Antifungal Activity of the Extract and the Active Substances of Endophytic Nigrospora sp from the Traditional Chinese Medicinal Plant Stephania kwangsiensis

To exploit a new source from medical plants for finding bioactive products, endophytic fungi DBR-5 identified as Nigrospora sp., was isolated from the root tubers of the traditional Chinese medicinal plant Stephania kwangsiensis Lo. The antifungal activities of the extract from its fermentation liquids were determined. The ethyl acetate extract of DBR-5 exhibited high and broad antifungal activities against plant pathogenic fungi, and showed high toxicity to Exserohilum turcicum, Bipolaris maydis, Ceratocystis paradoxa, Alternaria oleracea and Cochliobolus miyabeanus with EC50 values respectively at 0.01 mg/mL, 0.02 mg/mL, 0.03 mg/mL, 0.03 mg/mL and 0.04 mg/mL. By a bioassay guided fractionation, three antifungal secondary metabolites were isolated from liquid culture of DBR-5, and identified as griseofulvin, deoxybostrycin and austrocortirubin on the basis of spectroscopic analysis. In vitro antifungal assay showed that griseofulvin displayed significant inhibition against the hypha growth of tested plant pathogenic fungi with EC50 values ranging from 0.0013 mg/mL to 0.0202 mg/mL, and showed the highest toxicity to E. turcicum and C. paradoxa with EC50 values both at 0.0013 mg/mL. Compared with the broad spectrum fungicide carbendazim, except that the toxicity of griseofulvin to Diaporthe citri and Pestalotiopsis theae was lower, the toxicity to the other eight pathogenic fungi was much higher. The inhibitory rates of griseofulvin against spore germination of A. olerace, C. paradoxa and P. theae were 100%, 100% and 94.39% respectively, at a concentration of 0.01 mg/mL. The other two compounds deoxybostrycin and austrocortirubin exhibited only weak antifungal activities. The results indicate the potential of Nigrospora sp. DBR-5 as a source of griseofulvin and also support that griseofulvin is a natural compound with high potential bioactivity against plant pathogenic fungi.