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Tree threats - the scale of the problem and what we are doing about it

Britain’s trees and the people who care for them are in the front line of a challenging struggle to manage a range of potentially very damaging plant pests and diseases, most of which have entered from abroad.

Often causing little trouble in their native habitats and ecological niches in other parts of the world, some of these organisms can be virulent, fast-spreading and unstable in new environments which have few of the environmental or biological controls which keep them in check “back home”.

We are so concerned for the future health of the UK's trees and woods that we are increasingly diverting expenditure towards researching and combating tree pests and diseases. We are also working closely with the Department for Environment, Food & Rural Affairs (Defra), Forest Research, the Animal and Plant Health Agency (APHA), Natural Resources Wales (NRW), the Welsh and Scottish Governments, the Northern Ireland Assembly, environmental organisations and the forestry and horticultural industries to implement recommendations of the Independent Tree Health and Plant Biosecurity Taskforce. One output of the taskforce recommendations is an integrated, cross-sectoral strategy for strengthening plants’ and habitats’ protection from pests and diseases.

The UK Plant Health Risk Register (Defra) records and rates risks to UK crops, trees, gardens and ecosystems from plant pests and pathogens. It forms an agreed, evidence-based framework for decisions on priorities for actions by government and plant health stakeholders.

Global trade

Britons' love of exotic plants and trees in parks and gardens is partly responsible for the massive global trade in live plants. “Instant landscaping” for projects such as roadside and urban developments also generates significant demand for semi-mature trees and shrubs, a high proportion of which come from overseas suppliers.

Despite increasing phyto-sanitary (plant health) controls and inspections at points of export and entry, which aim to match the level of control with the level of risk to facilitate trade, some unwelcome guests are difficult to detect, and still get in. Some pathogens can enter the country, for example, in the soil of potted trees, plants and shrubs. Insect pests, such as moth and beetle species, can enter as larvae or live adults in freight containers or on vehicles, or as eggs laid on or in the plants in one country before they are exported to another. Oak processionary moth (Thaumetopoea processionae), for example, is believed to have entered London and Berkshire as eggs already laid on semi-mature oak trees imported from continental Europe for landscaping projects.

Despite being separated from continental Europe by the sea, some pests and diseases can also reach us through wind dispersal. Chalara dieback of ash, for example, is thought likely to have entered Britain in the form of wind-blown spores of the Hymenoscyphus fraxineus fungus as well as in infected ash plants for planting.

Climate change might be playing a part in some cases, because a warmer climate can make it easier for some exotic pests and pathogens to survive and become established in Britain.

Much can, and is, being done about these biological assaults on our islands’ ecosystems. Government strategy is founded on three basic principles:

1. keep it out if we can;

2. if we can't, eradicate it before it spreads and becomes endemic; and

3. if eradication is impossible, control and manage it to keep it below epidemiologically significant (that is, seriously damaging) levels.

There was general agreement that the EU’s plant health regime needed strengthening to meet the challenges of the global trading environment of the 21st century. The regime has been reviewed, and the UK was fully engaged in the review.

As a result, a new EU Plant Health Regulation was published in December 2016. The result is a more robust, transparent and sustainable regulatory framework. This should make it more difficult for exotic pests and pathogens to enter the EU or individual Member States in the future, while preserving the reasonable rights of citizens to engage in free trade and enjoy exotic plants.

Even without the EU review, much can and had been done by the UK and the international community to make it more difficult for pests and pathogens to enter the UK and other countries. For example, the international plant health authorities moved quickly to counter the threat of Asian longhorn beetle (Anaplophora glabripennis) and pinewood nematode (Bursaphelenchus xylophilus) when the former began infesting trees in the USA, and the latter in Portugal. Anecdotal evidence pointed to their having entered the USA and Portugal from Asia in the wood packaging used by industries such as the automotive assembly industry. International Standards for Phytosanitary Measures (ISPMs) were quickly agreed in 2002. These required all dunnage (timber packing used in ships’ holds to prevent cargoes moving about), and wood packaging materials (such as pallets, crates and boxes), to be either heat treated or fumigated. These measures ensured that the pathways by which these pests could have entered many more countries were largely closed. 

