Like most cultivated plants, shrubs and trees have their enemies, whether in the form of caterpillars or fungi. But speaking generally, there is not so much anxiety on this score as in the case of fruit trees. The Rose is the most harassed, and the enemies of that popular shrub are dealt with in the companion volume, "Roses and Rose Gardens."
Occasionally branches of Rhododendrons, as of other plants in the order Ericaceae, die from canker, for which there is no known remedy.
Young trees of the softer wooded kinds, such as Willow, Chestnut, Lime, Birch, Ash and Plane, sometimes show cracks in the bark, which on being removed reveals hard, dry wood. A considerable area of the stem of a tree may be affected in patches in this way, and it may dwindle and die. This condition is often caused by a burst of sunshine after a spell of dull damp weather. It is, in fact, a case of what the grape-grower would call "scorching," the "enemy" being the sun. It will be noticed that it happens most often in the case of trees that are quite exposed. Those of which the stems are shaded by their own foliage, by undergrowth, or by other objects, do not suffer. If a young tree should be found suffering from this cause, haybands might be made, twisted round the stem and kept moist. They are also useful when a tree has sustained accidental injury.
In case of any canker-growth, the patch should be cut out and gas tar painted on.
The upper part of a tree may sometimes break off and reveal the tunnellings of a caterpillar, such as that of the Goat Moth (Cossus ligniperda) or that of the Wood Leopard Moth (Zeuzera aesculi). The former is so called from its disagreeable odour. It makes large borings and does serious damage to timber if not destroyed. If the bark of a tree is observed to be injured in any way, it should be examined, and if any trace of wood chippings is found a hole should be searched for and a piece of wire passed in to the end, where it should be worked about so as to destroy the caterpillar. The same plan may be adopted in the case of an attack by the Wood Leopard Moth caterpillar.know
One of the most remarkable diseases in trees is the "Witch-knot" of the Birch, a bundle of twigs that at a short distance might be taken for a crow's overturned nest, but which proves on a close inspection to be attached to the tree. It is due to the irritation caused by a species of gall-mite. Such mites are numerous, and their work is generally shown in swellings on the leaves; but in this case twiggy clusters develop. An attack should be watched, or it may spread to other Birches in the vicinity. The safest plan is to cut the bunch of twigs out and burn it.
Larches are often grown in large plantations, and the trees may suffer severely from the aphis, Chermes laricis, which absorbs sap both through the leaves and the tender bark of young trees. A simple remedy is a wineglassful of paraffin oil in a gallon of water. They are also subject to other diseases, but these troubles attach rather to the domain of the forester than the flower-gardener. The same may be said of the Pine Sawfly, the Pine Weevil, the Spruce Gall Aphis, the Willow Beetle and other tree enemies. It is rare that a few trees well grown in gardens for ornamental purposes in fertile, well drained soil, are seriously injured or disfigured.
A similar remark applies to the vast majority of evergreen and deciduous shrubs, and it is quite unnecessary to take them one by one, as would be the case with the principal fruits, vegetables and florists' flowers, and deal with individual enemies.