SECONDARY SPREAD OF HETEROBASIDION-ANNOSUM IN WHITE FIR ROOT-DISEASE CENTERS

Tree mortality caused by Heterobasidion annosum Fr. (Bref.) in white f ir (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 call spread from tree to t ree through root contacts, it is often confined to the initially infec ted trees or stumps. We devised a field inoculation study to determine comparative virulence of fungal isolates, rates and modes of fungal c olonization, preferential direction of fungal colonization, and effect of root size on fungal growth in white fir roots. Fifty trees were in oculated with eight H. annosum isolates, and sampled at 4 and 12 month s. Heterobasidion annasum caused purple-brown staining and incipient w ood decay within 4 months. Isolates from stumps were as virulent as is olates from trees. Fungal colonization was less in smaller than larger roots. Fungal colonization was greater in the proximal (towards the h ole) than in the dis-tal direction. Most fungal colonization in the di stal direction occurred during the 4 months postinoculation, while col onization in the proximal direction occurred throughout the year.