Pinus monticola Dougl. seedlings were grown under eight light × nutrient treatments in a greenhouse for 5 months. One month after treatments began, seedlings were exposed to Armillaria; they were removed from their cells and inoculated by sandwiching the primary root between blocks of infected P. monticola. A small lateral root also was severed on each seedling. Seedlings grown with a complete nutrient supply and full light had significantly lower Armillaria infection rates than seedlings grown under light, nitrogen or phosphorus limitations and had significantly higher root, shoot, and total biomass and shoot: root ratios. However, root or shoot sugar or starch concentrations did not differ regardless of treatment. Seedlings given complete nutrient supply but limited light took up more Al, B, Cu, Fe, Mg, Mn, Na, P, and Zn than those given complete nutrient supply and full light; those given complete nutrient supply and full light contained significantly lower concentrations of Cu, K, Mn, Mo, and P but higher concentrations of Al, B, and Fe than seedlings given nutrient-deficient treatments. In the greenhouse, resistance of Pinus monticola to Armillaria infection depends largely on adequate light and nutrient resources; thus, creating conditions that favor tree vigor could reduce incidence of Armillaria infection, especially where light is a limiting factor.