Antimicrobial activity of plumeria alba (pokok kemboja) against anthracnose disease in guava, papaya and mango

Abstract

Plant species in the genus Plumeria of Apocynaceae family are well known for their uses in traditional medicine. Plumeria alba can be found in tropical and sub-tropical regions and in Malaysia it is locally known as Pokok Kemboja. In this study, the phytochemicals and antimicrobial activity of different parts (flower, leaves and stem bark) of P. alba were analyzed. Extraction of phytochemicals was carried out using organic solvents of various polarities (hexane, dichloromethane, ethyl acetate, methanol and water). Phytochemical screening of the crude extracts of flower, leaves and stem bark revealed the presence of phenolic compounds, alkaloids, tannins, phytosterols, cardiac glycosides, terpenoids, steroids, saponins, reducing sugar and flavonoids. The phytochemical results showed that there were no significant differences (p <0.05) between the different plant parts. Antimicrobial activity of crude extracts was evaluated using disc diffusion method at a concentration of 20 ?g/disc against Bacillus subtilis, Bacillus cereus, Pseudomonas aeruginosa, Candida albican, Fusarium soleni, Colletotricum gleosporoides and Saccharomyces cerevisiae. The extracts showed significant antimicrobial activity against bacteria and fungi strains at p< 0.05. The inhibition zones ranged from 7 to 15 mm. The next stages of this study were focused on investigating the potential of P. alba as a natural fungicide in vitro and in vivo against Colletotricum gleosporoides which cause anthracnose disease in selected fruits (guava, papaya and mango). Complete inhibition of C. gloeosporioides by active fractions of P.alba from flower (MAR-f, leaves (MAR-l) and stem bark (MAR-s) was observed at the lowest concentration tested (11mg) using sporulation test. Radial Mycellium Growth Rate test showed no mycelial growth at the concentration of 5% Mar-f indicated the highest mycelial growth inhibition at 40.5%. Minimum inhibitory concentration (MIC) of MAR-f, MAR-l and MAR-s against C. gloeosporioides were investigated and MIC value was determined to be 0.3 mg/mL. In-vivo study, using 32000 mg/L, MAR-f, MAR-l and MAR-s significantly reduced both anthracnose disease incidence and disease severity on guava, papaya and mango. Significant physicochemical changes were observed in fruits when treated with MAR-f, MAR-l and MAR-s as in weight loss, Titratable Acidity (TA), Soluble Solid Content (SSC) and pH compared to control groups. All positive fractions were preceded with isolation using column chromatography techniques and Chromatotron. The active compound isolated from MAR-l was identified as (3ß)-Urs-12-en-3-ol (?- Amyrin) and MAR-f yielded hexadecanoic acid (Palmitic acid), and both were active against C. gloeosporioides (at concentration of 0.3mg/ml MIC value). These results proved that P.alba extracted (MAR-f, MAR-l and MAR-s) was found to be promising as an antifungal agent and to control anthracnose disease in fruits tested.