American Pomology. Apples

by J. A. Warder

Light is the great stimulus of vegetation, an essential element to its existence: its withdrawal is followed by an arrest of some of the most important functions of vitality, and yet we find that there is a great difference among different species, as to their requirements of this element, and also that various parts and several products of vegetation require very different degrees of light for their perfection. It is also found that a sudden exposure of parts from which it had been withheld, is often attended with disastrous consequences. Its withdrawal does not so immediately destroy the plant, being attended with the etiolation of the parts that are usually colored, but a sudden re-exposure to the sun's rays will now destroy the plant. So the removal of a portion of the foliage from a tree, or the exposure of the bare stem of one that had been previously sheltered, is often attended with severe effects, known as sun-scald—for which there is no remedy, but very easy modes of prevention. The best of these is to provide against the evil by reserving the lower branches to shade the stem. There are other excellent reasons for this practice, which will be brought forward in the chapter on Pruning.

Frequently, however, the nurseryman, or perhaps the injudicious efforts of the planter himself, may have removed all the side branches of the young tree, and as these cannot be replaced, we may substitute for them a shelter from the scorching sun to which the newly planted tree is exposed. This may be done by tacking two narrow boards together at their edges, like a gutter spout, and setting them upright on the south side of the tree to shade it. A wisp of straw, tied loosely to the stem, will answer a very good purpose; but both of these appliances are objectionable, because they furnish a shelter for insects, and thus they fall short of the natural shading of the stem by the foliage of its own branches.

It is not only the scorching suns of summer that damage our young trees that are thus exposed by injudicious trimming. Even the bright rays of a mid-winter sun, falling upon the frozen stem, will often effect the most serious damage, and should be guarded against with equal care; but here the natural protection will answer, for the shade of the naked spray of the laterals is found all-sufficient in the well-trained tree.

2d—To resume the consideration of Lankester's causes of disease, it must be admitted that some diseased conditions may be produced by poisonous gases, but the usual result will be the death of plants confined in such an atmosphere. The natural power of diffusion of all gases among one another in the open air, prevents the danger that would ensue in a confined situation. The accidental production of sulphurous and other poisonous gas, or the escape of smoke from the flues or from the tobacco-pan in the green-house, sometimes produces the most disastrous effects upon the plants subjected to their action. So, in crowded cities, it often happens that the effects of coal smoke and other gases, generated in the furnaces and manufactories, are very injurious to vegetation. The coal soot falls in flakes like lamp-black, which covers the surface and obstructs the transpiration of the stomata, and thus seriously affects the health of plants in such situations.

The action of miasmata, suggested by Lankester, is as obscure in the effects produced upon plants as in those upon animals. The presence of these atmospheric conditions cannot be detected by any of our tests, nor can their effects be prevented by any means in our power; we know little or nothing about their characters, yet we cannot deny their existence: finally, they serve as a very convenient explanation, though a very unsatisfactory one, for the incursions of maladies that are of an obscure or unknown character. Whether of a miasmatic nature or not, no one can deny the existence of certain atmospheric conditions, which appear to produce disastrous effects upon some of our vegetable productions whether these be inherent to the air itself, or are only conveyed by it from one place to another. The inexplicable potato disease may owe its origin and diffusion to such a cause, and the grape malady, which appears to be dependent upon atmospheric causes, may at least be carried from one vine to another upon this medium, in the form of the minute spores or seeds of the fungi that are believed to be the cause of the trouble.[18]

Poisons in the soil are frequently very deleterious to vegetation, and we often find extensive injuries to our plants produced by this class of agents. When these are of a chemical nature, as is usually the case, they may be satisfactorily treated by applications that will neutralize their effects. In cities the escape of the illuminating gas, that is carried in subterranean pipes, has often so poisoned the soil as to destroy the shade trees by the side of the streets.

An excess of certain saline and alkaline ingredients often produces barrenness in the soil, by a sort of poisoning, even with those articles that in smaller quantities are used as manures with the happiest effects.

3d—The influence exerted upon vegetation by the growth of parasitic plants, cannot be observed without forcing us to the conclusion that they are prejudicial to the health of the plants they infest—since they either cover and smother the foliage by twining upon it, as is the case with the Dodder; or fasten themselves upon a limb, appropriating the sap that was intended for its support, and thus starve it, as does the Mistletoe; or attaching themselves to the bark, they interfere with its functions, as is done by the lichens and mosses; or, following the descending scale, in the size of these parasites, but meeting in them foes of much greater importance, we find the minute but innumerable fungi attacking the wood, the bark, the foliage, and the fruits, of our gardens and orchards, and committing incalculable damage—thus entailing serious disease. A very important question has arisen, however, as to whether the inroads of fungi were the cause or the consequence only of disease. A question which it will be necessary to leave to wiser heads, only observing that these epiphytes do appear, under certain atmospheric conditions, to invade some plants that had previously seemed to be in perfect health. That they are transported upon the air, in the form of very minute sporules, is unquestioned, and that their growth is dependent upon certain atmospheric conditions, is equally admitted, but whether they induce disease, or are only able to take possession of a plant that is not in a perfectly healthy condition, does not yet appear so clear. The very eminent Mr. Solly is of the opinion, that in the potato at least, the existence of parasitic fungi is a secondary result of previous disease. So it may be with our fruits, and there is considerable testimony to favor such a belief in many cases, where we find, with the appearance of these fungi, other causes of unhealthiness.

The leaves of the apple trees in some seasons become coated with a black efflorescence, that gives the tree a very sombre appearance, and seems to affect its health. I am not aware that any one has yet made any microscopical investigations of this condition of the foliage, which looks as though it were dusted with coal-smoke. It has been supposed, however, to be the result of a fungous growth.

Pear Blight.—This is a subject upon which so much has been said and written, that any one may well shrink from its discussion. The condition in which the invasion of the malady finds the tree has been pretty thoroughly ascertained, and the sad state in which it is left after the attack, is too well known to need any learned description. It is well called the blight, for nothing short of scorching by fire can more effectually destroy the life of the tree and blight our hopes of its usefulness. The varied theories and suggestions that have been advanced in attempted explanation of this state of things are altogether unsatisfactory so that it may be said we know nothing about the disease, nor whether it be occasioned by frozen sap, by fungous invasion, or by insect attacks, all of which have been set forward as causes of the difficulty. None of these explanations have been clearly proved, and they seem rather guesses than established facts in the history of the disease, which breaks out in the midst of the season of growth, and attacks those trees that are in the midst of the most vigorous production of succulent shoots; but it is not confined to the young wood; on the contrary, it appears first in the hard bark of limbs, that are two or more years old. This turns brown, becomes desiccated, and thus the circulation is arrested, and the foliage as well as the bark is affected. The outer extremities of the leaves wilt, die, and turn suddenly brown and then black, and often remain adhering by their petioles for months—sad testimonials of the destruction caused by the blight. The disease appears to extend in some instances, but it is not proved that there is any poisonous matter generated by a blighted limb that could have entered the circulation, and then have been transmitted to other parts of the tree. The apparent extension of the disease is rather believed to have been the successive development of the trouble from different foci, which had successively invaded so much of the bark as to have more or less completely arrested the flow of the sap. In some limbs of small size, a patch of dead tissue of moderate dimensions would entirely arrest healthy action early in the season, and destroy the portion of the branch beyond it; in other branches of greater size, quite a large patch of the dead bark might exist for a long time without entirely surrounding them, and arresting the circulation, which would thus be kept up until a later period, when at length this occurred, the symptoms of blight would appear.