2d--Those produced by poisonous agencies, as by injurious gases, miasmata in the air, or by poisons in the soil.

3d--Those arising from the growth of parasitic plants, such as the various Fungi, Dodder, Mistletoe, etc.

4th--Such as are caused by mechanical injuries or wounds, and by the attacks of insects.

These may be considered separately: 1st--It may be a.s.sumed, and has been already well established by botanists, that every plant has its own peculiar const.i.tution, adapting it to certain atmospheric conditions, and that for its healthful and successful culture, these must be understood and adhered to, within comparatively narrow limits.

Tropical plants, as is well known, cannot be cultivated beyond their natural limits, except under circ.u.mstances where their natural conditions are nearly imitated by the gardener; and even in our stoves and hot-houses, these plants do not compare in vigor with their fellows that luxuriate in the hot and steaming atmosphere of the tropics, under the stronger light of such a clime as is natural to them. On the contrary, the plants of northern lat.i.tudes will not grow and produce seeds where temperature is too elevated. Those from a humid atmosphere suffer in an arid clime, and those which thrive in dry sandy regions suffer equally when introduced into a humid atmosphere.

Thus we find, that where there is too much moisture for some of our cultivated plants, they are inclined to be too succulent, and this very excess may produce a dropsical condition that is really a state of disease. Thus we suffer in a loss of fruit, which will fall badly before its period of maturity, and that which remains its full time is found to be thin and watery, deficient in the high spicy aromatic flavor which is so highly appreciated by the connoisseur of these choice products. When, on the other hand, the arid character of the soil and climate prevail to an extent that is uncongenial to any particular fruit, we shall find that its growth is arrested, and that its highest qualities are not adequately developed: this is frequently observed in an unusually dry season--and in California, where irrigation is required to enable the orchardist to produce some of the succulent fruits, the most remarkable size and beauty have been attained, but we are told that it was often at the expense of the desired flavor that the same varieties acquire, under circ.u.mstances more advantageous to the development of their superior qualities.

So in many of our fruits, the successful results depend upon the hygrometric condition of the atmosphere, and Liebig suggests that a very prolific source of diseased action in plants, arises from the suppressed evaporation and transpiration consequent upon such atmospheric conditions.

Too much moisture prevailing at the time of the blossoming of our fruits, especially moisture precipitated in the form of rain during this period, is sometimes disastrous to our crops, both of cereals and of orchard fruits. Continuous showers prevent the development of the pollen-grains, and their transfer to the stigmas of the blossoms, so that the fruit does not set well. Fortunately this does not often occur in our glorious climate, which is so highly favored by an abundance of light and sunshine, which are the great and essential stimuli of the higher orders of plants. The loss of our fruit crops in some parts of the Ohio Valley in the years 1862, 1865, and 1866, was fairly attributed to this cause.

We must not overlook the unhealthy influences produced by an excess of moisture in the earth. Many plants that naturally delight in a dry porous soil, become weak, unfruitful, or even seriously diseased when they are planted in low wet grounds, or upon such as are underlaid by a very tenacious sub-soil, while an opposite condition is equally unfavorably to those that are naturally more aquatic in their tastes and habits. In the former case we learn to avoid such soils and situations, unless we are able to change their character in this respect by thorough under-draining, which will completely remove the evil, and the remedy becomes merely a question of expense.

A certain amount of temperature may be a.s.sumed as requisite to every plant, or rather it may be affirmed that some plants cannot exist and thrive except within a certain range, and it has been a.s.serted that each cla.s.s of plants requires a mean temperature for the year that shall not vary many degrees: the range of this variation has perhaps never been satisfactorily ascertained. But it is well known, that both heat and frost act injuriously upon vegetation. Mr. Lindley tells us that "the extreme limits of temperature which vegetables are capable of bearing, without destruction of their vitality, have not been determined with precision." When the temperature is maintained at a higher point than is natural, the plant is excited to undue activity of growth; but this is attended with an enfeebled condition, often seen in badly managed green-houses. Mr. Knight found that certain plants were rendered abortive by the production of male flowers only, when exposed to too great heat, and by an opposite treatment, when subjected to a low temperature for a long time, others produced only female flowers. In some plants a high degree of heat, with moisture, results in the production of leaves only, and Humboldt found that wheat was grown about Xalapa, Mexico, as a fodder plant, because it produced an abundance of gra.s.s, but did not form ears nor grain.

