Protecting Tilia trees: How members are helping to research tree diseases

---

Tilia trees are an important part of the UK's woodlands and urban forests. They provide a range of ecosystem services, such as improved air quality, stormwater runoff avoidance, and carbon storage. However, in recent years, there have been increasing reports of bleeding cankers in Tilia trees. The cause of this disease is unknown, but it is thought to be caused by a combination of factors, including climate change and pathogenic microorganisms.

Background on Tilia trees

Tilia are generally considered fairly healthy species due to the absence of serious pests and diseases threatening their survival in woodlands. In terms of pests caused by invertebrates, Tilia growing in woodlands rarely suffers serious damage from insects. In contrast, Tilia trees planted in urban environments tend to be more severely affected by insect infestations. Worth mentioning is the lime aphid, Eucallipterus tiliae which may cause extensive damage to the leaves through feeding. Leaf damage caused by mites of the genus Eriophyes or gall midges is reported to be the most widespread leaf pest affecting the native population of Tilia. Microorganisms such as fungi, bacteria and viruses are known to cause disease in trees. In the UK, the oomycete Phytophthora (fungi-like organism) has been reported in Tilia. The symptoms may depend on the species of Phytophthora infecting the tree, but they commonly include necrotic stem and branch cankers, root rot and foliage blights which can lead to mortality. Perhaps of particular interest is the species Phytophthora plurivora. The pathogen was likely introduced to Europe from overseas on living plants and has within the last century become widespread in the UK. Infection of Tilia by P. plurivora has been reported in Britain and the Tree Health Diagnostic and Advisory Service (THDAS) noted that from 48 recorded cases, 24 were identified from bleeding lesions on Tilia. Given the rising profile of Phytophthora pathogens of trees in Europe since the 1990s, the disease may pose an increasing threat to Tilia in the UK.

Importance of studying diseases that affect Tilia trees

There is currently no effective way to manage or control diseases affecting Tilia trees. In severe cases, the tree may be felled, especially if it poses a risk to health and safety. The lack of control measures for diseases affecting Tilia highlights the importance of continued research in the field.

Despite the resistant nature of Tilia trees towards pests and disease, climate change is predicted to have a negative impact and accelerate the spread and damage of pathogens. Periods of drought, high temperatures and excessive rainfall induced by the shifting climate are likely to weaken the trees leaving them vulnerable to pathogen attacks.

Introduction to Helene

My name is Helene Kile, and I am a first-year PhD student at the University of the West of England (UWE) under the supervision of Dr Carrie Brady.

I have always envisioned myself working in a plant-related field. This interest can be traced back to my early years, growing up in the Norwegian countryside surrounded by pine forests which lead to a deep-rooted interest and respect for nature. Aspiring for a more ambitious career, I left my job as a veterinary nurse and biological sciences became my natural choice of study. My area of interest has gradually become more focused through the years in academia, and by the end of the undergraduate degree, my interest in plant pathology had grown strong. I pursued this interest by undertaking an MRes degree to develop further knowledge and experience in the field. The MRes project (offered by Carrie at UWE) was based on the comparison of the bacterial microbiome of healthy and symptomatic Tilia trees to pinpoint potential bacterial pathogens involved in a disease of unknown cause. This was my first introduction to Tilia trees, and several interesting discoveries were made during this project, including the discovery of a novel and potentially pathogenic bacterial species (Brenneria tiliae). After completing the MRes we were eager to continue this research, and with funds generously awarded by FoWA and matched funding from UWE, we were able to create a PhD project that builds upon my previous discoveries.

Interest in studying Tilia tree diseases

Experiencing the effects of tree disease first-hand has opened my eyes to the devastating consequences. I recognise the importance of protecting and preserving our forests. A mission that is becoming increasingly pressing considering factors such as climate change leading to the increased survival and spread of pathogens. The ecosystem services provided by forest trees should not be underestimated as they may be vital contributors to mitigating the effects of climate change. Through extensive research during my MRes, I learned about the importance of Tilia trees and the ecosystem services they provide. I also discovered a lack of research regarding diseases affecting these trees. Through increased research we can improve our understanding of the threats posed and inform planting strategies to enhance future woodlands, overall making us better equipped to face the challenges. These aspects underpin the importance of future work in the field and reflect my motivation for undertaking this research project.

