Tilia Trees and Wild Streets

Tilia is a genus of broad-leaved temperate forest trees consisting of around 23 species, however only three species are native to the UK, including the large-leaved lime (Tilia platyphyllos), the small-leaved lime (Tilia cordata) and their hybrid the common lime (Tilia x europaea). These Tilia species 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 Tilia is further highlighted by the range of ecosystem services they provide to the UK woodlands and urban forests, including enhanced soil nutrient status and nutrient cycling, improved air quality, support of biodiversity and carbon sequestration. However, the naturally occurring population of Tilia is considered highly fragmented due to a declining distribution, which leaves them vulnerable to ecological deterioration.

Due to the changing climate in the UK, there has been a rise in the emergence of plant pathogens and pests on new hosts. Currently, there are 30 risks to Tilia listed on the UK Plant Health Risk Register; and over the past ten years, there have been increasing reports of Tilia species and hybrids with symptoms of bleeding cankers in several counties in the UK. In 2020 a novel bacterial species, Brenneria tiliae, was isolated from bleeding cankers of Tilia along with the bacteria known to cause acute oak decline. These cankers also tested positive for the presence of Phytophthora. Given the pathogenic potential of the bacteria associated with AOD, particularly the genus Brenneria, and the devastating environmental effects caused by Phytophthora species, either or both microorganisms may constitute a new threat to an already deteriorating Tilia population in the UK. The aim of the current research project is to (1) develop a detection protocol for symptomatic Tilia, (2) determine the pathogenicity and incidence of Brenneria and Phytophthora spp. on Tilia, (3) describe the symptoms of the potential novel disease observed in Tilia and (4) determine the current and future threats posed by pests and diseases to Tilia trees. Overall, this research will be key to increasing our understanding of the occurrence and impact of Phytophthora infections in Tilia trees and could provide novel insight into the role played by bacteria such as B. tiliae in the disease cankers. This could contribute towards better planting, conservation and protection strategies for Tilia and may reveal the future threats posed by these microorganisms to the declining population of Tilia in the UK. The results could inform foresters, landowners, and stakeholders about the choice of resilient Tilia planting stocks.