Written by Katherine Hinton. This is the report from a 2020 BSPP Lockdown Bursary. Click here to read more/apply for one yourself.
During the corona virus pandemic, I worked on a systematic literature review of the bacterial diseases currently affecting trees in the UK. Plant diseases are heavily influenced by their environment. With worsening climate change, host specific diseases are a large cause for concern. Recent epidemics in the UK such as horse chestnut bleeding canker and acute oak decline have led to a renewed interest in bacterial tree diseases in the UK. Research into this area is rapidly evolving and the technological advancements being made allow a greater understanding of disease mechanisms and plant-microbe interactions. However, many bacterial diseases of trees remain understudied compared to in agricultural plant health and a large proportion of the literature has focused upon taxonomy and symptomology. Understanding epidemiology, diagnostics and control is essential in reducing damage to forests and orchards, which hold economic, cultural and ecological importance. This review focused on two main groups of bacteria present in the UK: Pseudomonas, affecting horse chestnut, ash and stone fruit; and enteric-like bacteria, affecting oak and willow. During this project I collated the relevant literature into one spreadsheet. The changes in taxonomic names (e.g. Erwinia to Brenneria) made researching papers time consuming and it was easy to miss important papers. Hopefully this can be a useful research tool for others interested in the field.
Recently, there has been rapid progress in acute oak decline, a disease affecting British pedunculate and sessile oaks – this has been a particularly interesting area of my research into this report. Decline diseases are caused by a succession of abiotic (e.g. water stress, temperature) and biotic (e.g. pathogens and pests) factors; however, it is often hard to pinpoint what those factors are exactly. It is now understood that a core biotic stress contributing to disease development is a community of bacteria, including Brenneria goodwinii, Gibbsiella quercinecans and Lonsdalea britannica. The composition of bacteria differs between affected trees and one bacterium never causes 100% of infection, although B. goodwinii appears to be the main causal pathogen causing tissue maceration. The interactions between species in the microbiome is something that we are only just starting to understand so this is a really exciting example of how bacteria can combine and cause infection. There could be an opportunistic component to the pathobiome, or alternatively a wide range of oak resistance. As well as microbial interaction it is important to consider environmental predisposing factors and the hosts response to infection, for example drought is known to exacerbate AOD. It will be exciting to see how technological advancements progress this integrated understanding of host susceptibility to disease.
Development of genetic tools in the last decade has rapidly improved our understanding of pathogenicity factors used by microbes during an attack. Pseudomonas syringae species have well defined virulence factors, including a hypersensitive reaction and pathogenicity (hrp) gene cluster coding for the type three secretion system, effectors and harpin genes, as well as toxins and phytohormones. Prunus spp. are targeted by a range of P. syringae including pv. syringae, pv. morsprunorum race 1 & race 2. A characteristic of these diseases are bark cankers and reduced fruit production. Intriguingly, bacteria infect multiple niches of the tree at multiple stages of disease lifecycle. The large variation in their type three effector repertoire could be responsible, as effectors suppress the host immune response. Less is known about the genetic components of enteric-like bacteria. B. goodwinii and Brenneria salicis, affecting UK oak and willow respectively, are aggressive species threatening our forest landscape. Genetic analyses have also revealed a similar T3SS in Brenneria species, giving an indication to how they manipulate their hosts.
Accumulating knowledge of phytopathogens assists in the development of control methods to prevent the spread of disease. In situ detection has improved drastically, with refined PCR techniques allowing researchers to distinguish each species present in the microbiome of infected as well as healthy trees. Development of cheaper and faster sequencing-based technologies could potentially be applied to control strategies, identifying pathogenic bacterial colonies in orchards or forests as soon as possible to minimise spread. Currently, copper-sprays are a commonly used preventative measure against Pseudomonas species, but this is being phased out in Europe and resistant strains are being reported. Reducing usage of chemical as well as antibiotic controls minimises this growing resistance. An alternative to bypass this is the use of biological controls. There are many potential avenues to go down, one of which is the use of bacteriophages. Phages, used in both human and plant heath, kill bacteria with accuracy and minimise damage to the host. This is currently being investigated against horse chestnut bleeding canker as well as cherry canker with some potential phages being identified.
Overall, an integrated understanding of the relationship between host, microbes, environment and pests is required to predict the predisposition of a tree to disease. Advancements in sequence-based technology and multi-omic studies can move us closer to this and will hopefully result in effective detection and control strategies to minimise damage to forests and orchards. This area of research is more relevant and progressive than ever with further questions being posed as research develops. I am grateful for the BSPP bursary as it enabled me to carry out research into an area of plant pathology I find particularly engaging and has reinforced my goal of doing a PhD in a similar field. I look forward to following this area of research and understanding the implications for plant pathology and everyday life as a whole.