Dr Terry Mabbett considers the varied issues which affect the UK’s Christmas trees.

THE Christmas tree cycle – comprising cultivation, marketing and use – features monoculture, a long post-harvest period and field-grown trees placed inside warm indoor environments. This should present ideal conditions for pest and disease development and spread, but, surprisingly, there is no single pest or disease which currently threatens the survival of UK-grown Christmas trees. However they do face a range of truly peculiar if not terminal problems.

Forestry Journal: Silent and serene with a covering of snow – home-grown Christmas trees.Silent and serene with a covering of snow – home-grown Christmas trees.

PINING FOR CHRISTMAS – ONE STRIKE AND YOU’RE OUT

Despite Pinus sylvestris being one of only three native British conifers (the others being Taxus baccata/English yew and Juniperus communis/common juniper), Scots pine has never featured as a mainstream Christmas tree across the UK. There are niche pine markets for Scots pine and lodgepole pine (Pinus contorta) within Scotland and parts of northern England, but I have never come across pines on the Christmas tree retail market in the south of England.

This is in stark contrast to other parts of the English-speaking world such as Australia, where radiata pine (Pinus radiata) reigns supreme, and North America, where native pines like Eastern white pine (Pinus strobus) and Virginia pine (Pinus virginiana) feature prominently in the Christmas tree market. Pinus sylvestris is not native to North America, but is a popular choice there nevertheless and predictably perhaps is called ‘Scotch Pine’.

So are there any potential pest and disease problems for landowners thinking of growing pines for the UK Christmas tree market? The only real potential problem is Dothistroma needle blight (more aptly called red band needle blight/RBNB), caused by the fungus Dothistroma septosporum. Not so much a risk for Christmas tree farms, but high risk on the forest nursery due to UK plant health authorities’ peculiar policy of drastic and destructive action if Dothistroma is found on pine seedling trees.

Nursery pine beds are inspected by the Forestry Commission, which orders total destruction of the entire bed (at least) when just a whiff of Dothistroma is detected – and without compensation, even though Dothistroma is now endemic in the UK, from John o’Groats to Land’s End. The only thing to benefit is fireweed (rosebay willowherb) and the rust disease which affects both it and a range of true firs.

Pine species subject to inspection (and destruction) include Scots pine, lodgepole pine and Corsican pine (Pinus nigra subspecies laricio), although the latter, not traditionally used as a Christmas tree, is hardly planted at all because existing plantings have been riddled with the disease for years. Forest nurseries must go to great lengths to keep their pine seedlings Dothistroma free, only to see them sold and planted in the wider UK environment where this foliar disease is widespread,. I wish someone would explain the logic behind this apparently over-the-top action against forest nurseries.

MISSED OPPORTUNITY OR NEAR MISS?

Eastern white pine (EWP) is native to North America and used there as a mainstream Christmas tree, so it should at least feature as a minor Christmas tree species in the UK, as does lodgepole pine (also a North American native). Eastern white pine’s credentials as a Christmas tree are at least as good as those of lodgepole pine, which recently featured in a Daily Telegraph article, ‘9 of the best real Christmas trees for 2019 and where to buy them’.

Positive points about EWP as a Christmas tree are fast growth (6–8 years to reach 1.8 m or 6 ft) and little to no aroma – a relief for those with allergies. EWP trees are typically sheared to produce the classic Christmas tree shape. Otherwise, the crown becomes too thick to hang large ornaments.

EWP has been in the UK for 400 years, brought to England in 1620 by George Weymouth, which is why many out there will know the tree as Weymouth pine. However, there are warnings for anyone contemplating this conifer as a Christmas tree or indeed as a timber tree. Experimental plantings failed due to a rust disease (white pine blister rust) native to Britain, lethal to EWP and caused by the rust fungus (Cronartium ribicola), with its peculiar life cycle. Otherwise, this premium softwood timber tree would surely have featured prominently in UK forestry, alongside other North American natives like Sitka spruce and Douglas fir.

Cronartium ribicola is a highly specialised fungal pathogen which must alternate on two totally unrelated plant hosts to complete its life cycle. The alternate host to Pinus strobus is the genus ‘Ribes’, which includes wild and cultivated currants and gooseberries. The pathogen and disease subsequently got into North America, showing what it would have done to EWP in UK. That was 50–80 per cent mortality in EWP, causing affected US states to promptly set about destroying all wild currants and gooseberries, at great cost to overall biodiversity.

Forestry Journal: Rosebay willowherb (fireweed) gone to seed in September 2019, but seedlings germinating in 2020 could be spreading rust disease among true firs like Nordmann fir on Christmas tree farms.Rosebay willowherb (fireweed) gone to seed in September 2019, but seedlings germinating in 2020 could be spreading rust disease among true firs like Nordmann fir on Christmas tree farms.

