Phytochemical characterisation and antifungal activities of five botanicals used by subsistence farmers to manage plant diseases

Fungal phytopathogens cause many plant diseases resulting in severe crop losses. The phytochemical profiles, antioxidant and antifungal potentials of Aloe ferox, Allium cepa, Capsicum annuum, Tagetes minuta and Tulbaghia violacea extracted using acetone and methanol were investigated. Phytochemical profiling was undertaken using spectrophotometry and Liquid Chromatography-Mass Spectrometry (LC-MS). Antioxidant screening was done using 2,2´-azino-bis(3-ethylbenzthiazoline-6-sulfonic acid (ABTS) and 2,2-diphenyl-1-picrylhydrazyl (DPPH) assays. Antifungal effect was evaluated against Pythium ultimum and Botrytis cinerea using agar well diffusion and poisoned food techniques, respectively. Total phenolic concentrations ranged from 66 to 845 mg GAE/g (acetone extracts) and 20 to 195 mg GAE/g (methanol extracts); flavonoid concentrations ranged from 4 to 65 mg QE/g (acetone extracts) and 4 to 95 mg QE/g (methanol extracts). LC-MS analysis yielded 106 compounds, with 9-[2-(piperidin-1-yl)ethyl]-9 H-purin-6-amine, azelaic acid, glutarylcarnitine and sporovexin C as the common compounds across the five plants. Aloe ferox methanol extract (EC₅₀ = 0.2732 mg/mL) was the most potent extract in the ABTS assay, while T. minuta acetone extract (EC₅₀ = 0.1199 mg/mL) was the most potent in the DPPH assay. Tagetes minuta acetone extract (zone of inhibition = 26.67 mm) was the most potent against P. ultimum, while T. violacea methanol extract (62.4%) exhibited the highest mycelial growth inhibition against B. cinerea. Overall, Tulbaghia violacea and T. minuta were considered the most potent plants with antifungal effects. To fully understand the potential of these botanicals as antifungal, especially against P. ultimum and B. cinerea, further studies focusing on the efficacy of the bioactive compounds under in vivo condition is recommended.