Release ofCronartiumcomandraePk. aeciospores from stem cankers onPinuscontortaDougl. var.latifoliaEngelm. in southwestern Alberta occurred from mid-May to late August, the peak period being between late May and mid-June, after which there was a gradual decrease. Diurnal aeciospore periodicity showed spores to be usually released between 0800 and 1900 hours, with some evidence for a double peak. Maximum daily concentration of trapped spores at a distance of 2.5 cm from the canker was 17 217 spores/m3, but on more than half the days fewer than 100 spores/m3were collected. The highest hourly concentration observed from an individual canker was 128 200 spores/m3, which coincided with the onset of heavy rain. Atmospheric turbulence was associated with all peak aeciospore concentrations. Little release occurred when temperatures were below 7 °C, when relative humidity levels were high, or when dew was present. Peak release occurred when air temperatures were above 20 °C and relative humidity levels were at their lowest. Heavy rains initially increased spore concentrations, but light rains did not have the same effect. Wind was important for initiation of spore release, but there was no close correlation between high wind velocities and high spore concentrations released from a canker.