Tree mortality caused by Heterobasidion annosum Fr. (Bref) in white fir (Abies concolor (Gord. & Glend.) Lindl.) often appears in clusters; symptoms in the infected trees include sapwood and heartwood decay in tree holes and roots. Although the pathogen can spread from tree to tree through root contacts, it is often confined to the initially infected trees or stumps. We devised a field inoculation study to determine comparative virulence of fungal isolates, rates and modes of fungal colonization, preferential direction of fungal colonization, and effect of root size on fungal growth in white fir roots. Fifty trees were inoculated with eight H. annosum isolates, and sampled at 4 and 12 months. Heterobasidion annosum caused purple-brown staining and incipient wood decay within 4 months. Isolates from stumps were as virulent as isolates from trees. Fungal colonization was less in smaller than larger roots. Fungal colonization was greater in the proximal (towards the bole) than in the distal direction. Most fungal colonization in the distal direction occurred during the 4 months postinoculation, while colonization in the proximal direction occurred throughout the year.