Bacteria: Advancement of Control and Knowledge to Save Threatened Oak and Protect them for Future Generations

Oak trees are iconic in Britain and important to our natural world and economy. Unfortunately, they are under threat from an emerging disease Acute Oak Decline (AOD). Affected trees have weeping stem patches caused by bacteria rotting inner tissues that carry food and water vital for tree life, and larval feeding galleries the oak jewel beetle (Agrilus biguttatus). Disease disrupts the flow of food and water in the tree often leading to its death. Solutions to manage and protect trees are vital to ensure the future of British oak. The role the beetle plays in AOD remains unanswered, but it is thought to be involved in disease process and transmission. If so, at what stage of its lifecycle? How does the beetle find suitable trees to feed and lay eggs on? Do the rot-causing bacteria attract the beetles? We will test these questions using controlled experiments. Trees produce various odours, and plant-feeding beetles such as A. biguttatus use their sense of smell to find suitable trees and mates for feeding and reproduction. By capturing gases given off by the tree, presenting them to beetles and monitoring their response we can find out which ones attract beetles. This interaction can be exploited to protect trees by using attractive odours to pull beetles into traps. Agrilus species can only colonise weakened trees and previous research shows that specific environmental stresses make oak more susceptible to AOD. Main stresses are drought and imbalanced soil nutrition, specifically nitrogen and phosphorous deficiencies. These effects on tree metabolism and function as well as on natural soil and tree microflora will be tested under controlled conditions. This will provide important information about possible microbial additives that could accompany soil nutrition amendments to hasten tree recovery. Until these questions are answered, control of the beetle cannot be implemented and soil amendments and suitable management practices to make trees more robust to attack cannot be introduced.Research will provide a better understanding of who has an interest and stake in oak health. The values, attitudes and actions of people, professionals and policy makers will be examined to help understand reactions and responses to control of the disease and how to work together to develop and implement appropriate management and control. We will collaborate with stakeholders and scientists, build on existing experience, identify values, attitudes and actions and explore knowledge and awareness associated with oaks in urban and rural environments that will be used to promote and facilitate engagement in collective action to protect oak trees.Finally, we will address the question of whether the bacteria present in AOD bleeds on oak are also found in disease symptoms on other broadleaf trees, and if so, are there similarities to AOD that would help understand the infection and rotting processes in AOD. Furthermore, as bacterial tree diseases are generally under-regarded and there is a lack of information available concerning them, the risks to tree host species are unknown as are the implications for control of bacterial spread. Records from Forest Research's Tree Health Diagnostic and Advisory Service (THDAS) show that in the past 5 years over 550 cases of stem bleeding have been reported, 40% of these were on oak but approximately 10% had undiagnosed disease cause. Through laboratory microbial culturing processes from a range of broadleaved trees with stem bleeding symptoms we will isolate, identify and test bacteria for their disease-causing ability. Information about bacteria on these trees would be helpful to manage AOD and also to assess the risks of bacteria to other important native forest tree species that might be susceptible to AOD and other bacterial species. This interdisciplinary project will provide new evidence that can feed into a better understanding of appropriate and acceptable management practices for oak health and resilience.

