH A BIG PROBLEM

Juniper woodland was already in deep trouble from overgrazing, burning and afforestation long before Phytophthora austrocedri was first found on common juniper (Juniperus communis) at Moor House–Upper Teesdale NNR (National Nature Reserve) in 2012. More outbreaks occurred in Scotland and Cumbria including some in the nursery sector. The disease then appeared to go off the radar, but not for long. In spring 2014, a map appeared on the FC’s website showing there were now 60-plus wider-environment outbreaks, mostly concentrated in Scotland.  

The FC claimed to have no idea how the pathogen got into the UK. Previous disease outbreaks were confined to Cordilleran cypress (Austrocedrus chilensis) in Argentina and Chile but imports of this and other junipers and cypresses from non-EU countries were banned. 

The best guess is it arrived via the European nursery sector where the pathogen likely resides to this day, undifferentiated from other Phytophthora species. The closest relative of P. austrocedri is Phytophthora syringae, which causes fruit rot of apples in Europe and the UK.

Common juniper is a small tree with a big problem. Re-establishment of juniper sites in the drier areas of southern England where the pathogen may find it more difficult to survive could prove to be the only practical solution to the problem. 

Ten years on there is not much more to say. The latest map (2020) published by Forest Research shows the same concentration of infected sites in the Cairngorms National Park and SSSI at Glen Artney, Perth in Scotland. The biggest concentration in England is in Cumbria with a smaller number of outbreaks in the north east and several in the south. 

PINE TREES IN PERIL

Dothistroma needle blight (Dothistroma septosporum), previously called red-band needle blight (RBNB), has infected pines in these islands since the 1950s. Corsican pine was first to succumb, then lodgepole pine and finally native Scots pine. Foresters carrying out surveys for Plant Health England in 2014 said that in spread, disease level and damage caused, native Scots pine (Pinus sylvestris) was already as badly affected as Corsican pine. Fearful for ‘wildwood’ P. sylvestris in the Caledonian Forest, FC Scotland initiated aerial spraying trials with a copper fungicide in 2013. 

Aerial spraying capability in the UK had dwindled from 40-plus operators in the mid 1970s to just two or three by 2014. The only commercial aerial spraying taking place was on bracken using asulam herbicide, threatened by regulations related to aircraft, pilots, application equipment and chemicals. FC Scotland’s decision to cancel the June 2014 trial was caused by related operational problems. Several years of aerial spraying trials were carried out before the programme was apparently abandoned.

Corsican pine being lifted and destined for the bonfire on a West Midlands forest nursery in August 2023 following a routine inspection by the UK plant health authorities who identified a trace of the disease. And hardly surprising since there are pine plantations just a few hundred metres away riddled with Dothistroma needle blight. (Image: FJ)

Dothistroma needle blight has effectively removed Corsican pine from commercial planting. Scots pine was initially described as only slightly susceptible, but now looks to be heading for the same fate as Corsican. Dothistroma is now endemic across the British Isles with no means of control, but the FC still makes forest nurseries destroy hundreds of thousands of Corsican, Scots and Lodgepole pine at the first hint of disease in nursery beds. 

While the FC pursues this perverse policy on Dothistroma, Forest Research has announced a new threat to Scots pine affecting thousands of trees across Scotland, caused not by a single pathogen but by what appears to be a ‘mini-ecosystem’. The main culprit is an exotic fungus called Curreya pithyophilia. The fungus appears to be in a symbiotic (mutually beneficial) association with an insect, a native adelgid species called Pineus pini. Affected trees develop abundant blackened cankers on the shoots and branches. These wound sites are subsequently colonised by a native fungus called Crumenulopsis sororia. The final result is black, slowly expanding cankers which disfigure the trees. No doubt what’s happening to Scots pine in Scotland will wake the world up to yet another problem for forestry.

LAST TREES STANDING

The list of lost conifers (Corsican pine, Japanese larch, Norway spruce and common juniper) and those at real risk, including Scots pine, western hemlock and Douglas fir, continues to lengthen. In 2014, I said: “The numbers of tree species now under terminal threat together with closely related species that may become susceptible after small changes in a pathogen’s genome point towards an increasingly desperate situation. Add on the threats from insect pests and plant pathogens not yet in the country, but waiting for their ‘visas’, and apparently no will or way to avoid, prevent or control these problems, then ‘Last tree standing’ doesn’t sound quite as alarmist as it may at first have seemed.” 

There’s no reason to change that view. In addition to Phytophthora pluvialis in 2021, we received an unwanted Christmas present in December 2018. That was Ips typographus, potentially the most industry-changing pest to hit UK commercial forestry so far. Ips is already established on Norway spruce (its primary host) in the south of England, and should it ever (heaven forbid) develop a liking for Sitka spruce, then we might as well all pack up and go home.