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The Bipolaris setariae NY1 strain, isolated from a diseased green foxtail plant in Henan Province, China, showed strong pathogenicity towards green foxtail. In order to clarify the role of phytotoxic substances in the fungal pathogenicity, bioassay-directed isolation and bioactivity assays of secondary metabolites produced by the fungal strain were carried out. Five ophiobolins were obtained: 3-anhydro- ophiobolin A, 6-epi-ophiobolin A, 6-epi-ophiobolin B, 3-anhydro-6-epi-ophiobolin B and ophiobolin I. Bioassays on punctured and intact detached leaves of green foxtail indicated that 3-anhydro-ophiobolin A was the most phytotoxic, followed by 6-epi-ophiobolin A. The other three ophiobolins appeared to be inactive against green foxtail. The effects of 3-anhydro-ophiobolin A and 6-epi-ophiobolin A were synergistic. The symptoms on green foxtail caused by 3-anhydro-ophiobolin A or its mixture with 6-epi-ophiobolin A resembled those caused by the fungus. 3-Anhydro-ophiobolin A and 6-epi-ophiobolin A are likely the main pathogenic determinants of B. setariae. 6-epi-Ophiobolin A caused cytotoxicity against five kinds of human cancer cells: human colon adenocarcinoma cells (HCT-8), human liver cancer cells (Bel-7402), human gastric cancer cells (BGC-823), human lung adenocarcinoma cells (A549), and human ovarian adenocarcinoma cells (A2780). The results provide information for the development of herbicides and antitumor potential of the ophiobolin sesterterpenes. [ABSTRACT FROM AUTHOR]