Phytochemical study and anti-oomycete activity of Ageratum conyzoides Linnaeus

Abstract

Cacao black pod disease caused by most virulent oomycete agent, Phytophthora megakarya is the most damaging disease of Theobroma cacao in Cameroon. The biocontrol method using the extract plants to control the disease is almost inexistent to date. This study aimed to evaluate in vitro and in vivo anti-oomycete activity of the crude ethanolic extract of Ageratum conyzoides L. against Phytophthora megakarya, to identify and characterize some bioactive compounds from it. The inhibition of the mycelia growth of P. megakarya was evaluated by the Agar dilution method at a given concentrations of extract (2, 5, 10, 15 mg mL−1), a negative control treatment (without extract) and Ridomil (lethal dose in the field) used as positive control treatment. All concentrations showed the inhibition of P. megakarya and the highest concentration was obtained at 15 mg mL−1 with maximum inhibition of 38.96% compared to the negative control. Ethanolic extract of Ageratum conyzoides L. significantly (p < 0.0001) reduced the symptom expression of cacao black pod disease as effectively as Ridomil. The phytochemical screening of the crude ethanol extract of A. conyzoides revealed the presence of flavonoids, phenols, alkaloids, coumarins, saponins, tanins, sterols, glycosides and anthraquinons. Moreover, two steroids 3-O-β-D-glucopyranosyl-β-sitosterol, a mixture of β-sitosterol and stigmasterol, and two linear long-chain primary alcohols undecan-1-ol and decan-1-ol were isolated from Ageratum conyzoides. Their structural elucidation was done based on 1D- and 2D-NMR spectroscopy techniques, as well as comparison with previous reports. These secondary metabolites could explain its anti-oomycete activity against P. megakarya, the most virulent pathogen agent of the cacao black pod disease.