Antimicrobial and phytochemical properties of stem bark extracts from Piptadeniastrum africanum (Hook f.) Brenan

Aqueous, methanolic and ethyl acetate extracts of Piptadeniastrum africanum (Hook f.) Brenan stem bark were tested against fungi and representatives of Gram-positive and Gram-negative bacteria. Aqueous and methanolic extracts showed a maximum activity against the fungus Pyricularia grisea, a weak activity against the bacteria Staphylococcus aureus and Streptococcus mutans, while no activity was observed against Gram-negative bacteria. Ethyl acetate extract resulted inactive against all the tested microorganisms. Phytochemical investigations of the most active methanolic extract highlighted the presence of tannins and saponins. After purification by means of Sephadex LH-20 and RP-18 the tannin fraction resulted more active against bacteria, while the saponin fraction showed a pronounced activity against Pyricularia grisea. Five different strains of Pyricularia grisea were used and results show that the saponin fraction is promisingly active on the whole population (MIC and MFC: 0.03–0.06 mg mL−1). The saponin fraction fungicidal activity was similar to that of Flutriafol (0.01–0.04 mg mL−1), a commercially available fungicide, indicating that P. africanum stem bark extract can be a good candidate for the control of Pyricularia grisea, the most diffused rice blast disease fungus of rice.