Incidence of Heterobasidion annosum in precommercial thinning stumps in coastal British Columbia

Coniferous stumps in 83 stands in coastal British Columbia were sampled 3-5 years after precommercial thinning. The percentage of stumps and surface a rea colonized by Heterobasidion annosum were determined for 25 stumps of ea ch species in each 5-cm diameter class present in each stand. There were si gnificant differences among species in the percentages of stumps and surfac e area colonized, with Douglas-fir (Pseudotsuga menziesii) having the lowes t values, amabilis fir (Abies amabilis) and Sitka spruce (Picea sitchensis) the highest and western hemlock (Tsuga heterophylla) being intermediate. F or stumps of each species 5-20 cm in diameter, both the percentage of stump s and surface area colonized increased with increasing diameter. In stumps that were grafted to an adjacent tree, there was decreased incidence of H. annosum for Douglas-fir and Sitka spruce and increased incidence for wester n hemlock and amabilis fir. There were trends in the percentage of stumps a nd area colonized for season of thinning and biogeoclimatic subzones with t he values for most species decreasing as the amount of precipitation increa sed. Colonization of precommercial thinning stumps by H. annosum occurs thr oughout the coastal region of British Columbia, and this will increase the amount of inoculum and will likely increase the incidence of butt rot. The results of this study suggest that the increase in inoculum can be minimize d by thinning before age 15, by cutting only trees less than 10 cm in diame ter and by thinning during low risk seasons.