Bacterial wilt of solanaceous crops: Diagnosis, diversity and management

Wilt disease of solanaceous caused by bacterial pathogen Ralstonia solanacearum (Smith) Yabuuchi et al. (1995) is a major devastating soil borne disease in the world, limiting the production of solanaceous vegetable crops, and affecting more than 450 plant species. The loss due to this disease is very high, ranging from 2 to 100% depending on environmental conditions and crops. The affected plants show a typical wilt symptoms and browning of vascular tissue in roots stems and tubers. The pathogen is a gram -ve, rod shaped, frequently occurs in pairs, motile with 1 to 4 polar flagella, aerobic, non-fluorescent, positive in catalase and oxidase tests and forms nitrites from nitrates and negative in levan production and starch hydrolysis. Virulent isolates on 2,3,5-Triphenyl Tetrazolium Chloride (TTC) medium develop fluidal, irregular colonies having peripheral white and pinkish in centre. The optimum growth of the bacterium on artificial medium occurs at temperature of 28 to 32oC. In India, race 1 and bvs 3 and 4 are prevalent in tomato chilli capsicum and brinjal. However, race 1 and 3 infect the potato. Presently, R. solanacearum strains are categorized into four genetic groups called phylotypes which reflect the geographical origin and also ancestral relationships between the strains. Besides cultural, morphological and biochemical tests, sero-diagnostic like ELISA (Enzyme-linked immunosorbent assay), immunofluorescent cell staining, lateral flow devices, serological kits are applied for detecting R. solanacearum from infected plant, tubers and soil. For detecting R. solanacearum, polymerase chain reaction (PCR) based techniques are usually applied using amplification of ribosomal sequences (16S RNA) and other conserved genes of bacteria. To improve the sensitivity, various modifications have been made like enrichment in SMSA broth, nutrient broth or CPG broth to allow bacteria to multiply prior to do conventional PCR, called as BIO-PCR and two round PCR as nested- PCR. Multiplex-PCR protocols have been developed to simultaneously detect either R. solanacearum and E. carotovora subsp. carotovora or Clavibacter michiganensis subsp. sepodonicus from tubers of potato and also R. solanacearum along with Xanthomonas perforans in tomato. Management of bacterial wilt disease of solanaceous crops is very difficult due to its soil borne nature. Different workers throughout the world have made efforts to control this disease by using cultural, chemical, biological and host resistance methods, either alone or in combination of these methods.