Fungous diseases attacking a cavity produce a mass of fibers, known as the “mycelium,” that penetrate the body of the tree or limb on which the cavity is located.  In eliminating disease from a cavity, it is, therefore, essential to go beyond the mere decaying surface and to cut out all fungous fibers that radiate into the interior of the tree.  Where these fibers have penetrated so deeply that it becomes impossible to remove every one of them, the tree or limb thus affected had better be cut down. (Fig. 118.) The presence of the mycelium in wood tissue can readily be told by the discolored and disintegrated appearance of the wood.

The filling in a cavity, moreover, should serve to prevent the accumulation of water and, where a cavity is perpendicular and so located that the water can be drained off without the filling, the latter should be avoided and the cavity should merely be cleaned out and tarred. (Fig. 116.) Where the disease can be entirely eliminated, where the cavity is not too large, and where a filling will serve the practical purpose of preventing the accumulation of moisture, the work of filling should be resorted to.

[Illustration:  FIG. 118.—­A Cavity Filled in a Tree that Should Have Been Cut Down.  Note how the entire interior is decayed and how the tree fell apart soon after treatment.]

Filling should be done in the following manner:  First, the interior should be thoroughly freed from diseased wood and insects.  The chisel, gouge, mall and knife are the tools, and it is better to cut deep and remove every trace of decayed wood than it is to leave a smaller hole in an unhealthy state.  The inner surface of the cavity should then be covered with a coat of white lead paint, which acts as a disinfectant and helps to hold the filling.  Corrosive sublimate or Bordeaux mixture may be used as a substitute for the white lead paint.  A coat of coal tar over the paint is the next step.  The cavity is then solidly packed with bricks, stones and mortar as in Fig. 119, and finished with a layer of cement at the mouth of the orifice.  This surface layer of cement should not be brought out to the same plane with the outer bark of the tree, but should rather recede a little beyond the growing tissue (cambium layer) which is situated immediately below the bark, Fig. 120.  In this way the growing tissue will be enabled to roll over the cement and to cover the whole cavity if it be a small one, or else to grow out sufficiently to overlap the filling and hold it as a frame holds a picture.  The cement is used in mixture with sand in the proportion of one-third of cement to two-thirds of sand.  When dry, the outer layer of cement should be covered with coal tar to prevent cracking.

[Illustration:  FIG. 119.—­A Cavity in the Process of being Filled.]

[Illustration:  FIG. 120—­The Same Cavity Properly Filled.]