Resin and the Resistance of Conifers to Fomes annosus

Roots of young trees of different conifer species, Scots pine, Corsican pine, Sitka spruce, and Douglas fir were inoculated with Fomes annosus. In the pines, infection was negligible but in the other genera the central wood was extensively invaded. The process of infection was studied further in Sitka spruce by means of a root-severing technique in which an inoculum block was forced between the cut ends of a root. In most trees a central rot developed which in some extended up into the butt: the mean extent of growth recorded was 30 cm in 14 months. In the severed root portions infection also occurred but F. annosus was subsequently replaced to a considerable extent by other micro-organisms, particularly in trees growing on a peat soil. A similar central rot developed after inoculations of Norway spruce. The resistance of pine sapwood and the outer wood of spruce and Douglas fir can be explained at least partially by the structure and viability of the resin duct system. Larch, Douglas fir, spruce, and pine form a series showing increasing complexity and activity of the resin system. In addition, studies carried out on the resin yields of pines growing under different conditions showed that the resistance of a tree to F. annosus is correlated with its ability to mobilize resin. Site factors, forest management, host age vigour, genetic constitution, and past history may all influence resin mobilization.