Ascospore discharge and conidium release of Nectria galligena Bres. from cankers on yellow birch (Betula alleghaniensis Britt.) were investigated under field conditions and were related to prevailing rainfall, relative humidity, and temperature. Release of conidia was associated with splashing rain whereas ascospores were discharged during or after rain. Conidia were trapped only from May through August during the period of active growth of the host, and showed a peak in May and June. In contrast, ascospores were trapped on rainy days throughout the year, including mid-winter when temperatures were around the freezing point. Maximum ascospore discharge occurred in August and September. In the laboratory, ascospores were discharged as wetted perithecia began to dry out. Both relative humidity and temperature affected the rate of perithecial drying and thus the rate of release and duration of discharge of ascospores. The results are discussed with respect to environmental conditions that prevail in eastern Canada during winter.

Although broken bmnches and bqsanch stubs are t h e m o s t frequently encoz~ntered external signs of defects in the t?.unlc of yellozu birch, t h e y zue?.e found t o be

No abstract

The inciting agent for typical cankers on the stem and main branches of hardwood trees, the fungus Nectria galligena Bres., may be responsible also for a crown dying of yellow birch owing to the development of numerous small cankers on branches and lesions without calluses which girdle twigs. From the ground, trees that are so affected may show a dying up to 30 or 40 percent of the crown and they look much like trees suffering from dieback.

Trees in two 45-year old stands of aspen were sampled for microorganisms in discolored and decayed wood. The main genera of fungi imperfecti isolated were Phialophora, Libertella and Ascocoryne. The commonest decay organism was Corticiumpolygonium followed in decreasing order of frequency by Fomesigniarius, Phlebiastrigoso-zonata and Pleurotusostreatus. Trichocladiumcanadense was usually found in the wood near the stump as was Armillariamellea. Bacteria and yeasts were common, especially in colored wood. From the location of the main organisms recorded, a succession of organisms is apparent. The presence of F. igniarius apparently is influenced by aspen clonal characteristics.