This disease is far more important in the forest than in the orchard. Among some of the trees attacked, besides the apple, are beech, aspen, balm-of-Gilead, willow, sugar maple, red maple, striped maple, silver maple, yellow birch, butternut, black walnut, oak and hickory. Most destruction is wrought in the beech and aspen. But as an apple-tree heart disease, white Heart Rot is perhaps the most prevalent and the most destructive trouble of this type. Although this disease has been known for about two hundred years, it was not thoroughly studied until 1878. Little has been learned since that time, in spite of the fact that the disease is world wide in its range. It has been found in practically every country of the globe as well as in all the more important islands. The causal pathogene does not appear to be limited in its geographical range by climatic conditions, being found not only in temperate zones, but in the frigid and tropical regions as well. Symptoms.
The characters by which white Heart Rot may be recognized are of two general types, external and internal. The first external evidence of the disease is the appearance of the characteristic sporophores of the parasite (Fig. 20) which grow out through knot-holes where branches have broken off (Fig. 21, right). These fruiting bodies are more or less hoof-shaped, hard, black, and checked on the upper surface (Fig. 20), and dark-brown and porous on the lower surface. The pores are extremely small, their diameters being not more than one-sixteenth of an inch. The size of the sporophores themselves varies greatly. The internal symptoms of disease are evident when the tree is cut or blown over. Cross sections of the diseased portion show symptoms quite distinctive of the trouble (Fig.21). Affectedtrees never become hollow, but the rotten wood remains in place with a few cracks. The central area of the diseased heart is whitish or light-yellow (Fig. 21). Bordering this area is a narrow black line; sometimes there are several of these black lines arranged concentrically with white areas between them, and a yellowish to reddish brown zone, with an indefinite border, just outside the outermost black ring (Fig. 21). The character of the wood in these areas is as follows: - (1) in the white central area the wood is soft and crumbly when rubbed between the thumb and finger; (2) between the concentric black lines it is similar to that in the center, except that decomposition has not progressed so far; (3) outside the outermost black line the wood is hard and firm and thus differs from the normal wood only in color. The brown discoloration is caused by decomposition products which diffuse into the bordering healthy tissue.
Fig. 20. - Fruiting body of the white Heart Rot pathogene.
Fig. 21. - White Heart Rot; cross section of an affected limb showing the rotted heart-wood, the black lines and at the right a young fruit - body of the pathogene.
The causal pathogene, Fomes igniarius, is one of the pore-bearing basidiomycetous fungi. It has been known for about two centuries and was formerly used as tinder or touch-wood, or beaten into soft square pieces to be used by surgeons for stopping bleeding arteries. It is sometimes called the false tinder-fungus, the true tinder-fungus being a near relative (Polyporus sulphureus). Its life-history is similar to that of other basidiomycetes of this type. The spores, produced on the basidia lining the pores, are matured and disseminated during the early part of the summer. Some of these lodge in wounds where they germinate and, setting up a food relationship with the host, initiate the rot. The most common point of entrance is a knot-hole, or a stub exposed by careless pruning operations. From the germtube, mycelium is developed which grows into the heart-wood of the tree, passing up and down and obtaining the food necessary for its further growth. The rate of spread of the mycelium is dependent on many factors, as, for example, the breadth of the annular rings and environmental conditions. The rate is more constant in the horizontal than in the vertical growth. The mycelium develops abundantly in the wood-parenchyma, medullary-ray cells, and sometimes in the interior of the sap-tubes. It passes as a brown fungal mass from the sapwood into the bark. From here it presses outward and upon reaching a wound or bark fissure begins to form its sporo - phores. These originate by the massing of mycelial threads into a more or less definite form. On the lower surface of this mass a layer of tubes is formed; in each tube basidia and spores are developed. The spores break away, fall through the tube and out, are caught by the wind and carried to new infectioncourts. The mycelium lives over in the tree from year to year and the sporophores live for several years. One case is recorded where the sporophore reached an age of eighty years. Each year the mycelium making the sporophore fills up the old tubes and builds on a new layer of pores just below the layer of the preceding year.
For the orchardist, it will be well to consider the possible source of the trouble. It has been pointed out that the disease may be found on a wide range of broad-leaved trees. Should the orchard stand in close proximity to the forest, more than the usual amount of trouble may be expected to ensue. In such an event the destruction of diseased forest trees and the removal of sporophores become important. Perhaps the most important matter after all is the avoidance of wounds. Pruning wounds should in no case be left uncovered; a wound-dressing should be applied within a short time after pruning operations. Coal - tar will be found entirely satisfactory.
The making and handling of cuts have been discussed on page 53. In cases where trees are already affected with Heart Rot the treatment of such areas is an important matter in the control of the disease. It should be borne in mind that the normal heart of a tree is practically dead tissue. It gives only rigidity, and may be completely removed without infraction on the normal functions of the tree beyond impairing its strength, which is a serious consideration, however, because the limbs may be broken, or the tree may be blown over by the wind. Decaying wood can be of no use to a tree; on the other hand, it may act detrimentally and should therefore be removed. The operation should be thorough, all decayed wood being removed. A mallet, a chisel and a gouge are the chief tools needed. If the rot extends for a considerable distance down the limb, a hole, which will serve as a drain, should be bored at the lower extremity. Cavities in shade and ornamental trees are often filled with cement or asphaltum blocks. Whether or not this practice is to be followed in the apple orchard depends upon the extent of the injury and the exposure of the orchard to wind. As a general rule the filling of cavities with any substance is not within the province of the practical orchardist; the operation does not pay. But in any case the interior surface of the cavity should be coated with a wound - dressing.
Schrenk, H. von, and Spaulding, P. Diseases of deciduous forest trees.
White Heart Rot caused by Fomes igniarius. U. S. Agr. Dept.
Pl Ind. Bur. Bul. 149: 25 - 37. 1909. Atkinson, G. F. Studies of some shade tree and timber destroying fungi. Polyporus igniarius. Cornell Univ. Agr. Exp. Sta. Bul.
193: 239-247. 1901. Hartig, R. Die Zersetsungserscheinungen des Holzes. Polyporus igniarius Fr., pp. 114 - 123. 1878.