Extracts and metabolites derived from the leaves of Cassia alata L. exhibit in vitro and in vivo antimicrobial activities against fungal and bacterial plant pathogens

The use of synthetic chemicals in agriculture has not only caused humans and animals’ health risks but also environmental pollution. Botanical pesticides have been considered as alternatives to harmful pesticides because of their low toxicity and easy biodegradable property. The objective of this study was to evaluate the antimicrobial activities of the extracts and metabolites from the leaves of Cassia alata L. against fungal and bacterial phytopathogens. Ethyl acetate-soluble extract derived from C. alata leaves has been found to remarkably control the plant diseases caused by fungi Magnaporthe oryzae, Phytophthora infestans, Colletotrichum coccodes and Puccinia recondita in vivo. To provide scientific evidence on the effectiveness of the ethyl acetate-soluble extract against plant pathogens, the bioassay-guided fractionation of the extract was conducted and thereby eight compounds including methyl 2,4,6-trihydroxybenzoate (1), aloe-emodin (2), kaempferol (3), (-)-epiafzelechin (4), rhein (5), kaempferol-3-O-glycoside (6), kaempferol-3-O-gentiobiside (7) and aloe-emodin-8-O-β-D-glucoside (8) were identified. The content of 2, 5, and 8 in the ethyl acetate-soluble extract and methanol extract of C. alata leaves were quantified by HPLC analysis. Compounds 3-8 displayed in vitro antifungal activity against M. oryzae and Phytophthora species. Out of the isolated compounds, rhein (5) displayed the best antifungal activity; it strongly inhibited in vitro the mycelial growth of Phytophthora species and effectively suppressed tomato late blight (TLB) by 57.1 % at 150 μg/mL and 87.9 % at 300 μg/mL on the tomato seedlings in vivo. Besides, rhein showed excellent in vitro inhibition for the bacterial growth of Acidovorax avenae subsp. cattlvae with an IC₅₀ of 2.5 μg/mL. From the results, it was confirmed that the ethyl acetate-soluble extract derived from C. alata leaves contains antimicrobial constituents against various plant pathogens and has potent in vivo efficacy for controlling plant diseases.