Broad-spectrum antifungal activity of dichloromethane extract of Waltheria indica stems and isolated compounds

The secondary metabolites produced by plants are important sources of fungicides with intriguing bioactivities and environmentally benign attributes (eg. low residual toxicity to mammals and readily biodegradable in soil). In this study, the dichloromethane extract of stems of Waltheria indica L. was found to exhibit broad-spectrum antifungal activity toward eight phytopathogenic fungi, particularly against carbendazim-resistant Sclerotinia sclerotiorum with inhibition rate of 100% at 500 μg/mL. Two quinoline alkaloids, antidesmone and waltherione C, were isolated and identified as fungicidal compounds by bioassay-guided isolation and spectroscopic analyses. Antidesmone displayed superior antifungal activities against eight phytopathogenic fungi to commercial botanical fungicides (osthole, carvacrol, and eugenol). Moreover, antidesmone was found to have significant negative cross-resistance with carbendazim to Sclerotinia sclerotiorum for the first time and exhibited much better antifungal activity (EC50 = 0.60 μg/mL) against carbendazim-resistant strains than carbendazim-sensitive strain (EC50 = 9.61 μg/mL) of Sclerotinia sclerotiorum, which was further supported by the molecular modelling result. In addition, the dichloromethane extract of the Waltheria indica stems could effectively control the development of eleven species of phytopathogenic fungi in vivo. These results suggest that Waltheria indica dichloromethane extract and antidesmone can be developed as promising broad-spectrum botanical fungicides.