A diminished temperature, on the contrary, removes the stimulus of growth, and leads to the suspension of all vital action in proportion to its reduction. At the freezing point it is probable that all such action ceases, though in this regard there is great difference among plants; the mosses and lichens will flourish, and the Chickweed will vegetate and blossom at a temperature very little above freezing. The access of frost, after vegetation has somewhat advanced, often proves very disastrous, and we not unfrequently lose our crops of fruit by such an occurrence during the period of blossoming, or even afterward.

Some plants in a dormant condition, will endure uninjured a great depression of temperature, while others will be destroyed by the slightest approach of frost. According to De Candolle, this may depend upon the greater or less amount of water they contain, upon the greater or less viscidity of their fluids, or the rapidity with which these fluids circulate. Those with larger cells he thinks most easily injured by frost, and those which contain a great deal of air are able to resist it best. The freezing point will vary according to the quality of the sap, for we know that different vegetable juices congeal at different temperatures. The manner in which cold acts upon plants depends upon their physical structure. Lindley says, freezing is attended with the following effects:--The fluids contained within the cells of tissue are congealed and expanded--this produces a laceration of the cell-walls, and impairs excitability by the unnatural extension to which the cells are subjected; the air is expelled from the air-vessels and introduced into parts naturally intended to contain only fluid; the green coloring matter and other secretions are decomposed, and the vital fluid or latex is destroyed, and the action of its vessels is paralyzed. The interior of the tubes, in which fluid is conveyed, is obstructed by a thickening of their sides. So we have as a result, both mechanical, chemical, and vital changes.[16]

Our hardy fruit trees are woody perennials that hybernate during the winter. Yet we find that even these suffer upon some occasions from a great depression of temperature; it has been a.s.serted that a certain degree of cold would inevitably destroy the blossom buds at least, and we often find that the bark is burst off from the wood, and in some instances the wood itself is so injured as to suffer from a kind of decomposition, and to become affected with a change generally known as the dry rot, losing its elasticity and hardness, and acquiring a whitish color, which is supposed may arise from the introduction of the mycelia of fungous growths. Now it is believed that these injuries do not arise so directly from the degree of cold to which the tree has been exposed, as to the condition of its circulation at the time of the exposure. If the sap have been excited by mild or warm weather, as is so apt to be the case in our changeable climate, the sudden depression of temperature will produce disastrous effects, even when the cold has not been very severe. This is manifested by the bursting of the bark in young trees in the early part of winter, while they are yet holding their leaves, and of course having a circulation somewhat active. Hence the importance, now very well understood by our nurserymen, of checking the growth of young trees in time to have their terminal buds thoroughly ripened before the approach of frost.

This, to a certain extent, is subject to our control; but we cannot foresee the character of the seasons upon which the safety of our orchard trees will, in a great measure, depend, and they are less easily managed. When the autumn is dry, and continued late into winter, as sometimes happens, we see a perfect ripening of the wood, with a great development of blossom buds, and then we may confidently calculate upon the safety of our fruits, provided they be not exposed to a warm period at mid-winter, that shall excite some activity in their circulation, which would suffer terribly from any sudden and great depression of temperature such as frequently occurs, carrying the mercury from summer heat to a point below zero, in a few hours.

