Inonotus tomentosus (Fr.) Teng. causes a root and butt rot of spruce in Canada. Quantification of the disease and its effects has been impaired by an inconsistent relationship between above-ground symptoms and root infection, and by a lack of knowledge of root rot effects, particularly on tree growth. Trees in an 8ha stand were rated for crown symptoms, then felled, stumped, and the roots examined. Of 1180 trees which had been rated as healthy, 27% proved to be diseased, while 44% of 158 trees which showed severe crown symptoms were diseased. Degree of crown symptom severity and percent of roots infected were also only weakly related (r hoch2=0.12). The percent of stump top area with butt rot was a better indicator of root disease severity (r hoch2=0.58). Analysis of 20 trees in each of four disease severity categories showed that stem volume of severely infected trees (>75% of roots infected) was reduced by 30% as compared to healthy trees. Stem volume in lightly infected trees (<25% of roots infected) was reduced by 13%. Losses due to butt rot in severely infected trees averaged 0.2m3 or 30% of merchantable volume. These results, and the scattered distribution of the disease, suggest (i) that root sampling, rather than surveys of crown symptoms, is necessary if disease levels are to be estimated accurately; and (ii) that Inonotus tomentosus can cause economically significant losses to current and future volumes of spruce, particularly if trees become infected at an early age by re-planting spruce on sites with a known history of root disease. Early infection will result in earlier deterioration of the tree's root system, and therefore greater losses to increment reduction and butt rot.
