How the global threat of pests and diseases impacts plants, people, and the planet

physical and mental health, the economic benefits of healthy plants, and food security. We discuss the UK’s risk-based approach to protecting the UK’s plant resource, and the approaches being taken to manage different kinds of risks, including import controls, management of threats once established, breeding resistant varieties, utilizing citizen science, and awareness raising.

policies and management responses to threats. The threats to plant health are growing: Figure 1 shows an upward trajectory over the last 100 years or so as an increasing number of plant (in this case, tree) pests and diseases have arrived in the UK. This is driven by globalization in trade, the movement of people and, crucially, the unprecedented volumes of international trade in plants and trees.
The UK is a net importer of plants and plant products. One kind of import with its own particular risk profile is that of mature trees ( Figure 2). The olive tree is currently a regulated species. Imported olive trees have to be accompanied by a plant passport and additional declarations, which mean that it has been inspected so that the authorities can be confident that it is free of pests and diseases.
However, the challenges are obvious; the trees are large, which makes inspection difficult, and they are transported along with large volumes of soil, which itself carries biosecurity risks. Olive trees are a particularly high risk, because they are hosts of the bacterial pathogen Xylella fastidiosa, as discussed below.
Some wood and wood products also pose a risk. They include not just sawn wood and timber, but also, for example, pallets and other forms of wood packaging, firewood and pieces of wood used for all sorts of other purposes ( Figure 3). It is known that in other parts of the world pests have been moved in firewood through the practice of taking firewood on camping trips into forests. Overcoming this challenge is proving to be difficult. There are very strict regulations on wood and firewood, but constant vigilance is necessary.
Direct sales and the Internet are also introducing new risks, requiring work with the online sales community. If prohibited material is found, the online auction is instructed to take it down, but many people think that just because they can buy something on the Internet it must be acceptable and legal. Assessing the degree of risk requires a lot of work with parcel and postal depots, and a combination of online intelligence and inspectors on the ground, sometimes supported by sniffer dogs that can detect prohibited products. Figure 4 shows examples of prohibited materials intercepted by plant health and seeds inspectors.
It is acknowledged that there are some very good Internet businesses; understanding how better to regulate them to manage risk is necessary for them to be able to trade in this new way. Sources of risk can be unexpected. Figure 5 shows a headboard that a family complained kept them awake at night because they could hear a scratching sound in it. They contacted the local council Pest Inspector and eventually a Plant Health Inspector became involved. The cause was familiar: the larva of the Asian Longhorn Beetle. These beetles lay their eggs in wood and their larvae, which are up to 5 cm long, noisily feed on the wood until they pupate, eventually emerging as adult beetles.

| The UK Plant Health Risk Register
The requirement for a way of assessing risk and managing it has led to a unique initiative, the development of the UK Plant Health Risk Register ( Figure 6). It was started in 2013 in the wake of the first report of Ash dieback in the UK. The aim is to be more proactive about the assessment and prioritization of risk and the preparation of contingency plans thereby maximizing the chances of early detection and response in order to avoid large-scale losses.
A team of Pest Risk Analysts led by the CPHO scours the world for new and emerging threats. The team works with scientists and practitioners to give every potential threat a risk rating based on the likelihood of it getting to the UK via known pathways and assessing what would happen if it did arrive. Among the questions it seeks to answer are: could it establish? Could it complete its lifecycle? Is it going to spread? How many years would it take to complete its lifecycle? What impacts would it have (covering economic, social, and environmental impact categories)? How severe would the impacts be?
Every risk is given a rating, on a scale of 0 to 125, which allows the portfolio of risks to be prioritized. The portfolio is large, about 1,050 at the moment, but many of them are low risk. There are about 60 entries in the highest risk category. It is significant that, since the Risk Register was launched, the rate of increase in the number of threats F I G U R E 1 Pest pressures have increased due to globalization of trade and travel (Defra)  Economists and statisticians are working with researchers to develop, review and refine these figures, because we need to understand and value what is at risk in order to make the case for funding, for research and for action to mobilize the community to act.
The question as to the wider impacts of plant health on people and the planet is a very interesting area. been trained to observe: take them anywhere and they will be looking up, looking down, looking for plants and the problems they may be facing; but a lot of people do not notice or value trees and plants.

| Plant health and public health
The have to think about the oak tree to appreciate its historical and artistic significance and its role in shaping British identity.
Plants and trees are at the heart of communities and society. Consider physical health. Physical inactivity causes 9% of premature deaths globally and is the cause of 6%-10% of the major non-communicable diseases (Lee et al., 2012). Encouraging people to use the natural environment can F I G U R E 2 Mature olive trees can be transported to the UK in a few days (Animal and Plant Health Agency, APHA) F I G U R E 3 Wood and wood products (APHA) have a considerable positive impact on physical health and well-being (Morris & O'Brien, 2019;Ward Thompson et al., 2012). More detailed analysis is needed, but it is estimated that there could be more than £2bn annual savings in terms of averting public health costs if people would get out into the natural environment (Lovell & Maxwell, 2016).
At a time of temperature extremes, when more than half the world's population live in cities, trees have a critical part to play in improving the urban environment by cooling due to evaporation of water from leaves, by providing shade and protection from UV, and by reducing heat-related stress and illness (Edmondson et al., 2016;Doick et al., 2014;Gill et al., 2007). To get a sense of the seriousness of this problem, the extreme heat in Europe during the summer of 2003 was responsible for 2,000 deaths in the UK.
Whilst the effects of trees on air quality is small, some trees are effective in removing pollutants from the air and the health effects can be significant. For example, the annual value of air quality regulation performed by woodland in the UK was valued at £0.77bn by one study in 2015, based on avoided health costs (Jones, 2017). The World Health Organization estimates that air pollution is responsible for 7.6% of deaths worldwide. The London Plane is an example of a common species that can have real benefits in the urban environment.
It absorbs particulates into its bark, which is shed and can be gathered and removed, thus eliminating pollutants from the air (Figure 9).
Trees can be a source of relief from chronic stress and psychiatric disorders, particularly associated with urban lifestyles. They can screen elements of city environments that act as stressors, such as high-rise  (Hodson & Sander, 2017). Proximity to, and views of, trees have been related to lower rates of criminal behavior and behavioral problems and to higher test scores, reading performance, and graduation rates (Matsuoka, 2010).

