The infestation will be favored in its growth if the spore can find plenty of food, water, warmth and darkness.  As these conditions generally exist in wounds and cavities of trees, it is wise to keep all wounds well covered with coal tar and to so drain the cavities that moisture cannot lodge in them.  This subject will be gone into more fully in the following two studies on “Pruning Trees” and “Tree Repair.”

[Illustration:  FIG. 110.—­The Birch-fungus rot. (Polyponis betulinus Fr.) Note the similarity in the color of the fruiting body and bark of the tree.]

While the majority of the fungi grow on the trunks and limbs of trees, some attack the leaves, some the twigs and others the roots.  Some fungi grow on living wood some on dead wood and some on both.  Those that attack the living trees are the most dangerous from the standpoint of disease.

The chestnut disease:  The disease which is threatening the destruction
    of all the chestnut trees in America is a fungus which has, within
    recent years, assumed such vast proportions that it deserves special
    comment.  The fungus is known as Diaporthe parasitica (Murrill),
    and was first observed in the vicinity of New York in 1905.  At that
    time only a few trees were known to have been killed by this
    disease, but now the disease has advanced over the whole chestnut
    area in the United States, reaching as far south as Virginia and as
    far west as Buffalo.  Fig. 111 shows the result of the chestnut
    disease.

The fungus attacks the cambium tissue underneath the bark.  It enters through a wound in the bark and sends its fungous threads from the point of infection all around the trunk until the latter is girdled and killed.  This may all happen within one season.  It is not until the tree has practically been destroyed that the disease makes its appearance on the surface of the bark in the form of brown patches studded with little pustules that carry the spores.  When once girdled, the tree is killed above the point of infection and everything above dies, while some of the twigs below may live until they are attacked individually by the disease or until the trunk below their origin is infected.

All species of chestnut trees are subject to the disease.  The Japanese and Spanish varieties appear to be highly resistant, but are not immune.  Other species of trees besides chestnuts are not subject to the disease.

[Illustration:  FIG. 111.—­Chestnut Trees Killed by the Chestnut Disease.]