FIRS, FIREWEED AND RUST DISEASE

Eastern White Pine is not the only Christmas tree conifer to suffer at the hands of a peculiar, plant pathogenic fungus causing a rust disease. Pucciniastrum epilobii, which causes fireweed-fir rust, is another of those peculiar Puccinia-type basidiomycete fungi that require two completely different plant hosts to complete their life cycle.

The main host is a group of weed species belonging to the Onagraceae (evening primrose family) with the alternate hosts as various Abies species (true firs). This group of weed plants, commonly known as willowherbs, includes Epilobium hirsutum (great or hairy willowherb) and Epilobium montana (broad leaved willowherb), but also Chamaenerion angustifolium (rose-bay willow herb or fireweed), a potentially pernicious problem for tree nurseries and plantations.

A clue to its importance is in the name ‘fireweed’. This otherwise attractive plant, with cerise-coloured flower spikes, comes up en masse wherever a fire has been burning, an all-too-common but necessary evil on forest nurseries. The unpredictable nature of tree species planting requirements, against a background of persistently poor tree-planting figures in UK commercial forestry, has meant nursery owners regularly torch tens of thousands of trees because they had grown too big for bare-root planting and/or the space was required to plant tree species that had started to sell.

Failure to control fireweed allows the disease to develop on these weed plants with spores infecting fir trees to cause foliar disease symptoms and needle damage. Abies grandis (grand fir) and Abies nordmanniana (Nordmann fir) are among the Abies species susceptible to this rust disease.

Forestry Journal: Norway spruce of this age and size might not be so lucky if faced with the newly arrived eight-toothed European spruce bark beetle (Ips typographus).Norway spruce of this age and size might not be so lucky if faced with the newly arrived eight-toothed European spruce bark beetle (Ips typographus).

NOT-SO-CURRENT-SEASON NEEDLE NECROSIS

Five years ago, an unlikely-sounding condition called ‘current season needle necrosis’ (CSNN) was all the rage on Christmas tree farms, causing extensive damage and threatening to become an economic catastrophe for Christmas tree growers across the UK. CSNN is highly unusual in being primarily a physiological problem that weakens cell and tissue structure. But it allows erstwhile non-damaging fungal microbes to exploit this decline in plant structural integrity to cause necrosis.

CSNN is initially caused by periods of prolonged high humidity in midsummer which depress transpiration and the free uptake of soil water and nutrients including tissue-strengthening calcium ions. Calcium is the metallic component of a gelatinous compound called calcium pectate, which sticks plant cell walls together. However, calcium is a notoriously lethargic divalent cation (Ca2+), which moves into plant roots in solution with difficulty at the best of times.

During highly humid midsummer conditions, when the atmosphere is saturated with water vapour, and evapotranspiration via leaf stomata is effectively curtailed, absorption of calcium in solution from the soil by the roots will essentially stop. The consequent shortfall in calcium weakens the foliage, which becomes susceptible to infection by fungal pathogens, of which Sydowia polyspora is thought to be a major culprit. 

True firs on which CSNN is frequently found include Nordmann fir, grand fir, and noble fir (A. procera). CSNN is recorded as a serious problem for Christmas tree growers in North America and much of Europe. It has been off the radar for a while in the UK, but one can only assume that the high-humidity mid-summer periods which trigger the problem have not been so prevalent over the last few seasons.

Forestry Journal: Grand fir is one of the newer species to spark interest in the UK Christmas tree industry and market.Grand fir is one of the newer species to spark interest in the UK Christmas tree industry and market.

SUPER SAP-SUCKING BUGS

Very few plant species escape the depredations of aphids, and conifer species used for Christmas trees are no exception. Aphids breed at a phenomenal rate to form explosive pest populations, assisted by adult aphids giving birth to live young (nymphs) in a process called parthenogenesis. Also called blackfly and greenfly, according to colouration of the adult and nymph stages, some aphid species are sufficiently serious on Christmas trees to require control by spraying with insecticide.

Nursery owners describe a particularly large black aphid beast that targets Nordmann fir, while Norway spruce is regularly infested with the common-or-garden green aphid and green spruce aphid (Elatobium abietinum). Aphids are sap-sucking bugs which extract cell sap using sharp needle-like mouthparts called stylets. And because they exist in such large numbers, they have a serious debilitating effect on Christmas trees.

However, perhaps one of most peculiar aphid events reported on Christmas trees did not occur in the field or on the farm but in the home of a Derbyshire couple. They found thousands of aphids had invaded their house after insects brought into the house on a Christmas tree reproduced in a flurry, in response to the warm indoor conditions.

