Evaluation of Six Medicinal Plant Leaf Extracts against Acidovorax Citriulli causing Bacterial Blotch of Watermelon

Abstract

Medicinal plants used in traditional medicine in Nigeria were tested for their antibacterial activities against Acidovorax citrulli the bacterial blotch pathogen of watermelon (Citrullus lanatus). Isolation of the pathogen was made from infected fruits on Nutrient Agar (NA) and pathogenicity was carried out on healthy watermelon fruits. The extracts from the leaves of these plants (Entada africana, Vitex doniana, Lawsonia inermis, Azadirachta indica, Acalypha hispida and Nauclea latifolia) were prepared using ethanol, cold and hot water.Phytochemicals analysis of the each extract was done using standard methods. Antimicrobial activities of each extract was evaluated at 50mg/ml using agar well diffusion techniques with bacterial inoculum of 2 x 106 cfu/ml. Control experiments were set up using sterile water as negative control and Gentamicin (10 ug/ml) as positive control. Minimum Inhibitory Concentration (M.I.C.) and Minimum Bactericidal Concentration (M.B.C.) were determined by a modification of the agar well dilution method. The results on phytochemical analysis revealed the presence of either one or more of the alkaloid, saponin, tannin, proanthocyanin, steroid and flavonoid in the six plants evaluated. Alkaloids, tannins and flavonoids were found in all the plant extracts. The weight of extract (g) obtained were significantly higher (P<0.05) with ethanol than cold and hot water extractants. The six plants exhibited varying degrees of antibacterial activity against the test organism. Ethanol extracts of all the plants gave the best inhibition of the bacteria with A. indica and V. doniana given the highest inhibition of 81% and 73% respectively against the pathogen. The ethanol extracts of L. inermis, A. indica and N. latifolia showed a broad spectrum of activity with MIC of 1.5mg/ml against the tested pathogen. Therefore, field trials of these six medicinal plants on the antibacterial activities against Acidovorax citrulli are recommended.