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Protecting Scotland’s trees in a changing world

Author:  Professor Gerry Saddler
Last Updated:  16/03/2023

Professor Gerry Saddler, Chief Plant Health Officer for Scotland

Maintaining plant health in the urban, farmed or natural environment is of paramount importance despite the growing challenge as the risks from plant pests and diseases are steadily increasing due to increased global trade, international travel and the rise in internet shopping. Climate change is exacerbating this situation still further by extending the range of pests and diseases and facilitating their spread through increasingly violent storms.

Professor Gerry Saddler, Chief Plant Health Officer for Scotland, SASA, The Scottish Government

Professor Gerry Saddler, Chief Plant Health Officer for Scotland, SASA, The Scottish Government.

Trees, due to their longevity, can be particularly vulnerable to the actions of pests and pathogens. The UK is no stranger to their impact as witnessed by the devastation wrought by Dutch elm disease to our urban environment and natural woodlands in the 1960s and ’70s. A more recent outbreak, and of a similar scale in terms of impact and rate of spread, is chalara ash dieback (Hymenoscyphus fraxineus). The disease was first recorded in Poland in 1992, spreading from there across Europe, and was recorded for the first time in the UK in 2012, most likely arriving before that time on imported nursery stock. Ash dieback is now present across much of Scotland and as a result we have moved from prevention/containment to managing the effects of the disease and mitigating safety risks from diseased trees, while allowing for natural regeneration of potentially disease-tolerant or resistant trees wherever possible. The Tree Council has produced an invaluable resource targeted at local authorities and other landowners in Scotland detailing how to manage the disease. (Links to this and other sources of information are listed at the end of this article.)

Since the beginning of this century we have faced new challenges to tree health, most significantly diseases caused by Phytophthora species, a group of fungus-like organisms. Phytophthora ramorum is principal amongst these as a highly damaging pathogen of larch (Larix species) affecting many western parts of the UK and causing major problems in the southwest of Scotland. Due to the significant damage to the natural environment and plant-based industries this pathogen has caused, and will continue to do so, the authorities across the UK require landowners to take swift remedial action whenever and wherever it is found.

A close relative, Phytophthora lateralis, which mainly attacks Lawson cypress (Chamaecyparis lawsoniana) is equally devastating for the host as few trees, once infected, recover. It was first detected in the wider environment in 2010, at Balloch Castle Country Park in West Dunbartonshire and although the pathogen is thought to originate in Asia, it probably arrived in the UK through the importation of infected plants from neighbouring European countries. The likely source and infectivity of this pathogen are evident as four confirmed outbreaks on mature trees in the UK have been on nursery sites or next to garden centres or plant sales areas.

More recently, Phytophthora pluvialis was discovered in a woodland in Cornwall in September 2021 where it was found to be affecting mature western hemlock and Douglas fir trees. This pathogen was originally reported in Oregon, USA, in 2013 on tanoak and Douglas fir and was subsequently identified as the pathogen responsible for ‘red needle cast’ of radiata pine in New Zealand. Following extensive surveillance, more outbreaks have been found in Cornwall, Devon, Cumbria, Wales and Scotland, and in the case of the latter, specifically at two outbreak sites in Ross-shire and Argyll and Bute. Restrictions were put in place to prohibit the movement of any wood, isolated bark and trees of the genera Tsuga, Pseudotsuga, Pinus and Notholithocarpus from these sites but ongoing research into this pathogen may allow the partial relaxation of some of these methods if evidence indicates it is not likely to spread as rapidly or efficiently as Phytophthora ramorum has done on larch.

In addition to the threat posed to trees by fungal and fungus-like organisms, the UK authorities have had to deal with a number of invertebrate pests in recent years. Oak processionary moth (OPM; Thaumetopoea processionea) was first recorded in Greater London in 2006 on oak trees (Quercus species). In July 2019 the UK Plant Health Services intercepted a number of cases on recently planted oaks imported from the Netherlands and Germany, with around 60 findings now identified in England, Scotland and Wales. OPM caterpillars are damaging pests of oaks, but also pose a hazard to human and animal health and as such are subject to an official programme of surveys and control measures to minimise their further spread and impact.