Such a change has amounted to 68 degrees in nine hours.[17]

The influence exerted by the soil upon the healthiness or unhealthiness of our trees has already been alluded to incidentally, but it is an important subject of inquiry whether this may arise from a redundancy or a paucity of some particular ingredients necessary to sustain the plants we desire to cultivate. Liebig has pointed out how chemistry may be brought to our a.s.sistance in solving such a question.

As all the inorganic elements found in a tree and its fruits, must have been derived from the soil in which it grew, he suggested that the ashes of the plant would show us exactly what it needed, and then an examination of the soils would inform us whether they contained all the necessary elements, and in the right proportion. Hence arose the doctrine and the practice of applying special manures, which has been so fashionable in our day. Though there be many doubters as to the efficacy of such investigations and practices, most sensible and enlightened agriculturists admit the truths which Liebig has propounded.

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 subst.i.tute 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 tr.i.m.m.i.n.g. 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 cla.s.s 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 quant.i.ties 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.

The treatment of this malady is quite unsatisfactory, and gives us no clue to the cause of the trouble. Various plans have been suggested, the most satisfactory is the removal of the affected limbs--not that it cures the disease, but because it takes from us the sad mementos of our loss. We have been advised to pare away the diseased portion of old bark with a spoke-shave, or some similar instrument; but it is apprehended that few persons would ever find this patch of dead bark until they have the fatal evidence of the blighted foliage, and no possible good can result from its removal at that time.

This trouble is connected, in many instances, with an excessively vigorous growth of shoots; indeed, some of those varieties which are most thrifty, suffer the most, while those which make firm and moderately short shoots, seldom blight. Hence it has been inferred by some, that if we can check this excessive vigor, and reduce the wood growth to a moderate amount, not exceeding ten or twelve inches, annual extension, we shall be able to prevent the occurrence of blight. This object is easily attained by root-pruning the trees severely in the spring of the year. So far, we can only say that trees so treated, have not blighted; but it does not follow that they would have suffered if let alone.

Another form of blight may often be seen in the peach and in the apple; it consists in a loss of vitality of small twigs and their foliage in several parts of the tree, especially in the inner portions that are not freely exposed to the air and light. In the peach, this disease is accompanied with the decay of the fruit upon these twigs, which rots and becomes moldy. This trouble is usually attributed to mildew, and it is probably owing to some form of fungus invasion.

Quite a different affection of the twigs is that known as the "blight"

in apples and quinces. This attacks only the young shoots of the current season's growth, which suddenly wither and become brown at mid-summer. The same condition occurs also in the shoots of the Italian mulberry. The cause of this malady is not very apparent; by some persons it is attributed to the punctures of minute insects, but they have escaped the scrutiny of other observers, who attribute this blight to atmospheric causes.

The true apple blight is a malady of very serious character, that invades many orchards in the Western States. In its nature, and in the mode of its invasion, it very much resembles the dreaded fire-blight of the pear, with which most orchardists have unfortunately become already but too familiar. Like it too, all the guesses which Solons have offered for the explanation of its cause, appear equally unsatisfactory.

A whole branch or limb of the tree becomes simultaneously affected; sometimes one quarter or even one half of the top is destroyed by the disease, and the removal of the dead portions is not followed by the reproduction of healthy branches. Certain varieties are more subject to this blight than others, and they seem to poison the grafts that are inserted into them, to produce a new top to the tree with a more healthy variety.

BITTER ROT.--Our excellent and observing friend, H.N. Gillett, of Lawrence Co., Ohio, furnishes the following description of this disease to the Ohio Cultivator:

"The disease generally presents itself on the skin of the apple in very minute brown spots, from one to a dozen or more in number, generally after the fruit is pretty well grown. These gradually spread and penetrate the flesh of the apple, producing a black rot, almost as bitter as aloes, but this taste is confined to the discolored portion.

The fruit ceases growing, and falls prematurely. The rot occasionally begins at the center, and extends outward, so that the fruit appears perfectly sound for some time," on which account he advises against too early gathering of the fruit.

