Abstract Koch's Rules of Proof were used to establish C. fimbriata as the cause of a target type stem canker of aspen in Minnesota. The fungus was difficult to isolate from suspected cankers. The addition of 200 ppm of streptomycin to the isolation medium facilitated isolation of the fungus. Cankers were evident six weeks following inoculations made in the spring. Similar cankers formed on Populus alba var. pyramidalis following inoculations with C. fimbriata .