Cankering by Cytospora chrysosperma (Pers.) Fr. was investigated in a poplar nursery at Lulu Island, B. C. Cankers were observed to be more numerous in early winter than at other times and more numerous on Populus trichocarpa T & G than on the P.x canadensis hybrid cultivars P. ‘Robusta Bachieleri' and P. 'Regenerata'. The upper part of the shoot appeared to become cankered before the lower part. Experiments with cuttings of the three poplars showed that cankers developed from inoculations only when the bark moisture content was below a critical level. The hybrids were found to have a lower critical level than P. trichocarpa. Also, the incubation period was longer in the hybrids than in P. trichocarpa and longer in the lower part of the shoot than in the upper. Cankers were arrested when the cuttings were placed in water, the arresting time being shorter in the lower part of the shoot than in the upper part. Inoculated P. trichocarpa plants in the greenhouse became cankered when they were subjected to drought but the two hybrids required both drought and low relative humidity conditions for cankering to take place. The rate of moisture loss from cuttings exposed to a uniform relative humidity was found to be faster in P. trichocarpa than in the hybrids and faster from the upper part of the shoot than from the lower part. A basis for a superior water economy in the hybrids was suggested by their anatomical characteristics which included larger pith, wider vessels, greater sieve tube zone, more numerous bark fibres, thicker periderm and fewer lenticels than in P. trichocarpa. The lower shoot had a larger sieve tube zone, a thicker periderm and fewer lenticels than the upper part. In the nursery, the hybrids were found to have a significantly higher shoot moisture content during winter than P. trichocarpa and the lower part of the shoot had a higher moisture content than the upper part. The moisture content minima for all species occurred in late autumn. The results suggested that the greater resistance of the hybrids to Cytospora canker observed in the nursery was attributable to their greater water storage capacity and to the slower rate at which water was lost by evaporation during dormancy. The occurrence of the heaviest outbreaks of the disease in early winter could be attributed in part to the decrease in shoot moisture content in the late autumn. / Science, Faculty of / Botany, Department of / Graduate