Evaluation of genetic diversity of Ralstonia solanacearum causing bacterial wilt of ginger using REP-PCR and PCR-RFLP

Thirty-three strains of Ralstonia solanacearum Yabuuchi (Smith) isolated from ginger, paprika, chilli, t omato, Chromolaena and potato from Kerala, Karnataka, West Bengal and Assam in India, were phenotypically and genotypically characterized. Phenotypic characterization for biovar revealed the predominance of biovar 3 in India. Molecular analysis by REP –PCR, ITS– PCR and RFLP–PCR classified the strains into three clusters at 70% similarity, where ginger strains are grouped in Clusters I and II. Strains from potato (biovar 2) clu stered in the III cluster. Molecular analysis also revealed that ginger strains isolated from diffe rent locations during different years had 100% similarity according to D ice’s coefficient. The analysis further revealed that the genetic diversity of Ralstonia is very low within ginger, confirming that the pathogen population is of clonal lineage and is distributed through ‘rhizome transmission’ of the inoculum between locations and also between seasons within the locality. BACTERIAL wilt caused by Ralstonia solanacearum Yabuuchi (Smith) is a disease widely distributed in trop ical, sub-tropical and temperate regions worldwide. The host range of the pathogen is very wide and ginger is one of the i mportant hosts of the pathogen. Geographical distr ibution of the pathogen is expanding in the recent years. Bacterial wilt of ginger is reported from India, China, Japan, Indonesia, the Phi lippines, Hawaii and many other ginger-growing countries. In India the disease is found in Kerala, Karnataka, Himachal Pradesh, Sikkim, West Bengal, Assam and other North Eastern States. In contrast to the r eport from Queensland, the strains causing bacterial wilt of ginger in India belong to biovar 3 that causes wilt in 5 –7 days in 45-day-old ginger plants 1 . The pathogen is primarily rhizome-borne and it is believed to be transmitted to many ginger-growing areas through latently infected rhizomes and secondary spread within the field and neighbouring l ocalities is through rain splashes and run-off water in the field. R. solanacearum belongs to the rRNA homology group II pseudomonads based on rRNA : DNA homology 2 and to the β-sub class of Proteobacteri a. R. solanacearum exhibits both phenotypic and genotypic diversity. The sp ecies is divided into five races and 5 bi ovars based on its host range and also on difference in the oxid ation/utilization of certain carbon sources respectively 3 . A number of different phenotypic and genotypic methods are presently being employed for the identification and classification of bacteria, including plant pathogenic bacteria like Ralstonia. Each of these methods permits a certain level of phylog enetic classification from the genus, species, subspecies, biovar to the strain level. Moreover, each method has its adva ntages and disadvantages with regard to ease of applic ation, reproducibility, requirement for equipment and level of resolution 4 . Modern phylogenetic classification is based on