Evaluation of antibacterial activity of some invasive alien species (ias) extracts against tomato bacterial wilt caused by Ralstonia solanacearum (Smith)

Abstract

Tomato (Lycopersicon esculentum Mill.) is one of the most widely grown vegetables in the world including Ethiopia. But, its production is constrained by different biotic and abiotic factors. Among biotic, bacterial wilt caused by Ralstonia solanacearum is one of the most important pathogens, threatening the production of tomato and potato in Ethiopia. So far there is no single means that would totally manage the disease and provide an absolute cure or fully protect host plants against the pathogen. This study was therefore, initiated with the objectives of evaluating the antibacterial activities of aqueous and two organic solvents (acetone and methanol) extracts of five invasive alien species (Eichhorina crassipes, Mimosa diplotricha, Lantana camara and Prosopis juliflora) under in vitro and in vivo against R. solanacearum. Completely randomized and randomized complete block designs with three replications were used for in vitro and in vivo experiments, respectively. In vitro antibacterial test was carried out in disc diffusion sensitivity test. After in vitro screening, four botanicals (aqueous extracts of E. crassipes, M. diplotricha, L. camara and methanolic extract of P. juliflora) with inhibition diameter >10mm were selected and Minimum Inhibitory Concentration was assessed under in vitro, and further evaluated under in vivo on tomato plants against R. solanacearum by applying the botanicals at three time of application (simultaneously, before 2-days and after2-days pathogen inoculation). It is evident from the result that most of the plant extracts exhibited significant inhibition of the bacterial growth compared with the control at 5 % level of significance. All the tested plant species bioactive compounds were relatively more extracted by water than organic solvents. Aqueous extract of E. crassipes provided the highest inhibition zone (26 mm), followed by M. diplotricha (14 mm). Results from in vivo test revealed that most of the treatment combinations significantly reduced disease incidence, disease severity index, R. solanacearum population density in the rhizosphere soil and increased biomass of tomato plants. The application of plant extracts at the same time as inoculation resulted in the highest reduction of disease development on tomato plants. More than 91 and 75 % reduction in disease severity index and wilt incidence, respectively, of bacterial wilt was observed in tomato plants treated with leaf extract of E. crassipes under greenhouse experiment when it was applied simultaneously with pathogen inoculation. The highest percent reduction of incidence Area Under Disease Progress Curve and severity Area Under Disease Progress Curve were exhibited from aqueous extract of E. crassipes with 67.52 and 71.35 %, respectively, followed by aqueous extract of M. diplotricha 64.08 and 68.69 %, and then L. camara aqueous extract with 59.48 and 66.19%. Aqueous extract of E. crassipes was found to be the most effective botanical in disease suppression and increase above ground biomass in terms of fresh and dry weight; 52.72 and 11.96 g, respectively, compared to inoculated control (8.79 and 2.79), respectively. The study revealed that aqueous extracts of E. crassipes and M. diplotricha and L. camara are promising botanicals whose effectiveness under field conditions and their mode of action should be explored