Two Ralstonia species associated with bacterial wilt of Eucalyptus

Abstract

Ralstonia solanacearum sensu lato causes bacterial wilt in many economically important agronomic crops and tree species. Eucalyptus is one of the recorded hosts but, despite the losses to forestry, little is known regarding the disease. The aim of this study was to identify the Ralstonia species that are associated with bacterial wilt of Eucalyptus trees growing in different parts of the world. Phylotyping, sequevar and multilocus sequence analysis were used to identify and determine the genetic diversity of the available isolates. The isolates obtained from Africa and Asia grouped in phylotype I, and those isolates from South America in phylotype II. Following the recent reclassification of R. solanacearum sensu lato, these results show that there are two Ralstonia species associated with bacterial wilt of Eucalyptus, namely, R. solanacearum and R. pseudosolanacearum. The isolates from South America represented R. solanacearum, while those from Africa and Asia were R. pseudosolanacearum. Analysis of the endoglucanase (egl), transcriptional regulator (hrpB) and DNA mismatch repair (mutS) gene regions supported this finding. In addition, one previously unidentified sequevar was identified using partial endoglucanase (egl) gene sequence analysis of those isolates obtained from Colombia. This study raises questions regarding the aetiology of the two Ralstonia species found on Eucalyptus, their importance relating to quarantine, potential impact and management of the disease with which they are associated.

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

“…In the case of Ralstonia species, artificial inoculations on Eucalyptus are rarely successful, i.e., inoculations either fail or inconsistent results are obtained (Cruz and Dianese, 1986; Dianese et al, 1990; Dianese and Dristig, 1993; Coutinho et al, 2000; Wei et al, 2014). Fonseca et al (2016) described what they termed an “efficient method” to test for resistance of Eucalyptus to R. solanacearum but when attempted elsewhere (Carstensen et al, 2017), the method failed to produce symptoms in a susceptible clone. The common failure to prove Koch’s postulates for Ralstonia species on Eucalyptus provides an indication that there are factors, other than the susceptible host and an aggressive pathogen, that play a role in this pathosystem.…”

“…Based on phylogenetic evidence, the two Ralstonia species causing bacterial wilt of Eucalyptus appear to have evolved along two primary lines of descent: one in the Americas ( R. solanacearum ) and the other ( R. pseudosolanacearum ) having both an Asian and African origin (Prior and Fegan, 2005; Carstensen et al, 2017). It is thus likely that the movement of Eucalyptus from its primary center of origin in Australia to many countries worldwide, has led to these trees being exposed to local populations of these two bacterial species.…”

Detection and characterization of Ralstonia pseudosolanacearum infecting Eucalyptus sp. in Brazil

“…As for the two Ralstonia species associated with bacterial wilt of eucalypt found in this study, further investigation into their pathogenicity mechanisms is necessary to provide researchers with enhanced knowledge that could facilitate the selection of resistant eucalypt clones (Carstensen et al, 2016). Possible differences in the pathogenicity strategies utilized by these two bacterial species should be taken into consideration in breeding programmes in order to design novel and effective disease control alternatives appropriate for each pathogen.…”

“…However, R. pseudosolanacearum (phylotype I) has also been found associated with solanaceous crops, such as tomato, aubergine, long pepper and bell pepper (Santiago, Lopes, Caetano‐Anollés, & Mizubuti, 2017). Until now, only phylotype II has been reported causing BW in eucalypt in Brazil, but some reports indicate that phylotype I can also cause wilt symptoms in eucalypt in Africa and Asia (Carstensen, Venter, Wingfield, & Coutinho, 2016).…”

“…A clear understanding of the geographic distribution of these two bacterial species is crucial for establishment of both worldwide and local containment strategies. Whether the pathogenicity of these two bacterial species towards Eucalyptus is similar is not known and requires further investigation (Carstensen et al, 2016). This work demonstrates that R. pseudosolanacearum is present in Brazil causing BW in Eucalyptus sp., information highly relevant for disease management.…”