Phellinus weirli (Murr.) Gilbertson is an important root rot of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) in western North America. The effect of site and tree species on the growth of P. weirii along roots can be measured by inoculation using naturally infected root material or P. weirii cultured on sterile wood. This thesis reports the development of an inoculation technique using naturally infected root material to infect Douglas-fir and western hemlock (Tsuga heterophylla (Raf.) Sarg.). Intact naturally infected root sections of Douglas-fir used as an inoculum source resulted in P. weirii growing on approximately 87% of roots inoculated. Further inoculations were made using infected root sections split longitudinally with the exposed wood surface placed in contact with the host root. Subsequently an attempt was made to evaluate the influence of xeric, submesic and hygric sites, within the Demonstration area of the UBC Research Forest, Maple Ridge, on inoculation and growth of P. weirii along roots of Douglas-fir and western hemlock. Dry soil conditions experienced during the summer of 1978 reduced the expected number of successful infections of host roots to approximately 20%, 26 weeks after installation. All inoculations of a third series established after heavy rains in the late summer infected the roots of both Douglas-fir and western hemlock, verifying that the technique was successful when conditions were cool and moist. P. weirii inoculum used was collected from two sources, Haney and Surrey. Laboratory studies indicated incompatibility between the two sources when raised on agar media and field results indicated a longer retention of viable P. weirii in inoculum blocks from the Surrey source. / Forestry, Faculty of / Graduate