Septoria leaf spot and stem canker, caused by the fungal pathogen Mycosphaerella populorum Thompson (Anamomorph = Septoria musiva Peck). An efficient greenhouse disease resistance screening is essential for the development of resistant clones. Fourteen clones of hybrid poplar were inoculated with spore suspension. A regression model with parameters (lesion number and proportion necrotic area) is consistently and accurately predicted field resistance categories of the most resistant and susceptible clones. In second experiments, the infection biology of S. musiva was examined at several time points by scanning electron microscopy (6 h, 12 h, 24 h, 72 h, 1-week and 3-week) and histological analysis (3-week, 5-week and 7-week). Results indicated that there are differences occur following penetration between the resistant and susceptible. Those differences provide the first clues elucidating resistance mechanism in hybrid poplar stems. The results from this thesis will be used to improve resistance to Septoria canker in breeding programs.