Abstract: Pathogenicity of tenuazonic acid isolated from Alternaria alternata on Alternanthera philoxeroides was carried out with isolated leaf puncture assay and isolated plant assay. The results suggested that, tenuazonic acid was strongly pathogenic to Alternanthera philoxeroides leaves, increasing spot diameter with the concentration increase from 31.25 to 500.00 μg·mL^-1, the spot diameter was 2.41 mm at 500.00 μg·mL^-1, and it was more pathogenic to the younger leaves in plant tip than the ones in other plant parts at 20.00 μg·mL^-1. Tenuazonic acid inhibited the growth of A. philoxeroides plants, growth length, fresh weight and average root length were significantly inhibited at concentration of 125.00 μg·mL^-1, root number was also significantly inhibited at concentration of 250.00 μg·mL^-1; at 500.00 μg·mL^-1, growth length and root number were both zero, fresh weight decreased 0.77 g. It was concluded that tenuazonic acid had the potential for bioherbicide development for the control of A. philoxeroides．

Key words: Alternaria alternata, tenuazonic acid, Alternanthera philoxeroides, pathogenicity