Ralstonia solanacearum and R. pseudosolanacearum on Eucalyptus: Opportunists or Primary Pathogens?

Teresa A Coutinho,Michael J Wingfield

doi:10.3389/fpls.2017.00761

Abstract

Ralstonia solanacearum and R. pseudosolanacearum are well known primary pathogens of herbaceous crops. Reports of wilt caused by these pathogens in tree species are limited other than on Eucalyptus species. Despite the widespread occurrence of so-called bacterial wilt on eucalypts in tropical and sub-tropical parts of Africa, Asia, and the Americas, there remain many contradictions relating to the disease. Our field observations over many years in most regions where the disease occurs on Eucalyptus show that it is always associated with trees that have been subjected to severe stress. The disease is typically diagnosed by immersing cut stems in water and observing bacterial streaming, but the identity of the bacteria within this suspension is seldom considered. To add to the confusion, pathogenicity tests on susceptible species or clones are rarely successful. When they do work, they are on small plants in greenhouse trials. It has become all to easy to attribute Eucalyptus death exclusively to Ralstonia infection. Our data strongly suggest that Ralstonia species and probably other bacteria are latent colonists commonly occurring in healthy and particularly clonally propagated eucalypts. The onset of stress factors provide the bacteria with an opportunity to develop. We believe that the resulting stress weakens the defense systems of the trees allowing Ralstonia and bacterial endophytes to proliferate. Overall our research suggests that R. solanacearum and R. pseudosolanacearum are not primary pathogens of Eucalyptus. Short of clear evidence that they are primary pathogens of Eucalyptus it is inappropriate to attribute this disease solely to infection by Ralstonia species.

Highlights

Bacterial wilt is one of the world’s most destructive plant diseases
While Ralstonia species are the primary cause of bacterial wilt in herbaceous crops, most available evidence suggests that is not true in the case of their association with Eucalyptus
Usually linked to poor root systems, inappropriate planting practices, poor site factors or infections by primary pathogens such as G. philippii appear to be necessary for infection to occur

Summary

Introduction

Bacterial wilt is one of the world’s most destructive plant diseases. Two of the causal agents, Ralstonia solanacearum and R. pseudosolanacearum, have a broad host range infecting plants in over 50 families (Hayward, 2000), with reports of new hosts appearing regularly (Denny, 2006). If bacterial exudation is observed from the plant material, the immediate assumption is rapidly made that a Ralstonia species is solely responsible for tree death. In the case of fungi, some are considered latent pathogens, e.g., members of the Botryosphaeriaceae, and when the tree is stressed by either biotic and/or abiotic factors, disease symptoms develop (Slippers and Wingfield, 2007).

Results

Conclusion