In vitro and in vivo antimicrobial activities of extracts and constituents derived from Desmodium styracifolium (Osb.) Merr. against various phytopathogenic fungi and bacteria

Phytopathogenic fungi and bacteria pose a threat to plant health and cause large losses in crop quality and production. Currently, green methods for controlling phytopathogenic fungi and bacteria have been paid attention to. In this study, in vitro and in vivo antimicrobial activities of Desmodium styracifolium and its metabolites against phytopathogenic fungi and bacteria were investigated to find a new solution for the green method. The result showed the crude extract of D. styracifolium exerts a potent suppression against fungal plant diseases in vivo. At 3000 µg/mL, n-hexane and dichloromethane-soluble extracts of D. styracifolium effectively controlled rice blast (84.6% and 53.8%) and tomato late blight (67.7% and 61.3%), respectively. Using in vitro antifungal bioassay-guided fractionation, sixteen compounds were isolated and identified. The occurrence of isoflavanones 5 and 7 in D. styracifolium, as well as pyrrole lactone alkaloid 14 in the genus Desmodium was firstly reported. Compound 9, a new isoflavone isolated from nature, displayed potent antifungal and antibacterial activities in vitro. The two remarkable bioactive groups of isoflavanones (5–7) and isoflavones (8–10) were determined as antifungal constituents from D. styracifolium with a wide-spectrum antifungal activity. Magnaporthe oryzae and Phytophthora species were susceptible to the isoflavonoid groups. Especially, in a range of 125–500 µg/mL, isoflavonoids 5 and 10 effectively inhibited M. oryzae in both in vitro and in vivo experiments. Compounds 5, 6, 8–11 significantly suppressed the bacterial growth of Acidovorax avenae subsp. cattlyae in vitro. Among them, 5 (IC50 = 33.1 µg/mL) and 9 (IC50 = 42.2 µg/mL) were the best inhibitors against A. avenae subsp. cattlyae. Our findings indicate that the isoflavanones and isoflavones of D. styracifolium are worthy of further consideration as lead molecules to develop a new class of inhibitors against phytopathogenic fungi and bacteria.