The number of tree pests and diseases arriving in the UK from other parts of the world has increased significantly in recent times.  Others are thought to be heading our way, helped by increased global movement of goods and people, and changing climatic conditions.

A non-native pest or disease can bring with it risks, costs and may pose a serious threat to plant species which may be a host, and more widely to tree and plant communities. Hosts which have not co- evolved alongside the pest or disease can also be at a disadvantage in resisting the new threat.

Within the UK, the historic impact of Dutch elm disease has removed mature elms from most of the English landscape. More recently, the arrival of Chalara die back of Ash prompted a national conversation and raised public awareness of the threats to plant health and the landscape. The impact of the arrival and spread of Chalara dieback of Ash continues to be seen across the UK.

However, if a pest or disease is detected early enough, there are measures which can be taken to eradicate them, or to prevent or slow the spread in a managed way. It may also give scientists time to research the issues and to advise and help develop suitable approaches.

An early detection led to the effective eradication of Asian longhorn beetle from a woodland in Kent. But early detection is most effective and wider reaching when more people are out looking for the pests and diseases of most concern.

In the autumn of 2013, Observatree was established to determine whether a network of specialist volunteers from across the UK could be trained in the identification and reporting of pests and diseases of concern. The Observatree partnership brought together tree and plant health inspectors, scientists, and charitable bodies with a shared interest in tree health to establish a volunteer network and equip them with the necessary skills and resources.

The first big success story came in the summer of 2015. Oriental chestnut gall wasp (OCGW) had just been found for the first time in the UK in a woodland in Kent and plans were being drawn up to limit its spread. An Observatree volunteer then reported it to the north of London, some 50 miles away. This prompted an increased surveillance by tree and plant health inspectors who found evidence of the gall wasps in many areas between. This altered our understanding of the distribution of the pest and how long it had probably been in the UK and provided an early indication of the potential of the Observatree project.

Throughout the project, Observatree volunteers have continued to submit other notable finds adding to our understanding of the distribution and spread of pests such as the Elm zigzag sawfly, Oak processionary moth and diseases including Chalara dieback of Ash and Sirococcus tsugae.

Whilst it’s important to know when a tree pest or disease is found in a new area, knowing where it is absent can be just as helpful. These absence data help us to understand the current extent of a pest or disease and, over time can be used to tell us when a pest or disease arrived in an area, and assist modelling of rates of spread. Observatree volunteers therefore not only provide information of tree pests and diseases found, but also information on trees that appear to be healthy and absent of any signs of ill-health. Recent updates to the online tree health reporting system TreeAlert, now include the option to record healthy-looking trees and any member of the public can register on the system and add trees that are near to, or important to them.

When Observatree began, it was necessary to guide our volunteers to where they could undertake their surveys. Both the National Trust and the Woodland Trust are Observatree project partners and permission to access the land managed by them broadened the scope of places for volunteers to survey. On many Woodland Trust sites in the southeast of England, our volunteers continue to monitor for Oak processionary moth (OPM), sharing their findings with Woodland Trust site managers and the tree health teams monitoring OPM at a national level. Opportunities for the volunteers, inspectors, and scientists to meet up at training days has also strengthened the surveillance being delivered in partnership.

Learning and experiences from the first ten years of Observatree continue to be shared through wider connections and activities, at home and abroad.  These collaborations and the dedicated volunteers are the key to Observatree’s continued success.

If you would like further information on key pests and diseases or links to the various plant health agencies please visit Reporting a pest/disease - UK Plant Health Information Portal (

If you wish to report a tree pest or disease please visit tree alert Tree Alert (