Toxicity of black nightshade (Solanum nigrum) extracts on Alternaria brassicicola, causal agent of black leaf spot of Chinese cabbage (Brassica pekinensis)

Root extracts of black nightshade (Solanum nigrum) were analyzed for activity against isolates ABA‐31 and ABA‐104 of Alternaria brassicicola, the causal agent of black leaf spot of Chinese cabbage (Brassica pekinensis). Preliminary results showed that dried root tissues of black nightshade extracted with 70% ethanol contained antifungal properties against A. brassicicola. Ethanol root extracts were used for further fractionations using ethyl acetate, n‐butanol and water. Among the three extracts, the n‐butanol fraction showed the strongest antifungal activity by its suppression of conidial germination of A. brassicicola. The n‐butanol extract of S. nigrum roots was fractionated further into six fractions (I–VI). Among the six fractions tested, fraction V showed a strong inhibitory effect on conidial germination of A. brassicicola and thereby suppressed lesion development of black leaf spot of Chinese cabbage at a concentration of 25 ppm or higher. Nuclear magnetic resonance analysis indicated that fraction V contained a mixture of saponins, and results of further bio‐guided fractionation and bioassay suggested that saponins in fraction V were key chemical components in the control of A. brassicicola. The potential of using black nightshade for developing natural products for the control of fungal plant diseases is discussed.