Iconic British oak are under threat from Acute Oak Decline (AOD) typified by stem bleeds, bacterial lesions and larval Agrilus biguttatus. The beetle-bacteria relationship is unclear and controversial, and until resolved beetle trapping as a control cannot be implemented. 1. The role of Agrilus in introducing and transmitting bacteria, and adult and larval beetle chemical interactions with bacterial metabolites, and underlying genetic mechanisms, will be assessed. 2. Data on drought impacts will guide stress management. 3. Public/stakeholder perceptions to management will inform policy and uptake of practical management. 4. Other broadleaf host susceptibility to AOD will be tested. Transmission microcosm experiments will detect presence of bacteria from adult beetles, eggs and larvae using metabarcoding, RT-PCR and culturing methods. Introduction and spread in stems will assess larval chemotaxis to bacterial semiochemicals, and lab and field behavioural assays and coupled gas chromatography-electrophysiology (GC-EAG) will assess adult beetle responses. Transcriptomics will evaluate Agrilus x bacterial tests for gene upregulation linkages between Agrilus and bacteria. Druoght effects will be tested in a field trial where drought and wounding by ring-barking (as a proxy for nutrient and water stress) and host & microbiome responses (metabolomics and RNA-seq) determined. High-resolution metabolomics (LC-MS/MS and GC-to-MS profiling) and Random Forest machine learning modelling will identify metabolites. Values and attitudes of a diverse group will be examined in relation to oak engaging with citizen scientists and land managers to gather data for evidence base for decision making. Finally, we will assess cross infectivity potential of AOD bacteria and investigate other bacterial species in unresolved tree cankers. Results will guide future management practices in terms of beetle control, tree stress management and cross infectivity of bacteria.

Oaks are iconic - a landscape feature, at the heart of our natural cultural heritage and the mainstay of the British timber industry. The National Forest Inventory reports 218,600 Ha of oaks in GB woodlands. From around the 1980s onwards a stem disease, Acute Oak Decline (AOD) emerged and is recognised as a serious threat to oak. At government level AOD is identified as a 'priority for future action' in Defra's Tree health Management Plan (April 2014) and Tree Health and Resilience Strategy (May 2018). Action Oak Knowledge Review (June 2019) and Defra ROG research document (April 2019) flag up research priorities which we propose to address here. The complex nature of the problem and the sectors involved necessitate practical management solutions. We aim to determine whether or not the beetle is essential to AOD. This will be of interest and benefit to ecologists and conservationists, foresters, land managers and policy makers because implications of a confirmed causal role for the insect would have ecological implications that need to be taken into account in management. Identification of larval elicitor molecules and deciphering the underlying gene activity will create the platform from which to conduct targeted studies on development of chemical and/or enzymatic inhibitors. New control possibilities, such as adult trapping by bacterial volatiles, could emerge leading to new research opportunities enhancing their specificity and reducing non-target effects and any undesirable effects of controls. Advances in evidence supporting functional roles at the transcriptomic level will be helpful to molecular pathologists in particular but would provide even further clarity and evidence of the role of the bacteria and its interaction with Agrilus and the host, which would give greater confidence in management strategies and policy advice. Evidence on the specific effects of drought and nutrient stress on AOD development will be a significant step forward in making the case for long term drought and nutrient stress alleviation research for silvicultural management and soil amelioration control. Information about the chemical signatures (volatiles and tree metabolites) will advance us towards developing a rapid diagnostic tool for tree stress detection which will be of use to foresters, arborists, tree carers and researchers alike, and knowledge of microbiome effects - with identification of potential antagonists will offer further management potential. All these tools and methods will be of enormous benefit and advantage to foresters, arborists and land managers in applying practical tree management, which is a constant need. Apart from the need for practical solutions evidence about how to reach out to those involved in management will drive its effective implementation. Insights into values and attitudes of land/woodland managers and publics will foreground the role that oak plays in the identity of actors and understanding the behavioural actions of those involved and engaging them in a co-design process will support integration of project findings into policy and uptake of management options as well as illustrate the complexities and benefits of engaging with a broad range of stakeholders. Lessons learned will be useful for future related ventures.
Information about bacteria in unresolved tree diseases will be of global interest to tree care takers, and involvement of AOD bacteria in bleeding cankers on other broadleaf tree species will allow us to focus our control efforts on a management solution across tree species. Rapid detection of novel bacteria identified in bleeding cankers of broad leaf trees will help us guide management practices to minimise the spread of these bacteria. Our work will thus impact cross sector groups at a national and local level but will also be of international interest (for example European countries such as France, Lithuania, Poland and Switzerland) where this problem has been under-researched.