Forestry Journal: Why UK plant health authorities feel the need to carry out such drastic action against Dothistroma in forest nurseries, when the disease is widespread, rampant and endemic in the wider environment, is peculiar to say the least. Corsican pine seen here devastated by Dothistroma 10 years ago in East Anglia.Why UK plant health authorities feel the need to carry out such drastic action against Dothistroma in forest nurseries, when the disease is widespread, rampant and endemic in the wider environment, is peculiar to say the least. Corsican pine seen here devastated by Dothistroma 10 years ago in East Anglia.

BEETLES ‘WITH TEETH’ – BUT NOT FOR CHRISTMAS TREES

In December 2018, just in time for Christmas, the Forestry Commission announced the eight-toothed spruce bark beetle (Ips typographus) had been found for the first time in the UK. This insect pest of spruce trees, with Norway spruce as its primary host, is not any old pest, but is described as one of the most destructive insect pests of spruce trees in Europe, where it has regularly caused colossal damage.

Two outbreaks were recorded on Norway spruce in the Ashford area of Kent; one on Forestry Commission land and the other in woodland belonging to a high-profile NGO. I am reliably informed of a third outbreak reported from the Surrey Hills area of Surrey on privately owned land, though this one has never been officially recorded and publicised.

Virtually nothing has been said about the progress of Ips typographus by the UK plant health authorities for almost a year. A saving grace for the Christmas tree industry is that Ips typographus does not attack Norway spruce trees younger than 15 years old, which rules out the vast majority of commercial Christmas trees. However, there are some very large specimens that could exceed this size and age including, perhaps, the Christmas trees traditionally placed outside Number 10 Downing Street. Now, wouldn’t that set the ‘calling birds’ among the ‘French hens’?

Forestry Journal: Pristine pines at Prees Heath Forest Nurseries in Shropshire, with owner David Gwillam. Some of these Scots pine seedling trees could end up as Christmas trees in northern UK markets.Pristine pines at Prees Heath Forest Nurseries in Shropshire, with owner David Gwillam. Some of these Scots pine seedling trees could end up as Christmas trees in northern UK markets.

PHYTOPHTHORA RAMORUM SPREADS ITS WINGS

As long as Phytophthora ramorum is preoccupied with deciduous conifers in the genus Larix (larch), evergreen Christmas tree conifers will continue escape lethal disease caused by this fungus-like pathogen. However, Phytophthora ramorum appears to be spreading its wings and is now recorded on an increasingly wide range of conifers, including some species grown for the Christmas tree market.

According to Forest Research, conifers which can be infected when growing near diseased larch include Douglas fir (Pseudotsuga menziesii), grand fir, noble fir, and western hemlock (Tsuga heterophylla). Western hemlock readily loses its needles after cutting and is therefore unsuitable as a Christmas tree. Douglas fir, while popular in its native North America, has never caught on as a Christmas tree in the UK due to perceived difficulties in decoration. Increasing interest is being shown in both grand fir and noble fir for the UK Christmas tree market.

ROBIN ‘RED-PEST’

Nothing is more lovingly associated with Christmas than the robin – a native and largely resident bird which topped a BBC poll with 34 per cent of votes as the UK’s unofficial national bird and is traditionally pictured sitting on a Christmas tree with the obligatory snow cover.

So I was amazed to find that robins can assume pest status on Christmas tree farms, through perching on top of tree leaders which snap under the strain. Christmas tree growers do not blast these iconic birds, but traditionally deploy something altogether more subtle. Visit a Christmas tree farm and you will see upper branches adorned with clothes pegs, considerately placed for robins to perch on, thus saving tree leaders from undue pressure and risk of breakage.

NEW PESTS AND DISEASES FOR CHRISTMAS

A recently published report warned how Christmas trees imported from mainland Europe could be providing a pathway for new pests and diseases to enter the UK. Using data gathered and analysed by the DEFRA plant health risk and horizon scanning team, the Grown in Britain report highlights 15 pests which it says could potentially reach the UK on imported Christmas trees. Every year, the UK imports around £3 million worth of cut Christmas trees from Scandinavia and the rest of Europe. Grown in Britain is now urging consumers and retailers to buy Christmas trees labelled with its mark, which certifies they are UK grown. However, what is not completely clear is the question around trees freely imported from Europe as seedlings to be grown on in Britain and whether, when sold, they are classed as ‘home-grown’ Christmas trees.

Given the highly integrated nature of European Christmas tree trading, with huge volumes of cut trees arriving from all over Europe during a brief period before Christmas, any monitoring for pests and diseases will prove to be a tall order indeed. Well-established, well-trodden and proven pathways for tree pests and diseases from Europe, including Ips typographus as the latest arrival, strongly suggest that any belated attempts to shut this specific stable door will have little effect other than creating more problems for a Christmas tree industry fighting an uphill battle against trees made from synthetic materials. Trees, woodlands and forests growing in Britain are already groaning under the strain of exotic pests and diseases introduced from and via Europe.