The research project

The three-year research project (jointly funded by the Friends of Westonbirt Arboretum and UWE) focuses on the investigation of bleeding cankers in Tilia trees (commonly known as Lime or Linden) in the UK. The cankers have a black to mid-brown colour and are present on the trunk of the tree, they are sometimes wet with exudate weeping out, hence the term bleeding cankers. Over recent years, there have been increasing reports of bleeding cankers in Tilia trees, but the cause has is unknown. The project builds on my discovery of a novel bacterial species, namely Brenneria tiliae, which was first isolated from a diseased Tilia x moltkei tree suffering from bleeding cankers at Westonbirt Arboretum in 2020. Interestingly, this species belongs to a group of tree pathogens known to cause bleeding cankers in multiple tree species. This led to the hypothesis that B. tiliae could be involved in the bleeding cankers observed. However, other pathogenic microorganisms including bacteria associated with Acute Oak Decline and the oomycete Phytophthora were also isolated from symptomatic Tilia trees. This may indicate that the disease is of polymicrobial cause.

Importance of the project

My research aims are to determine the agent(s) responsible for the bleeding cankers, the occurrence and the potential threat posed to Tilia trees in the UK.

The large-leaved lime (Tilia platyphyllos), the small-leaved lime (Tilia cordata) and their hybrid the common lime (Tilia x europaea) occur naturally across the UK, but due to their poor ability to colonise, they are often rare or absent in new forests and are more frequently found in old woodlands. Therefore, they are considered ancient woodland indicators, and their presence can be used to recognise these biologically important woodlands. The value of these species is further highlighted by the range of ecosystem services they provide to the UK woodlands and urban forests. Tilia trees have been reported to contain high concentrations of nutrients in their leaves, decompose rapidly and positively influence the soil base saturation, leading to enhanced soil nutrient status and nutrient cycling. In urban areas such as parks and avenues Tilia species are among the most frequently planted tree species where they provide improved air quality, avoid stormwater runoff and store large amounts of carbon. They also support the pollinating community by providing valuable floral resources. Old, hollow Tilia trees in parks provide valuable habitats for a range of saproxylic beetles including red-listed species.

Over the last 2000 years, the distribution of Tilia in European woodlands has declined, mainly due to the exploration of beech but also from farming expansion and low seed fertility. The naturally occurring population is now considered highly fragmented leaving them vulnerable to ecological deterioration. Additionally, climate change is predicted to negatively impact forest trees and accelerate the spread and damage of pests and pathogens. However, due to the resistant nature of Tilia towards severe disease, their broad ecological amplitude and tolerance towards drought and shifting temperatures increased inclusion of these species in mixed forests have been suggested to enhance the forest ecosystem resilience to a changing climate. The valuable ecosystem services provided by Tilia coupled with their resistant nature make them promising candidates in future planting schemes. An inclusion as such could aid to reverse their decline in woodlands habitats. Nevertheless, for the species to be included, the cause of the bleeding cankers and the potential threat this disease may pose should be elucidated.

Current progress and findings

The research is still in its infancy as I only started my PhD in January, however, from the samples I have analysed so far, several potentially pathogenic bacteria including B. tiliae have been isolated from new sites in Gloucestershire, Wiltshire, and Shropshire. Going forward we will continue to sample Tilia trees with bleeding cankers present across the UK to aid the data collection. Based on what we find, pathogenicity trials will be set up to determine whether these bacteria can cause disease in Tilia trees. This will help us elucidate the causal agent(s) responsible for the bleeding cankers observed.

The research project described in this blog is investigating the cause of bleeding cankers in Tilia trees. The results of this research will help us to understand the cause of bleeding cankers in Tilia trees and to develop strategies for managing the disease. This is important because Tilia trees are an important part of the UK's ecosystems. By protecting these trees, we can help to maintain the health of our woodlands and urban forests.

---

Our charity supports the arboretum in a number of ways, including funding research projects. This research project into the cause of bleeding cankers in Tilia trees is being part-funded by the University of West England and the Friends of Westonbirt Arboretum, thanks to the generous donations of our members through membership.

The Friends are committed to supporting the research and conservation of trees, and we believe that this project is essential to understanding the threat posed by the cause of bleeding cankers to Tilia trees in the UK. The project is still in its early stages, but The Friends are confident that it will make a significant contribution to our understanding of this disease.

If you would like to support us and help to fund important research projects like this one, you can become a member or make a donation today. Your membership will help to ensure that the arboretum can continue to be a centre for tree research and conservation.