Fungi are flowerless vegetable organisms that multiply by spores. The mushrooms are the familiar fruiting organs of under ground species. Rust, mildew, blight and rot of fruit or of 5wood are also among the well-known fungous growths that disfigure trees. The shelf fungi are the largest. Many kinds of destroying fungi may attack a single tree. Every enfeebled tree is increasingly vulnerable. Dead trees are gradually devoured by fungous organisms.
Protection against fungous diseases is not practicable yet in the forests. In orchards and home grounds and parks spraying is used as a preventive. Compounds of copper destroy the spores of fungi. It is asserted that one part of copper sulphate in ten thousand parts of water will prevent the germination of a spore of apple scab or pear-leaf blight. Lime water is added to keep the copper sulphate from burning the foliage. Copper, lime, and a large proportion of water make the so-called "Bordeaux mixture"— the standard fungicide in the orchards and vineyards of Europe and America. Two or three sprayings a year, the first just before the leaves open, will keep a healthy forest tree free from fungous troubles, while neighbour trees and their fruit are badly damaged by rot and other fungous attacks.
Bacterial diseases that enter the growing shoots of trees and develop within them are well illustrated by the "fire blight" of the pear. No fungicide can reach and check this disorder. The affected parts should be cut off and burned. Often burning the whole tree is the only safe method, as otherwise contagion will spread to other trees.
Constitutional diseases are found among trees, as well as in the human family, and no explanation of their causes nor hint of proper treatment has been discovered. "Peach yellows" is an example. It is the moral, if not the legal, obligation of every owner of a tree thus afflicted to dig it out and burn it, root and branch, in order that the disease may be kept from spreading. Tree diseases are not all disseminated by the wind. Some live underground, carrying infection by contact of root tips from unsound to sound trees.
Insects form a large body of the enemies of trees, inflicting untold damage each year upon orchards and forests, and upon trees everywhere. Each species has its insect enemies, not one, but more—often many. There are borers that infest the solid wood, channelling it and ruining it for timber, or working just under the bark, sapping the cambium, which is the tree's life. Some borers work in the twigs, causing the young shoots to die and snap off. Black locust, one of the most valuable post timbers, is ruined wherever it grows now in the East by the locust borer.
Sucking insects are a vast aggregation of species whose bond of similarity is the beak or proboscis, by means of which they puncture the skin of fruits, leaves, twigs or roots and suck the juices there found. To this class belong the deadly scale insects,
the plant lice, bark lice, true bugs, weevils, etc.
Chewing insects eat the substance of the leaf or other parts. The caterpillars of many butterflies, moths and beetles are chewers. Borers belong to this class.
It is quite out of the question to attempt in this volume a discussion of a subject so vast as the insect enemies of trees and the methods science has devised to combat them. Horticulture has led the way, of course. Publications covering all that is known on the subject are issued by the Department of Agriculture. Experiment stations in the different states are investigating this subject and reporting progress in bulletins, which anyone within the state may have for the asking. Besides, a growing body of literature on the subject is being issued by various publishers of scientific books.
Spraying and fumigation are the two methods now in use for the wholesale destruction of insects. They are developed to a high degree by fruit growers. Power spraying has been intro duced by park commissioners in a few large cities for the protection of shade trees. It promises to grow in popularity wherever public spirit is strong and trees are threatened, as they are with the gypsy-moth plague near Boston. Study of the life history of different insects and fungi reveals their various weak points and helpless stages. The principles and practice of spray ing depend for success upon this intimate knowledge.
Boring insects cannot be reached by spraying. They are dug out of fruit trees or destroyed by running a wire up the burrows. It is the grub that does the damage.
Chewing insects that live on trees are killed by spraying poison on their food. Paris green, dissolved in water, and arsenate of lead are commonly used. The younger insects are sprayed the better.
Sucking insects are killed by spraying with kerosene and water, or with an emulsion of whale-oil soap, and with lime and sulphur washes. The oil chokes the breathing tubes which are along the sides of the body. The whale-oil soap chokes and is also injurious to the delicate body wall. So is the lime and sulphur solution. Scale insects, plant lice and all soft-bodied insects of whatever eating habits are thus treated.
Fumigation chokes the insects with poisonous gas. Hydro cyanic-acid gas, confined by a canvas tent that completely covers a tree, destroys all insect life in a few minutes. This is an ex pensive method, but it is used in orange groves in California as the best means of checking scale insects. As these insects do not fly nor walk, but settle down after birth, a tree once cleared of the nuisance is not likely to become infested again for some time.