Surveillance for Acute Oak Decline: Developing new methods to analyse and add value to volunteer reports

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By Nathan Brown, Matt Combes, Sandra Denman, Peter Crow, and Stephen Parnell

Large oak tree with volunteers and clipboards walking around it
Volunteers assessing tree health during a training workshop (Forest Research)

Increasing invasions of exotic pests and diseases

The introductions of exotic pests and diseases are already having large scale impacts on trees and woodlands across Great Britain. Diseases such as ash dieback and ramorum dieback have hit the headlines and are causing tree death at a scale that is altering woodland composition, forcing land managers to adapt their management plans and halting the wide scale planting of ash and larch trees. The unfortunate fact is that these arrivals are not isolated, in fact, due to increasing global trade the number of arrivals has been increasing each year.

Tree with dark spots on the trunk due to stem bleeds
Severe stem bleeds caused by AOD bacterial lesions (Forest Research)

Volunteer surveys to find the needle in the haystack

Early detection increases the chances that a pest or disease population can be successfully controlled before causing widespread problems. Spotting new pests and diseases quickly after they arrive is however difficult, because of the large areas of forests and complex landscapes involved, making detection something of a needle in a haystack problem. This makes volunteer surveys for plant pests and diseases an extremely useful addition to statutory surveillance activities. Reporting trees that you are concerned about and receiving diagnosis of the causes of ill health is a straight forward process, Forest Research manages TreeAlert a web based app that guides users to make contact with the relevant experts in their Tree Health Diagnostic and Advisory Service (THDAS).

Trunk of a tree with cracks through it's bark
Older bark cracks (Forest Research)

Acute Oak Decline

In addition to enabling the detection of new pests and disease TreeAlert gives an indication of common disorders currently affecting trees allowing the extent and impact of known issues to be monitored. One example where public reporting has furthered current scientific understanding is Acute Oak Decline (AOD). AOD affects the main stems of native oak trees. In 2018 we produced a risk map for AOD across England and Wales using a designed survey and 207 TreeAlert reports of AOD. This analysis revealed that the location of AOD affected sites is strongly correlated with environmental variables that are likely to predispose trees to further decline. Areas that experience higher temperatures and lower rainfall are especially susceptible to oak decline. Since 2018, we have continued to receive new reports of AOD affected trees through TreeAlert. Each new record helps to improve our understanding of the distribution of AOD and the site factors that make oak trees more likely to be predisposed to the bacterial and insect species associated with AOD.

Close-up of bark with small D-shaped holes left from beetles
D-shaped emergence holes of Agrilus biguttatus (Forest Research)

Join one of our volunteer groups!

This summer we plan to run training courses to aid the detection of affected trees among volunteers, forestry professionals and members of the public so that we can better understand how likely it is that symptoms are reported when they are present and further improve our risk maps.

The symptoms of AOD are readily identifiable, on the outer bark black, vertically arranged, weeping stem lesions are a distinguishing feature and, in at least one third of cases, the D-shaped exit holes of the adult native two-spotted oak buprestid Agrilus biguttatus are also present. Below the surface (in the inner bark) larval galleries and bacterial lesions are visible close to the cambium. The degradation of live bark tissue has been attributed to a bacterial pathobiome (multiple bacterial species that act together to hasten tissue decay). Three species of bacteria play key roles in stem decay, primarily Brenneria goodwinii, which is supported by Gibbsiella quercinecans and Rahnella victoriana. These species become especially active in the galleries of A. biguttatus and a link between the beetle and the pathobiome is currently being investigated.

Small shiny beetle on the tip of someone's finger
Adult Agrilus biguttatus beetle (Forest Research)

During the summer of 2022 we worked with a group of 23 people who had a range of prior experience of AOD, from novices to Observatree-trained experts. We held workshops at Richmond Park, London and Hatchlands park in Surrey where we provided training on the day and then held an activity following a set route through the parks where they could check up to 175 trees for symptoms of AOD. This was quite an undertaking on some very hot summer days, so thank you to everyone who took part. In total we collected over 7500 observations of the three symptoms types and our observers were so keen eyed that they even spotted one of the illusive Agrilus biguttatus beetles visiting the oak stems. This autumn (2024) we will extend this work and plan to hold further events in London, Surrey, Shropshire and Norfolk.

If you would like to find out more about one of our free workshops this Autumn, please contact Dr Matt Combes, to register your interest! We will run a series of one day workshops and will shortly be confirming dates and times of these events.

Click here to find out more about Acute Oak Decline or check out the Forest Research website at

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Posted On: 03/05/2024

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