Control and manage

Clearly, though, a number of the pests and pathogens which have entered the UK are here to stay, so we are necessarily in “control and manage” mode with them.  Phytophthora ramorum (P. ramorum) is an example of a very damaging pathogen whose permanent presence in the UK we accept is now inevitable.

P. ramorum is a fungus-like pathogen which is particularly damaging to economically important larch trees (Larix species) and some other species associated with woodland. Among them are the introduced and highly invasive Rhododendron ponticum and the ecologically important bilberry (Vaccinium myrtillus; known as blaeberry in Scotland and winberry in Wales). Larches can succumb very quickly, and produce extremely high levels of infective spores, which can be spread widely from tall trees by the wind and moist air currents. We therefore require infected trees to be felled or killed, preferably before sporulation. We are also urging heightened vigilance for symptoms on larch, Vaccinium and Rhododendron ponticum.

Ramorum disease is also known, confusingly, as “sudden oak death”, because different strains of P. ramorum present in the USA have killed millions of native oak and tanoak trees there. Fortunately, Britain’s native species, sessile and pedunculate oak (Quercus petraea and Q. robur) appear to be much more resistant than their American cousins, with only a tiny few having been found to be infected to date.

Management of damaging Phytophthora pathogens such as P. ramorum is tailored to our understanding of the epidemiology (behaviour) on each host, and is required to keep their incidence below epidemiologically significant levels. Infected plant material must be destroyed, and the surrounding area monitored for recurrence. The application of biosecurity measures will also help to minimise spread.

The only available treatment for infection by P. ramorum is to destroy affected, sporulating host plants as quickly as possible after they become infected to limit further spread of the disease.

In the case of larch, which was not known as a P. ramorum host until 2009, the requirement to fell or kill infected stands of trees and buffer areas around them is causing locally significant landscape change as well as economic impacts for owners. It is a drastic step which can be heartbreaking for owners and disappointing for local communities and woodland visitors for whom the trees can be a socially important feature of their lives and landscapes. However, the principle driving the strategy is to sacrifice trees now to contain and significantly slow down the progress of the disease, and we greatly appreciate the public-spirited co-operation we have received from woodland owners. Without it, P. ramorum would almost certainly have had a significantly greater and more rapid impact than it has had.

Research is key

A rigorous and rapid response from the scientific research community is essential to successful control and management. We are fortunate that our Forest Research agency as well as APHA, Fera Science, SASA (Science & advice for Scottish Agricuuture) and several other British academic and scientific institutions employ some of the world’s leading biologists, plant pathologists and entomologists working on these problems. Through their work, we are increasing our understanding of the organisms and mechanisms involved, thereby potentially unlocking the keys to their successful management.

This scientific effort has led to success stories which encourage us to be confident that we need not fear catastrophe. For example, when accidentally imported on timber imported from continental Europe in the 1980s, it was feared the great spruce bark beetle (Dendroctonus micans) would inflict serious damage on our softwood timber industry, which, in northern and western regions, is dominated by Sitka spruce (Picea sitchensis).

Our initial response was to declare a quarantine zone in control areas where it first became established. This prohibited movements out of the quarantine zone of logs with bark still attached to prevent the pest being moved into new regions. This was followed by intensive research into potential longer-term control measures, which resulted in our ability to deploy a biological control agent (BCA) in the form of Rhizophagus grandis, a tiny predator beetle of the great spruce bark beetle. R. grandis predation was proved to be highly specific to great spruce bark beetle, so it poses no risk to benign and ecologically important insect species, and it has an impressive ability to “sniff out” the tiniest populations of the beetle. Now when an outbreak of great spruce bark beetle is detected, a number of R. grandis can be released into the affected woodland to quickly reduce the beetle population to minimal levels.

Inexpensive BCAs such as R. grandis can seem an ideal control tool for pest species which cannot be eradicated, but they are not always a magic bullet. As any Australian land manager plagued by cane toads (Bufo marinus) can testify, introduction of the wrong BCA can prove even more damaging than the pest it was introduced to control. They can therefore only be permitted for use after thorough research to ensure that the introduction of such a species will not itself be damaging, and introductions must first be approved by Defra's Advisory Committee on Releases to the Environment (ACRE).

