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Armillaria ostoyae causes considerable loss in forest productivity in both immature and mature stands within the Interior Cedar Hemlock (ICR) and the Interior Douglas-fir (IDF) zones of the southern interior of British Columbia. Two studies concerning the impacts of this pathogen where conducted near Salmon Arm, BC; one was within four plantations age ten to twenty-five years on Larch Hills, and the other was within mature stands on Hunter’s Range and Larch Hills. In the plantation study the relationship between the levels of A. ostoyae infection in mature tree stumps and the regeneration was examined. The evidence of past A. ostoyae infection in stumps remains visible on the inner bark for at least thirty years. This evidence may be used to estimate the extent of the disease in the former stand. The relationship between A. ostoyae in stumps and A. ostoyae-caused mortality in regeneration was significant, though not strongly. Three measures of stump inoculum were compared: the proportion of stumps infected, the number of stumps infected, and the basal area of stumps infected. The number of stumps infected was most closely associated with the proportion of regeneration infected. The relative rates of infection incidence were compared among the eight regeneration species present in the four plantations. A quantitative means of comparing the incidence of infection among species was developed. The number of infected Douglas-fir trees was chosen as a standard measure of disease incidence for each plantation. The incidence of infection in the other seven species were then compared to the Douglas-fir standard. The probability of a young …