Horse Chestnut Bleeding Canker: A Twenty-First Century Tree Pathogen

Abstract

European horse chestnut is an important amenity tree species which has recently been devastated by an emerging epidemic of bleeding canker disease. Symptoms include bleeding cankers on the stem and branches which can lead to tree mortality. The causal agent of this disease is the pathogenic bacterium, Pseudomonas syringae pv. aesculi, which is believed to have originated in India on Indian horse chestnut. This bacterium probably spread to Europe in the early 2000s via an unknown pathway, with the epidemic in Britain resulting from the introduction of a single bacterial strain. The development of a real-time PCR diagnostic test for P. syringae pv. aesculi has facilitated its rapid detection in symptomatic trees and provides a useful tool to study host infection and survival outside the host. Lesions caused by P. syringae pv. aesculi developed on woody branches of horse chestnut during the host dormant season and were centred mainly on lenticels, leaf scars and nodes. The lesions developed in the cortex and phloem and extended into the cambium to cause longitudinally spreading cankers. To better understand the evolutionary history of P. syringae pv. aesculi and its genetic adaptation on to a novel host species, draft genome sequences were generated for a strain from Britain and a type strain from India. Genomic comparisons with other P. syringae pathovars showed that P. syringae pv. aesculi has readily gained and lost genes during its recent past. Potentially important genetic gains include a pathway for the enzymatic degradation of plant-derived aromatic products and two genes involved in the metabolism of nitric oxide which may enable the bacterium to disable an important host defence response. Future genomic comparisons combined with functional analyses of genetic pathways will help unravel the key host-pathogen interactions which underlie bacterial diseases of trees.