By the kindness of the forest manager I was allowed to fell one of these trees. It was chosen at hazard, after the men had struck a large number, to show me how easily the hollow trees could be detected by the sound. The tree was felled by sawing close to the roots; the interior was hollow for several feet up the stem, and two of the main roots were hollow as far as we could poke canes, and no doubt further. The dark-colored rotting mass around the hollow was wet and spongy, and consisted of disintegrated wood held together by a mesh work of the rhizomorphs. Further outward the wood was yellow, with white patches scattered in the yellow matrix, and, again, the rhizomorph strands were seen running in all directions through the mass.
Fig. 16.—Sketch of the base of a young tree (s) killed by Agaricus melleus, which has attacked the roots, and developed rhizomorphs at r, and fructifications. To the right the fructifications have been traced by dissection to the rhizomorph strands which produced them.
Not to follow this particular case further—since we are concerned with the general features of the diseases of timber—I may pass to the consideration of the diagnosis of this disease caused by Agaricus melleus, as contrasted with that due to Trametes radiciperda.
Of course no botanist would confound the fructification of the Trametes with that of the Agaricus; but the fructifications of such fungi only appear at certain seasons, and that of Trametes radiciperda may be underground, and it is important to be able to distinguish such forms in the absence of the fructifications.
The external symptoms of the disease, where young trees are concerned, are similar in both cases. In a plantation at Freising, in Bavaria, Prof. Hartig showed me young Weymouth pines (P. Strobus) attacked and killed by Agaricus melleus. The leaves turn pale and yellow, and the lower part of the stem—the so-called "collar"—begins to die and rot, the cortex above still looking healthy. So far the symptoms might be those due to the destructive action of other forms of tree-killing fungi.
On uprooting a young pine, killed or badly attacked by the agaric, the roots are found to be matted together with a ball of earth permeated by the resin which has flowed out; this is very pronounced in the case of some pines, less so in others. On lifting up the scales of the bark, there will be found, not the silky white, delicate mycelium of the Trametes, but probably the dark cord-like rhizomorphs; there may also be flat white rhizomorphs in the young stages, but they are easily distinguished. These dark rhizomorphs may also be found spreading around into the soil from the roots, and they look so much like thin roots indeed that we can at once understand their name—rhizomorph. The presence of the rhizomorphs and (in the case of the resinous pines) the outflow of resin and sticking together of soil and roots are good distinctive features. No less evident are the differences to be found on examining the diseased timber, as exemplified by Prof. Hartig's magnificent specimens. The wood attacked assumes brown and bright yellow colors, and is marked by sharp brown or nearly black lines, bounding areas of one color and separating them from areas of another color. In some cases the yellow color is quite bright—canary yellow, or nearly so. The white areas scattered in this yellow matrix have no black specks in them, and can thus be distinguished from those due to the Trametes. In advanced stages the purple-black rhizomorphs will be found in the soft, spongy wood.
The great danger of Agaricus melleus is its power of extending itself beneath the soil by means of the spreading rhizomorphs; these are known to reach lengths of several feet, and to pass from root to root, keeping a more or less horizontal course at a depth of six or eight inches or so in the ground. On reaching the root of another tree, the tips of the branched rhizomorph penetrate the living cortex, and grow forward in the plane of the cambium, sending off smaller ramifications into the medullary rays and (in the case of the pines, etc.) into the resin passages. The hyphæ of the ultimate twigs enter the tracheides, vessels, etc., of the wood, and delignify them, with changes of color and substance as described. Reference must be made to Prof. Hartig's publications for the details which serve to distinguish histologically between timber attacked by Agaricus melleus and by Trametes or other fungi. Enough has been said to show that diagnosis is possible, and indeed to an expert not difficult.
It is at least clear from the above sketch that we can distinguish these two kinds of diseases of timber, and it will be seen on reflection that this depends on knowledge of the structure and functions of the timber and cambium on the one hand and proper acquaintance with the biology of the fungi on the other. It is the victory of the fungus over the timber in the struggle for existence which brings about the disease; and one who is ignorant of these points will be apt to go astray in any reasoning which concerns the whole question. Any one knowing the facts and understanding their bearings, on the contrary, possesses the key to a reasonable treatment of the timber; and this is important, because the two diseases referred to can be eradicated from young plantations and the areas of their ravages limited in older forests.
Suppose, for example, a plantation presents the following case. A tree is found to turn sickly and die, with the symptoms described, and trees immediately surrounding it are turning yellow. The first tree is at once cut down, and its roots and timber examined, and the diagnosis shows the presence of Agaricus melleus or of Trametes radiciperda, as the case may be. Knowing this, the expert also knows more. If the timber is being destroyed by the Trametes, he knows that the ravaging agent can travel from tree to tree by means of roots in contact, and he at once cuts a ditch around the diseased area, taking care to include the recently infected and neighboring trees. Then the diseased timber is cut, because it will get worse the longer it stands, and the diseased parts burnt. If Agaricus melleus is the destroying agent, a similar procedure is necessary; but regard must be had to the much more extensive wanderings of the rhizomorphs in the soil, and it may be imperative to cut the moat round more of the neighboring trees. Nevertheless, it has also to be remembered that the rhizomorphs run not far below the surface. However, my purpose here is not to treat this subject in detail, but to indicate the lines along which practical application of the truths of botanical science may be looked for. The reader who wishes to go further into the subject may consult special works. Of course the spores are a source of danger, but need be by no means so much so where knowledge is intelligently applied in removing young fructifications.