During the process of finding a bioherbicide agent, we found that a rust infected and killed plants of common cephalanoplos herb Cephalanoplos segetum (Bunge) Kitam in Qingdao, Shandong Province, P.R. China. The infected plants are necrotic or stunted. The symptom of rust disease is the orange-coloured pustules or uredia appear on the lower surfaces of the leaves and their rupture to expose masses of reddish-brown uredospores. Pustules appear first on the abaxial surface in orange-coloured speckles, and then developed into 0.3-0.6 mm pustules; most of them are 0.5 mm in diameter. Uredospores are round or subcircular in shape, dark cinnamon brown in colour when mature, diameter 26-27.5 µm in size, wall is brown in colour, with mostly 1 germpores. Telia and teliospores are also chiefly hypophyllous; their colour is darker than pustules. Telia size is 0.3-0.6mm, most of them are 0.4 mm in diameter. Teliospores are 2 celled, 32-39×21-27 µm, short pedicel, hyaline. Based on morphology, the pathogen was identified to be Puccinia obtegens. Conditions suitable for uredospore germination were determined and showed that germination is not susceptible to light, the optimum temperature for uredospores germination is 15-22 °C, and germination needs water. Although C. segetum (Bunge) Kitam is an herb of value in traditional Chinese medicine but it is also one of the most harmful weeds in crops; competing for fertilizers, sunshine, water, and growing space. In order to investigate the possibility that a mycoherbicide might be developed to kill C. segetum (Bunge) Kitam we surveyed the weed pathogens for it and found a lethal rust pathogen. In this paper we describe some experimental results about the use of the pathogen as a bioherbicide agent.