| Case study: ash dieback
It is now recognized that plants have an essential role in addressing the great social and environmental issues of the age. There are large numbers of ash trees in the landscape and they are a common sight by roadsides as amenity trees (Maskell et al., 2013).
There are at least 120 million mature ash trees in the UK, providing Resilience Strategy (Defra, 2018). It is considered as an option when a pest or disease has established such that a tree population is unable to recover, and where a genetic basis for resistance is demonstrable in a proportion of the tree population. Ash dieback is one such example, and Defra funded the largest experimental ash screening in the world. To identify sources of resistance, 155,000 ash seedlings from 15 provenances over 14 sites in the UK were screened, leading to the first archive of putatively tolerant trees, which will be planted in 2020.
There is reason to be optimistic, therefore, that there will still be ash in the landscape in the future. Tolerant trees will set seed and regenerate, supplemented by a breeding program to reinstate them. Similarly, a range of resistant cultivars of elm have recently been developed, and it is hoped that these will soon start to re-establish this species as well.

| Case study: emerald ash borer
A further threat to ash is the emerald ash borer (EAB), a pest in the highest risk category of the Plant Health Risk Register. It is native to Asia but has been in North America since 2002 to devastating effect (Donovan et al., 2013;Klooster et al., 2014). It has wiped out billions of ash trees, and has been particularly evident in urban environments. For several years, it has also been established in Russia and may now have The annual value of UK Plant Health to society (Defra)

F I G U R E 9
The bark of London Plane reduces urban pollution by removing particulates from the air (tctrca.ca) spread to the Ukraine (Baranchikov, Mozolevskaya, Yurchenko, & Kenis, 2008;Drogvalenko, Orlova-Bienkowskaja, & Bieńkowski, ;Valenta, Moser, Kapeller, & Essl, 2017). The main pathway for introduction into the UK is wood rather than trees, so although a ban on importing ash trees remains in place, EAB could be introduced in firewood or other wood products. In contrast to ash dieback, which can take many years to kill ash trees, EAB larvae feed on the inner bark of the tree, damaging the vascular system and usually leading rapidly to death within two or three years (Klooster et al., 2014). A study by Donovan et al. (2013) used EAB as a natural experiment to investigate the impacts of largescale tree loss on human health. The findings were striking: the loss of 100 million trees was associated with increases in mortality related to cardiovascular and respiratory-tract illnesses (15,000 and 6,000 additional deaths, respectively, in the 15 states investigated). The study suggests that plant diseases in the wider environment can have direct and significant impacts on public health. Similar effects may be occurring with other tree health epidemics elsewhere.

| Case study: Xylella fastidiosa
Olive trees have already been mentioned as an example of the difficulties associated with the movement of whole mature trees, but there are specific and significant threats facing the species itself. Figure 10 shows

| Public awareness and the role of citizen science
What can people do to help the plant health effort? Anyone flying in to a UK airport this (2019)

F I G U R E 10
Olive grove in Puglia, killed by Xylella fastidiosa (Defra) People can also play a role in helping to spot and report plant health problems. Figure 11 provides an overview of one initiative, "Observatree," a citizen science approach to tree health reporting.
Two hundred expert volunteers have been trained to identify 25 tree pests and, by working with a range of partners such as the Woodland Trust and Forest Research, they report occurrences through an online tool called TreeAlert. One of the successes of this network of observers has been the second finding of Oriental Chestnut Gall Wasp, reported by a volunteer and her daughter. Finding two occurrences was significant because it alerted the authorities to the fact that it might be not an isolated instance but the start of an outbreak.
Forest Research's tree health reporting tool, Tree Alert (https ://treea lert.fores trese arch.gov.uk/) is a way in which anyone can upload a report of a sick-looking tree. A triage process behind the reporting tool may be followed by a visit from inspectors to a tree of concern to establish what might be happening.
Another development is the launch of a Biosecurity Standard which, by working with industry, should lead to a plant health standard that will apply to plants in garden centers and nurseries so that customers will know whether the plants they are buying have been either UK sourced and grown, or properly imported and inspected, and are biosecure. The Plant Healthy website (planthealthy.org.uk) provides information about plant health and a self-assessment tool for businesses.

| CON CLUS ION
The risk from invasive plant pests and pathogens is significant and constantly growing as a result of increasing globalization and environmental change. The UK's plant health regime aims to manage that risk to protect the enormous value of plants and trees; from the value of crops and forestry products, to ecosystem services and societal benefits. There is also a role for industry and society through risk and responsibility sharing and risk reporting. Volunteers and citizen science initiatives can also help to identify and report threats so that timely action can be taken. Together we can help protect our plants and trees so that they can continue to deliver extensive economic, environmental, and social benefits to society.  Barton, J., & Gladwell, V. (2013). Viewing nature scenes positively affects recovery of autonomic function following acute-mental F I G U R E 11 Observatree, a citizen science project to monitor tree health (https ://www.obser vatree.org.uk/)