In vitro and in vivo antibacterial effect of African star apple (chrysophyllum albidum linn.) extracts on bacterial wilt of tomato
This study was carried out to investigate the antibacterial activity of leaf and stem bark extracts of African star apple, Chrysophyllum albidum on Ralstonia solanacearum, the causative organism of bacterial wilt of tomato; the antibacterial effects of leaf and stem bark extracts of Chrysophyllum albidum on pathogenicity of Ralstonia solanacearum infecting tomato and to determine the phytochemicals present in the extracts. This was with a view to providing information on the antibacterial effects of the extracts on R. solanacearum. The sensitivity assay was carried out using agar well diffusion method at 200 mg/ml, 100 mg/ml, 50 mg/ml and 25 mg/ml of each extracts and 1 mg/ml, 0.5 mg/ml and 0.25 mg/ml of streptomycin. The minimum inhibitory concentration was determined using broth dilution method while the minimum bactericidal concentration was determined using spread plate technique. The effect of the extracts on the pathogenicity of R. solanacearum on tomato was carried out in the screenhouse using randomized complete block design with fifteen treatments in four replicates and two trials. The treatments included two levels of each extract (1000 mg and 500 mg), 20 mg streptomycin, inoculated control and uninoculated control. The data obtained were subjected to analysis of variance using SAS statistical package. Significant means were separated using LSD at 5% probability level. The results obtained showed that the extracts have antibacterial effect on the test organism in vitro except for aqueous leaf extract at 25 mg/ml. Methanolic stem bark extract compared favourably with streptomycin, a synthetic bactericide. The minimum inhibitory concentration of the extracts ranged from 6.25 mg/ml to 50 mg/ml while the minimum bactericidal concentration ranged from 25 mg/ml to 50 mg/ml. The extracts significantly reduced the extent of pathogenicity of the test organism on tomato in comparison with the inoculated control. For the concentration of the extracts used, 1000 mg gave a lower disease severity in comparison with 500 mg. For the diluents used in extraction, ethanolic extracts gave the lowest disease severity followed by methanol and then water. For the plant part used, stem bark extracts gave a lower disease severity in comparison with leaf extracts. The extracts reduced the severity of the disease on tomato. Tomato plants treated with the extracts gave a higher shoot weight in comparison with the inoculated control. Phytochemicals such as saponins, tannins, flavonoids, terpenoids, steroids, glycosides and alkaloids were found in African star apple extracts. In conclusion, the extracts had both in vitro and in vivo antibacterial effect on Ralstonia solanacearum and this can be attributed to the phytochemicals in the extracts.