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. Introduction The use of synthetic chemical fungicides has become less accepted in modern agriculture as many of these fungicides are harmful to non-target species in the agroecosystem and hazardous to the environment (Zadoks, 1993). Natural plant substances such as alleochemicals are considered less harmful to the environment as these chemical compounds are readily broken down in the soil and have no long-lasting residual effects to the environment (Chou, 1999; Huang and Chou, 2005). The genus Solanum contains about 1400 species of plants distributed throughout the temperate and tropical regions of the world (Eltayeb et al., 1997). The species Solanum nigrum L. (black nightshade) grows abundantly in the wild in open fields in the northeastern region of Asia and it is known to have medicinal importance (Roddick, 1991). Plants of S. nigrum have been traditionally used in the Orient as herbal medicine. In India, S. nigrum is used as hepatoprotective agent, and in Mexico, the fruits of this plant are used in a tonic for treating various nerve disorders (Perez et al., 1998). A previous study revealed that ethanol extracts of S. nigrum suppressed the oxidant-mediated degradation of calf thymus DNA (Sultana et al., 1995). Other studies showed that the plant exerted cytoprotection against gentamicin toxicity on Vero (African green monkey kidney) cells (Kumar et al., 2001) and anti-neoplastic activity against Sarcoma 180 in mice (Yen et al., 2001). Alternaria brassicicola (Schw.) Wiltshire, the causal agent of black leaf spot of Brassica species, is a cosmopolitan fungal pathogen. It is responsible for severe yield losses of crucifer crops in commercial fields in Taiwan (Huang and Chung, 1993). Alternaria brassicicola produces a host specific AB-toxin during conidial germination and the toxin plays a major role in infection and colonization by the pathogen of Brassica spp. leaves, resulting in the formation of dark brown circular leaf spots with concentric rings (Otani et al., 1998). In addition to AB-toxin, other enzymatic components such as cutinases and lipases may also contribute to the pathogenicity of A. brassicicola during infection (reviewed in Thomma, 2003). www.blackwell-synergy.com J. Phytopathology 154, 45–50 (2006) 2006 The Authors Journal compilation 2006 Blackwell Verlag