- Studies to identify the environmental factors that influence the rate of ectotrophic mycelial growth of Phellinus weirii (Murr.) Gilbertson along Douglas-fir roots were undertaken both in the laboratory and in the field. The effects of soil pH, nitrogen, moisture, temperature, microorganisms and bulk density were investigated. In the lab, the rates of ectotrophic mycelial growth from Alnus inoculum blocks onto 1.5 cm diameter Douglas-fir branch sections, glass rods and wood dowels buried in 475 cm³ soil tubes filled with autoclaved, pasteurized or untreated forest soils were compared. Maximum ectotrophic mycelial growth occurred along Douglas-fir branches. The rates of growth along Douglas-fir branches were similar in autoclaved, pasteurized and untreated soils, but the fungus only grew profusely into the sterilized soils. Ectotrophic mycelial growth averaged 9.0 cm after 25 days. Penetration of the pith from the cut end of the branch section occurred more rapidly than penetration through the xylem. Ectotrophic mycelial growth of P. weirii was greatest at 15% soil water content. Maximum growth occurred at 20-25 C. Field inoculations to determine the effects of soil environment on the rate of ectotrophic mycelial growth of P. weirii were performed in two Douglas-fir plantations on the east side of the Oregon Coast Range, near Corvallis, Oregon. Roots were inoculated by tying inoculum blocks to living roots, and test treatments were produced by altering the soil around inoculated roots. After one year, the roots were removed and inoculation success and ectotrophic mycelial growth were measured. Ectotrophic mycelium was observed on 58% of the inoculated roots. However, on 10% of the roots, the fungus failed to grow beyond the plastic ribbon that was wrapped around the inoculation area. The majority of ectotrophic mycelial growth appeared to have taken place during the first few months after inoculation. Although ectotrophic growth was dramatically enhanced in the vicinity of the wounds, the rate of growth away from the wound area was not significantly affected (90% confidence). Penetration of the fungus into the root was observed in 53% of the successful inoculations. P. weirii was isolated from the phloem of 24% of these roots. Colonization of roots was markedly enhanced by root wounding. The average total extent of P. weirii along inoculated roots, where any growth had occurred, was 15.5 cm. However, variation was high, so conclusions must be tentative. Soil pasteurization and elevated soil temperatures markedly decreased inoculation success. In contrast, changes in soil pH, moisture and bulk density had relatively little effect. The extent of P. weirii growth along inoculated roots decreased with changes in soil pH, moisture and nitrogen levels, both above and below ambient levels. Relatively little change was associated with pasteurization, compaction and temperature treatments.