In a similar vein, pine processionary moth (PPM; Thaumetopoea pityocampa) caterpillars are a pest of pine (Pinus species) and other conifers, as well as a hazard to human and animal health. During 2022 emergency legislation was introduced in response to a recent interception of PPM on pines imported from France. These measures were introduced to protect against further introductions and establishment of PPM in Great Britain by restricting the importation of pine and cedar trees.

Clearly government has a key role to play in protecting plant life, through the implementation of an effective import regime, pest and disease surveillance, outbreak management and response, oversight of trade in plants and plant products, advice to industry and the general public, etc. The UK’s Plant Health Service, made up by Defra, the Forestry Commission, the governments of Northern Ireland, Scotland and Wales, and their agencies work effectively together as they recognise that pests and diseases do not respect borders. It is inevitable, however, that even with an ever-vigilant inspection service, pests and pathogens may evade detection, enter the country and spread.

Largely as a response to increasing concern over chalara ash dieback, the authorities launched their first strategy to protect plant health in 2014 (Protecting Plant Health: A Plant Biosecurity Strategy for Great Britain). This was followed soon after, in 2016, by the Scottish Government’s own Plant Health Strategy. The latter aimed to build on the GB Strategy by reflecting local conditions, marshalling and focussing resources in order to protect Scotland’s agriculture, horticulture, forestry and natural environment. The strategy, amongst other things, made two major commitments: the appointment of a Chief Plant Health Officer for Scotland and the creation of a virtual centre of expertise in plant health (The Plant Health Centre). The purpose behind each was to bring greater focus to the maintenance of plant health at a national level and to draw together expertise from across agriculture, forestry, horticulture and the study of the natural environment to research, advise and ultimately counteract the multiple threats that exist, now and in the future, to Scotland’s plant life.

Although much has been achieved since the publication of these strategies, much work still remains and plans have been set out in a refreshed GB plant health strategy (Plant Biosecurity Strategy for Great Britain) published in January. The strategy builds on the consultation launched in September 2021 to inform our approach to plant biosecurity over the next five years. It aims to build yet closer working relationships between the authorities, those working in the plant-based sectors and members of the public, to foster greater understanding and ownership of plant health. It seeks to encourage industry-led initiatives, such as the Plant Healthy scheme, to increase awareness and highlight the importance of protecting plants from pests and diseases. It also seeks to raise greater awareness amongst the general public of plant health, through campaigns such as ‘Don’t Risk It’ that urge the public to play their part in protecting plant health by not bringing back flowers, fruit and vegetables when returning home from holidays abroad.

In summary, the threat from pests and pathogens to plant life in Scotland is ever-present and likely to grow in future. Only by maintaining vigilance and developing lasting partnerships between government, research institutes, industry and the general public will we succeed in protecting plant life now and in the years to come.

For more information

Ash Dieback Toolkit for Scotland – The Tree Council. www.treecouncil.org.uk/what-we-do/science-and-research/ashdieback/ash-dieback-toolkit-for-scotland

Don’t Risk It! www.sasa.gov.uk/dont-risk-it

Plant Biosecurity Strategy for Great Britain (2023 to 2028). www.gov.uk/government/publications/plant-biosecurity-strategyfor-great-britain-2023-to-2028

The Plant Health Centre. www.planthealthcentre.scot

Plant Healthy Certification Scheme. www.planthealthy.org.uk

Protecting Plant Health: A Plant Biosecurity Strategy for Great Britain (April 2014). www.assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/307355/pb14168-plant-health-strategy.pdf

The Scottish Plant Health Strategy (March 2016). www.gov.scot/publications/scottish-plant-health-strategy/

This article was taken from Issue 200 Spring 2023 of the ARB Magazine, which is available to view free to members by simply logging in to the website and viewing your profile area.