Spermagonia and ascocarps of Mycosphaerella populorum Thompson were produced in vitro on a poplar leaf decoction agar. Ascospore and conidial release and infection of foliage and stems of two Populus clones by the pathogen were monitored in a hybrid Populus plantation. Ascospore collection commenced in April at approximately the time of bud swell of P. deltoides and continued for a period of 5-6 months. When using a Burkhard spore trap, most ascospores were trapped during daylight after rains. Cumulative ascospore productivity, monitored in the plantation with vaseline coated slide traps and an ascospore liberation tunnel and in controlled temperature cabinets with the ascospore liberation tunnel, was related to degree days base 0 C. A Gompit transformation linearized cumulative ascospore productivity-degree day curves. Utilization of historic degree day data and the equation developed from controlled temperature chambers allowed prediction of seasonal ascospore productivity;Conidia of Septoria musiva were collected from overwintered pycnidia in April. Conidia produced from infected foliage during the growing season generally increased on a weekly basis starting in June. Vaseline coated slide traps collected most conidia in weeks with low levels of rain, while a water collection spore trap monitored highest conidial levels in periods with high rain fall;Foliar infection of a susceptible clone was found whenever measurable levels of conidia or ascospores were collected. Foliar infection ratings were highly correlated with ascospore densities while correlations with conidial densities were generally nonsignificant. Stem infection of a susceptible clone (NC5272) occurred in periods of peak ascospore and conidial release while a moderately resistant clone (NC5271) was infected only during peak periods of ascospore collection. Environmental parameters monitored in the field were highly correlated with foliar and stem infection and were used to develop regression equations. In laboratory experiments, both foliage and stems were infected after a four-hour post inoculation wetness period;Bottoms of leaves were more susceptible to infection than tops of leaves of both clones. Inoculation of foliage and stems with various inoculum densities showed that significant isolate-clonal interactions were present on the moderately resistant clone. These interactions were generally nonsignificant on the susceptible clone.