Pine trees under fire

One disease, Dothistroma needle blight of pine, until recently known as red band needle blight, is infecting pine trees across Britain. It inhibits their growth and timber yield, and in some cases eventually kills them. It causes distinctive reddish-brown bands on the needles, hence its previous name, and in Britain it is caused by the fungal pathogen Dothistroma septosporum. It can also be caused by another pathogen, Dothistroma pini, which is present in parts of continental Europe, including France, but has not yet been detected in the UK.

D. septosporum was first observed in the UK on pine plants in a Dorset nursery in 1954, but outbreaks were rare until the late 1990s. Since then it has affected all three of Britain’s main commercial pine species. Until recently Corsican pine (Pinus nigra ssp. laricio) was the species most affected in the UK, particularly in England. However, surveys have shown that a significant number of lodgepole pine (Pinus contorta var. latifolia) stands are also infected, and there is now evidence that Scots pine (Pinus sylvestris) is more susceptible than was initially thought. The disease has also been found on a number of other pine species and several other conifer species. We are particularly concerned about Scots pine, Britain's only native pine tree, and there is growing concern that our iconic Caledonian pinewoods might become increasingly affected.

Like all fungal agents, D. septosporum thrives, and spreads most readily, in humid conditions, so we advise forest managers to thin their trees well to encourage air movement to reduce humidity in the forest canopy. Inoculum levels should be reduced where disease is already present. The felling of heavily infected stands of lodgepole pine (particularly those of ‘inland’ origins) will also play a significant role in reducing inoculum pressures on other, less-susceptible pine species.

Treatment with copper fungicides to suppress symptoms might be practicable in some situations, such as Christmas tree plantations. Small trials have begun in Scotland to assess the potential efficacy and environmental impact of aerial application of approved fungicides. Regular surveys of Forestry Commission woodland are helping to develop our knowledge of its extent. Research continues to further develop long-term management strategies, and to determine the distribution and ratio of two mating types known to be present in Britain, which could, together with genotypic variation, lead to genetic exchange and the appearance of a new, even more-virulent form of the fungus.

Threat to native ash trees

A recent new threat is Chalara dieback of ash. This is a serious disease of ash trees caused by a fungus called Hymenoscyphus fraxineus (previously known as Chalara fraxinea). The disease causes leaf loss and crown dieback in affected trees, and often leads to tree death, particularly in younger trees.

Chalara ash dieback has caused widespread damage to ash populations in continental Europe. The long-term consequences of its entering the natural environment in the UK are likely to be similar, although the degree of mortality cannot yet be predicted with any certainty in our maritime climate. Experience on the Continent indicates that the disease can kill young ash trees relatively quickly (sometimes within one growing season of symptoms becoming visible), while older trees can survive initial attacks, but might succumb after several seasons of infection, particularly when exposed to other pathogens, especially honey fungus (Armillaria).

Chalara is especially destructive of common ash (Fraxinus excelsior), including its ‘Pendula’ ornamental variety. Narrow-leaved ash (Fraxinus angustifolia) is also susceptible. A great deal of research is being undertaken to identify potentially ‘resistant’ ash trees for future breeding purposes.

A Great Britain chalara management strategy has been agreed. Country response plans have been drawn up to implement the strategy in England, Scotland and Wales, while Northern Ireland has joined the Republic of Ireland in adopting an all-Ireland approach.

New era

It is clear that everyone needs to help in the fight against tree pests and diseases. If you manage or work with trees in woods and forests please:

  • be vigilant – regularly inspect the trees that you are responsible for, be alert for plant health problems, and report unexplained symptoms to us
  • be diligent – adopt ‘biosecurity’ and hygiene measures in daily routines to minimise accidental spread of pests and disease. For example, clean and, where practicable, disinfect items such as tools, equipment and boots before leaving infected woodland sites

If visiting woodland for sport and pleasure you can help by arriving with clean boots, clothing and equipment. Brush off mud and plant debris from boots, clothes, bikes, buggies and pets before leaving the forest, and give everything a more thorough clean when you return home.

In other words – don’t take an unwelcome guest into the woodland with you, and don’t bring one out.

Top tree pests and diseases


Last updated: 9th January 2018