Phytobeneficial properties of tomato (<i>Lycopersicon esculentum L.</i>) rhizospheric bacteria against bacterial wilt pathogen (<i>Ralstonia solanacearum</i>) under greenhouse conditions

Abstract

Bacterial wilt caused by Ralstonia solanacearum is the second most important pathogen causing bacterial wilt diseases in horticultural plants. In Ethiopia, the pathogen is widely distributed, affecting the production of tomatoes. This study was designed to isolate and  characterize the pathogenicity of the local strains and study the potential antagonistic rhizobacteria (biological control agents) for  potential control of the homologous pathogen using standard methods. All isolates displayed fluid and irregular colonies with pink or  light red colours at the centre on the TTC medium typical features of R. solanacearum and showed a 60 to 96% wilting disease index  percentage after 4 weeks of inoculation. The tomato varieties showed variation in disease resistance to the pathogen, with disease index ranging from 0.34 to 0.86, of which 30% of the tested tomato varieties such as the local Awash, Cochoro, Melkashola, and the commercial  variety (Venise) were moderately resistant. Out of the 120 rhizobacteria isolates, 27 (23%) were antagonistic to the test pathogen and  classified into their respective taxa based on their cultural and biochemical characteristics in relation to standard reference strains. Thus,  the isolates were tentatively identified as Bacillus cereus, Bacillus pumilus, Bacillus subtilis, and Bacillus thuringiensis; Pseudomonas  fluorescens, Pseudomonas putida, and Pseudomonas aeruginosa. The isolates were further characterized for additional plant growth  promoting properties (PGPP). Most isolates were capable of producing hydrogen cyanide (88%), indole-3-acetic acid (IAA) (83%) (147–645  ug/ml), ammonia (76%); whereas 53% of the isolates solubilized inorganic calcium phosphate (280–471 ug/ml). The isolates Bacillus  subtilis AAUB14, Bacillus pumilus AAUB13 andBacillus pumilus AAUB11 showed the highest number of PGPP as potential candidates for plant protection and health promotion provided that they are validated under greenhouse and field conditions.