The late Dr. Barker, of McConnellsville, Ohio, who was one of our most observing pomologists, referring to this disease as peculiar to certain varieties, concludes in an article in the paper above quoted, vide Vol. VI., p. 283, that this malady is different from what is called Bitter Rot in other places, and which affects other varieties with a discoloration of the flesh and a bitter taste. He thinks this malady is different from that described by Mr. Gillett, and that it, the true Bitter Rot, is caused by a fungous growth, the spores of which are carried on the air from tree to tree, like a similar fungus producing mold in the cherry, plum, and peach. He also traces a resemblance of this disease to the vaccination in the human subject, except that the scab does not separate and fall off. Hence he suggested the name of _pock_, instead of _Bitter Rot_. High culture, manure, lime, tr.i.m.m.i.n.g, and pasturing hogs in the orchard, have all been recommended as remedies.

CRACKED FRUIT--MILDEW.--Certain fruits become partially covered with what appears to be a fungous growth, which occupies the skin in such a manner as to prevent the development of the succulent tissues beneath it. This may result in a deformity consequent upon the irregular growth, and the fruit is called scabby, or it may strike deeper into the tissues, which become dry and corky and crack open, being thus utterly worthless. Some varieties, which formerly produced the most beautiful fruits, have been so severely affected by this malady as to yield absolutely nothing in certain localities, and are only rendered profitable by top grafting with other sorts that are not affected with the cracking. That this is not caused by the wearing out of the variety, as has been suggested, it may be added that the same fruit ripens perfectly and is quite fair in other regions of the country.

The trouble, however, is extending, and it is hardly safe to plant largely of those varieties that have proved subject to the malady. No explanation has been satisfactory as to the cause, nor has any treatment been successful.

Dr. Kirtland addressed the Ohio Pomological Society upon this subject, and an abstract of his remarks is here given:--

"The disease known as the blight or the fire-blight, is at this day proving the most serious obstacle to the successful cultivation of the pear, in many sections of the country. Early in the present century it prevailed extensively in New England, coincidently with the spotted fever, and other disorders of a low grade of action, which at that period swept epidemically over that region of the country. It was a popular opinion that all these diseases, both of the human family and vegetable kingdom, arose from one cause;--an opinion not, however, tolerated by medical men and men of science in that day.

"Various theories have been advanced to account for the origin of this blight. Insects, frozen sap, electricity, excessive evaporation, and exhaustion of the soil, have, at different times, been a.s.signed as the cause. Investigation of each fails to meet and explain the phenomena attendant on the rise, progress, and results of that disease. It is time they all should be abandoned, and that researches for a cause be extended in some other direction.

"As a starting point in this undertaking, I will suggest another hypothesis, which may perhaps explain the pathology of the blight, and call into use an effectual remedy or preventive. _Pathology_, Dr.

Webster defines to be 'the doctrine of the causes and nature of diseases.'

"1. The Pear-tree blight is produced by the poisonous impression of the seeds (_sporules_) of a microscopic fungus.

"2. Several combinations of iron, especially the sulphate (_copperas_), will, to some extent, counteract that impression.

"It will be understood that these two propositions are merely hypothetical. If sustained by a.n.a.logies, subsequent observations, and experience, they will be accepted as truths; if not thus sustained, they will of course be rejected.

"The extensive prevalence of the cholera, over large portions of the globe, commencing in the year 1818, led medical men to seek for its cause. Dr. Cowdell, of London, in 1848, published 'A Disquisition on Pestilential Cholera, being an attempt to explain its phenomena, nature, cause, prevention and treatment, by reference to an extrinsic fungous origin.'

"In 1849, Prof. J.K. Mitch.e.l.l, of Philadelphia, issued a more elaborate work, 'On the Cryptogamous Origin of Malarious and Epidemic Fevers.' It abounds in numerous facts and correct reasoning, and should be consulted by every investigator of disease, animal and vegetable.