Updating our understanding of Acute Oak Decline

On 16 February 2022 over 100 participants gathered online, from as far afield as Latvia and South Africa, to hear about the latest research on Acute Oak Decline (AOD). This online webinar included research updates from both the BAC-STOP and Future Oak project teams. Topics ranged from chemical signalling and the oak microbiome to environmental stressors and cultural attitudes. One thing was clear throughout – Acute Oak Decline (AOD) is a complex multi-factor disease. Bacterial species (particularly Brenneria goodwinii), the bark boring beetle Agrilus biguttatus, and environmental stresses all contribute, but there is no single cause or easy solution.

The project teams are tackling this challenge from many different angles. After a welcome from BAC-STOP project lead Sandra Denman, the first two talks from Gareth Thomas (Rothamsted Research) and Marine Cambon (Bangor University) discussed chemical signalling. Gareth shared research identifying volatile odour compounds released by AOD-associated bacteria and oak bark, which are attractive to egg-carrying female beetles. Marine’s research report had a similar theme, but a different angle. Brenneria goodwinii, the main bacterium associated with causing necrosis of the live bark in AOD, can be present on trees without causing disease. But in the presence of Agrilus larvae, Brenneria becomes more damaging. So, what triggers it to become pathogenic? Marine has been working on identifying compounds from wounded and beetle-infested oak that might switch on and off pathogenicity genes in the bacteria. In both cases further work is needed to precisely identify and test the volatile compounds and genes, this research might lead to new control methods that would interrupt how the disease-triggering signals are transmitted.

Sandra Denman (Forest Research) and Nathan Brown (Woodland Heritage) discussed the environmental factors that can make oak trees more susceptible to AOD. There is a clear link between AOD, drought (including low rainfall and high temperature) and nutrient stress (high nitrates and low sulphur). Sandra’s team are exploring the impact of these conditions in a field trial where they can mimic drought and nutrient stress. They are then deliberately introducing bacteria and beetles to wounded and non-wounded trees to monitor what happens. Nathan’s approach involves monitoring oak trees in natural settings and then correlating tree condition, AOD incidence and environmental conditions. Nathan works with citizen science volunteers to do the surveying, and has been developing and assessing the reliability of visual tree health scoring systems.

The next major theme was the oak microbiome. Oak trees, like all plants, are constantly interacting with a plethora of bacteria, fungi and viruses that surround their leaves, trunks and roots. This microbiome includes beneficial, benign, and pathological micro-organisms. PhD student Usman Hussain (Bangor University) explained that the entire oak microbiome shifts during AOD. Whether this is the cause or a symptom of AOD is not known yet, but Usman’s research is working on characterising those changes. Future Oak project lead James McDonald (Bangor University) explained how their research is also working on characterising the oak microbiome. Their ultimate aim is to engineer the microbiome, increasing the numbers of beneficial or benign bacteria to outcompete or inhibit pathogenic bacteria. They have collected samples from 350 trees across the UK and are developing novel techniques to identify the full breadth of micro-organisms – not just the most common or fastest growing ones. Bridget Crampton explained research which is transferring the learning from oak trees to other broadleaf trees. Looking particularly at lime, beech and London plane they have been sampling trees with bleeding cankers all over the country, with support from a team of enthusiastic citizen scientists and Tree Health Officers. Their analysis has revealed a variety of bacteria, mainly from the Pseudomonadaceae and Enterobacteriaceae families. Lab studies are now helping them understand the potential significance of these bacteria in causing tree cankers.

The BAC-STOP project is truly interdisciplinary, including not just physical and life scientists, but also social scientists. Mike Dunn (Forest Research) explained the work he has been doing to understand public attitudes towards oak trees. 6000 members of the public were surveyed to understand what they value about oak trees, how much they know about oak diseases, and what kinds of control interventions would be most acceptable. Mike is also collecting more creative tributes to oak trees via the Odes2Oaks campaign. Submissions so far include a poem from Alan Titchmarsh and the team are welcoming contributions until 31 March.

Sandra Denman said “Acute Oak Decline is a serious emerging disease of native oaks causing many tree deaths each year. Together with environmental disturbances (such as increased pollutants and competition for landuse) and increased pest attacks (e.g. oak processionary moth), as well as horizon disease threats (Xylella), this iconic broadleaf tree species is under an increasing stress load, which leads to weakened, more vulnerable trees. We need all the help we can get from our stakeholders and the public. We thank all those whose give this so generously. They are the true heroes in helping us find solutions to ensuring the good health and resilience of our oak trees for future generations.”

To find out more about this research please visit the project pages for BAC-STOP and Future Oak.

If you are interested in learning more about any of these talks you can download the presentation slides below.

• Using the odour of bacteria to tackle Acute Oak Decline – Gareth Thomas (Rothamsted)
• Bacteria-beetle interaction in Acute Oak Decline: chemical cues as potential triggers of Brenneria goodwinii pathogenicity – Marine Cambon (Bangor)
• Acute Oak Decline Predisposition Drivers affect host susceptibility, and disease infection and severity: an update – Sandra Denman (Forest Research)
• Are droughts and nutrient stress impacting the oak microbiome composition and increasing oaks susceptibility to Acute Oak Decline (AOD)? – Usman Hussain (Bangor)
• Investigating the role of bacteria in broadleaf tree cankers – Bridget Crampton (Forest Research)
• Monitoring oak condition and predisposition to decline – Nathan Brown (Woodland Heritage)
• Public attitudes towards oaks and their management – Mike Dunn (Forest Research)
• Update on Future Oak fieldwork and microbiome analysis – James McDonald (Bangor)