Characterization of indigenous bio-control agents against wilt complex pathogens of tomato

Abstract

The main objective of the study was to find out potential indigenous bio-control agents active against tomato wilt complex pathogens like Ralstonia solanacearum and Fusarium oxysporum and various strains of the pathogens were first isolated from wilted tomato plants grown in different areas in Bangladesh. Then different indigenous strains of bacteria and fungi, known for their antagonistic activity against wilt complex pathogens, were isolated and tested against the pathogens. It was found that twenty strains of R. solanacearum and three strains of F. oxysporum demonstrated infectivity in tomato plants. Based on the ability to infect and severity of infections, one strain of each organism was then selected for further study. Bacillus spp., Pseudomonas spp. Rhizobium spp. and Trichoderma spp. are known to have potential antagonistic effect against R. solanacearum and F. oxysporum. To determine their ability and efficacy as bio-control agents, all these organisms were isolated from rhizosphere except Rhizobium spp. which were isolated from root nodule of lentils. In vitro screening test carried out for antagonistic property revealed that five Bacillus strains- Bacillus subtilis 1, Bacillus subtilis 2, Bacillus pumilus, Bacillus licheniformis, Paenibacillus polymixa, one strain of Pseudomonas fluorescens, one strain of Rhizobium leguminosorum and two strains of Trichoderma harzianum were effective against R. solanacearum and F. oxysporum f. sp. lycopersici as bio-control agents. The findings indicated that R. leguminosorum inhibited the maximum growth of F. oxysporum f. sp. lycopersici in dual culture method with 65.56% inhibition while in case of fungi versus fungi, T. harzianum and T. harzianum (BAU) showed strong antagonism against F. oxysporum f. sp. lycopersici pathogen inhibiting 78.37% and 83.78% of radial mycelial growth, respectively after 7 days of incubation. On the other hand, B. subtilis, and P. fluorescens were found to be more potent against R. solanacearum. Four different methods of inoculation viz. collar region, root dipping, soil drenching and sick bed were evaluated to determine the degree of pathogenicity of R. solanacearum and F. oxysporum f. sp. lycopersici. It was found that collar region inoculation method was best for evaluation of pathogenicity of both pathogens. In the field experiment B. subtilis 2, P. fluorescens and R. leguminosorum showed similar effect against bacterial wilt when seedling roots were treated separately before transplanting. In case of fungal wilt, B. subtilis 2 and T. harzianum BAU were found to be most effective in reducing wilt severity. However, when two pathogens were combined together with the same root treatments, B. subtilis 2, P. fluorescens, R. leguminosorum and T. harzianum showed better results. It results indices that the optimal temperature and pH for the growth of the indigenous bacterial bio-control agents were 350C and pH 7, respectively. Growth of the bacterial strains seemed to be affected by varying the temperatures and pHs. From the present study, it suggests that B. subtilis 2, P. fluorescens, R. leguminosorum and T. harzianum (BAU) have great potential as bio-control agents not only active against the tested tomato wilt complex pathogens but also they hold a promising future in the development or formulation of an effective bio-control strategy against other plant pathogens.