Nature of the disease fungi and their action.
Fungi are a low class of plants, consisting of fine threads. called hyplue. many hyphiE forming the mycelium. The mycelium grows in the dead or living parts extracting certain food substances therefrom. After varying periods, fruiting bodies are formed, which develop spores. These fruiting bod ies have various shapes, varying from microscopic structures to the large punks or toadstools so com monly found on older trees. The spores are discharged into the air, and are dis tributed from one tree to another by the wind ; they are also carried from tree to tree by insects, rain, or, when the fungi grow under the ground, by burrowing animals, such as moles and mice.
When the fungus causes a disease of the leaves or branches, the spores usu ally germinate directly on the leaves or branches, the fungus penetrating into the living tissue, and growing there. When the fungus attacks the heart-wood of the tree, the spore must get into some wound. During the early life of the tree these wounds are very few in number, but as a tree grows older many wounds are formed, and the tendency to close these wounds, either by the formation of callus or by the exudation of gum or resin, is very much reduced. Wounds are made by deer and other browsing animals, by wood peckers, but chiefly by the breaking off of large branches by the wind or snow. Wherever a wound is made, the spores from numerous wood-rotting fungi enter and germinate, and the mycelium of the fungus grows down into the heart-wood of the tree. When it has reached the heart-wood, it grows both up and down in the tree trunk, and results in the partial or total destruction of the wood, as shown in Figs. 492, 493 and 494. When a sufficient amount of nutritive material has been absorbed from the trunk, a punk or toadstool forms on the outside, bearing new spores, as shown in Fig. 492. Fig. 497 illustrates a different type of injury. It shows the way in which mistletoe forms a "bird's-nest" on lodge-pole pine.
The fungi that attack leaves and branches are rarely present in sufficient number to kill a large tree, although they may stunt its growth. They are very much more dangerous to extremely young trees. The so-called " damping-off " fungi belong to this group, and they are particularly active in seed-beds. As the tree grows older, the wood-rotting fungi become more important, and the older the tree gets the more liable to disease it becomes. For most kinds of trees, a certain age usually will mean an almost certain attack by one or the other of the wood-rotting fungi, and it is generally well, when such trees are used for lumber, to cut them shortly after this age has been reached. For pines this may be about eighty to one hundred years. It is the latter class of fungi that are of particular interest to the lumberman and forester.
Some of the important fungi which produce disease in forest trees are the red heart fungus (Trametes pini, Fig. 496), found on all coniferous trees; the false tinder fungus (Polyporus igniarins, Fig. 493), found on beech, apple, oak, poplar and other hardwoods, where it produces a white, soft rot of the trunk ; the sulfur mushroom, which causes a brown rot of many coniferous trees, and also of oak, walnut, cherry and other deciduous trees.
The fungi that attack hewn timber and produce decay belong to a separate group. The factors which favor their development are, a certain amount of heat, oxygen, water and food supply. Dry wood will last very much longer than green wood. A post set in the ground with its bark removed will outlast one with the bark on. Sap wood is very much more liable to attack than heart-wood. The rate at which different kinds of wood will decay differs, and woods are accordingly classed as long- and short-lived. Long-lived woods are such as white oak, cypress, cedar, chestnut and redwood ; and short-lived woods are such as fir, hemlock, beech, red oak, gum and the soft pines.