Molecular Characterization And Aggressiveness Of The Ralstonia Solanacearum Species Complex From Eucalyptus Spp. In Brazil

Bacterial wilt, one of the world's most destructive diseases in many crops, including eucalypt, is caused by four distinct phylogenetic lineages of the Ralstonia solanacearum species complex, recently classified in three distinct species: R. solanacearum (phylotype II), R. pseudosolanacearum (phylotype I and III) and R. syzygii (phylotype IV). In this study, we characterized 93 Ralstonia isolates obtained from eucalypt grown in different Brazilian regions using phylotype and sequevar designations and genomic fingerprinting with BOX-PCR. In addition, we evaluated the aggressiveness of a select group of isolates in two eucalypt clones differing in resistance. We found that 89 isolates belong to R. solanacearum (phylotype II) and four to R. pseudosolanacearum (phylotype I). Unlike R. pseudosolanacearum, R. solanacearum was phylogenetically diverse and no correlation was found between sequevar and geographic origin. Most isolates grouped with reference isolates of phylotype IIA sequevar 41, whereas a few others clustered in phylotype IIB, mainly sequevar 4NPB, which is an emergent variant described affecting eucalypt for the first time. Isolates of R. solanacearum phylotype IIB were less aggressive to clone CLR371 (susceptible) whereas the R. pseudosolanacearum isolate tested was the only one pathogenic to the CLR172 (resistant) clone. Isolate aggressiveness varied between the eucalypt clones tested. The results of this study reinforce the importance of conducting molecular and aggressiveness characterization of the pathogen population to develop management strategies aimed at the deployment of host resistance in eucalypt breeding programmes.