Screening of Beneficial Rhizospheric Bacterial and Fungal Isolates for Control of Bacterial Wilt of Tomato (Solanum lycopersicum L.) in Kenya

Abstract

Ralstonia solanacearum is a soil-borne pathogen causing bacterial wilt disease in solanaceae crops, including tomatoes. Available control strategies have been limited because of pathogen’s ability to develop resistance against pesticides, its wider host range, and ability to break host’s innate resistance. Management strategies including chemical and cultural practices have all failed causing the pathogen to be a threat to production. Although this pathogen is reported to result in 100% yield losses in the fields, pockets of uninfected plants are always visible. Little is known about the rhizosphere microbiome that would contribute to successful survival of plants amidst the highly virulent pathogen. In the current study, purposive sampling of rhizospheres from healthy tomatoes from Bomet, Kiambu, Kajiado and Kirinyaga was done. Pour plate method was used to screen for potential beneficial bacterial using nutrient agar media. Sprinkling method was used to screen for potential beneficial fungal isolates using potato dextrose agar amended with chloramphenicol at rate of 25mg/l. Purification of bacteria and fungi was done 48 hours and 72 hours, respectively, post incubation at 28 0C. Population of isolates revealed 40 bacterial isolates distributed as Bomet (48%)>Kiambu (27%)>Kajiado (18%) >Kirinyaga (8%). Similarly, fungal isolates were distributed as Bomet (30%)>Kiambu (28%)> Kajiado (23)>Kirinyaga (19%). Biochemical, microscopic and morphological characterization for bacterial isolates revealed Bacillus sp. (27.5%), Micrococcus sp., (27.5%) and Burkholderia sp., (15%) as the most dominant bacterial organisms inhabiting the rhizosphere of healthy plants. Macroscopic and microscopic description of fungal isolates also identified Aspergillus, Trichoderma, and Fusarium species as the most dominant fungal isolates in the rhizospheres of healthy tomatoes. Hierarchical cluster analysis done using DARwin software V6 clustered fungal and bacterial isolates into four and three clusters respectively. Finally, greenhouse experiments studied the efficacy of four bacterial isolates prequalified in the lab bioassays, in controlling bacterial wilt in Rio-grande tomatoes. Data on disease incidence, severity and population of R. solanacearum in roots and rhizosphere soils from each treatment revealed that both Bacillus sp. KMB16 and Pseudomonas sp.KJ2 significantly (p<0.05) reduced bacterial wilt incidence and severity on tomatoes. Further, treatments with the two antagonists resulted in a significant reduction of R. solanacearum in the roots of the tests plants. Bacillus sp.KJ4, Pseudomonas sp.KJ2, and Bacillus sp.BMT16 significantly (P<0.05) reduced population of R. solanacearum within the soil as compared to Burkolderia sp.KRN2. The result of this study contributes to knowledge of rhizosphere microbiome, diversity, and their potential. Further work needs to be done to formulate and utilize identified effective as an innovation that will shape the future of sustainable control of plant pathogens.