Popular Science Monthly/Volume 25/July 1884/Diseases of Plants
By D. P. PENHALLOW.
LECTURER IN BOTANY, MCGILL UNIVERSITY, MONTREAL.
STUDIES in vegetable pathology are by no means a recent development of science. So long ago as 1795, Schreger issued a work treating of the various diseases then known, the work being in reality a compilation of the literature of the subject, which, up to that time, had been very much scattered. Then came a rather wide gap, until in 1833 linger issued his work entitled "Die Exantheme der Pflanzen und einige mit diesen verwandte Krankheiten der Gewächse." From that time until the present we find the well-known names of Meyen, De Bary, Sorauer, Hartig, Frank, and others linked with a tolerably copious literature on this subject. We find the Germans among the first, if not the very first, to recognize the desirability of pursuing questions of this kind from a scientific stand-point, though, aside from purely scientific considerations, these questions were forced upon the general attention of the country from an economical stand-point. It was recognized that the important interests involved in forest-growths were liable to be seriously impaired through the operation of disease, and that, even were this not the case, the interests involved could be most fully protected by the development of that knowledge which should secure the best oversight and care of forests in all respects. A wise policy, therefore, dictated the establishment of forestry stations, the duties of which included a study of the various diseases affecting trees.
In America hardly a serious thought has yet been given to such considerations, so far as they extend to the protection and preservation of our forests; but it seems probable that the movement to protect our forests from ruthless destruction at the hands of man, which is each year assuming more tangible shape, must ultimately embrace also an effort to have our trees studied according to strict scientific methods, for the purpose of determining their relation to disease and protecting them from injury. But, while we find the question unconsidered from the exact stand-point which first developed in Germany, we do find that it has been forced upon our attention in another and, for those immediately interested, more unpleasant way.
For the last hundred years or more, under the influence of the peculiar methods of cultivation which have been employed by our fruit-growers, various diseases have appeared from time to time in several of our important fruits, and to such an extent have some of them developed within the last ten or fifteen years that they have completely destroyed the fruit industry in some sections, and now threaten a more general annihilation of one of the most enticing and profitable occupations for the farmer. For the last hundred years we have heard of the "blight" in pear-trees, and the best records show unmistakably that the disease has been on the increase during that period. So badly is it developed in some fruit sections, as through Southern New York, that it is a matter of extreme difficulty to find a really healthy tree. For the last eighty years we have also heard of the "yellows" in peaches, and here again we find that history records a constant development of the affliction. So serious have its ravages proved that whole sections have been deprived of the very important industry of peach-culture. Not only this, but the disease is now so thoroughly established, and has come to be so much a matter of inheritance, that the life of the tree is greatly modified and even determined by it. The peach is naturally a long-lived tree, instances brought to my notice showing that it may live for upward of one hundred or more years, and, if well cared for, it will certainly produce fruit for a long period. At the present time, however, as in the great peach districts of Delaware and New Jersey, we find that, owing to the certainty of disease appearing, or the inherently weak constitution resulting from its previous operation, the period of a profitable life is limited to nine years, at the end of which time the trees are rooted out of the soil as worthless.
Twenty-five years ago the Hudson River Antwerp came into cultivation in Southern New York, and for a long time was a famous berry, and made money for those who cultivated it. Within a few years a disease has appeared, and to-day it is considered worthless to the fruit-grower. And so it is with others of our important fruits. Diseases are yearly becoming a more and more familiar foe for the horticulturist to deal with, and a great deal of alarm is felt, and with reason, lest the fruit industry in some directions be completely destroyed. Thus it is that within a few years it has become imperative that something be done, looking to the acquisition of facts which will enable us to successfully cope with these disorders, to ward them off or arrest their progress. Fortunately, the question is an important one, and so is yearly claiming more careful attention from scientific men.
Diseases arise from such a variety of causes, and are so various in their effects, that we can not judge them all from the few given as illustrations. Moreover, there seems to be such an inadequate conception of the nature and extent of these disorders in plants, as well as of their influence, that it seems desirable to present a general outline of the subject according to our present knowledge. We will, therefore, pass over a special consideration of the various scientific investigations of the last few years, and deal with the conditions under which the diseases develop, as well as some of the most important results of their action. First, let us briefly consider what constitutes a disease.
In the animal the system is considered diseased when the functions of the body cease to be performed in a normal manner, and the disease is more or less serious, according to whether one or more functions are involved, as well as the degree to which the impairment of a particular function is carried. In instituting pathological comparisons between animals and plants, we have to keep in mind that there are important structural differences and physical peculiarities which may favor the development of disease more in one case than the other. In animals the mass of their structure is composed of highly vitalized and actively growing cells and tissues; while in the higher plants, where the differentiation of structure is carried to a high degree, there is a very considerable portion of the body which has become incapable of further growth, and is virtually dead. In many cases, as in trees, the permanent structure predominates, and the vitalized tissues are relatively few. Furthermore, through a delicate nervous system which penetrates the most remote parts of the body, the whole animal organism is brought into more or less active sympathy with the diseased portion, even though the disorder be one of a strictly local nature, while the blood as a general medium of circulation tends to distribute the affection and thus bring the entire system into a diseased condition. In the vegetable kingdom we find no fluid which would be strictly equivalent to the blood of animals and capable of disseminating disease through the organism in a similar manner. Recent researches by Hillhouse, however, seem to strongly confirm the previous observations of Gardiner, Strasburger, Frommann, and others, that there is a well-defined continuity of the protoplasmic substance between adjoining cells through their walls, thus rendering it highly probable that, in plants, the protoplasm may act in a manner similar to the nerves of animals to bring somewhat remote parts into more or less active sympathy, and this consideration must have weight in the future, as giving us a more correct insight into the operation of disease and the possibilities of its complication.
If we clearly recognize that the physical basis of life is the same in both plant and animal, and that it is through disturbance, primarily, of the protoplasmic functions that the functions of the organism as a whole are disordered, then from this and what has already been stated it becomes evident that the pathology of plants and animals is the same, chiefly involving greater degrees of complexity in the latter, and that we must apply the same principles for the recognition of disease in each case. This view was expressed so long ago as 1846, and receives confirmation in the expressions of some of our best pathologists of the present day. Frank tells us that "disease is every deviation from the normal condition of the species"; while Sorauer says, "We must recognize as a disease every disturbance of the organism which detracts from the final end of its labor, the accomplishment of its purpose."
In considering the diseases of plants, it is important to bear in mind that we have to deal with subjects which on the one hand are cultivated, and on the other hand not. In forest-trees there has been no modification through cultivation, and disease would not be likely to become complicated from this cause. In cultivated trees, and plants, as in the peach, pear, strawberry, raspberry, etc., a high degree of cultivation has resulted in a corresponding modification upon which the pecuniary value directly depends. This strong divergence from the original type involves a debility in one or more directions, and is quite parallel with the changes known to occur in more highly civilized communities of men, by reason of which diseases are not only likely to be more prevalent but more complicated. This analogy, as well as general principles, would show us that the more highly cultivated the varieties of fruits or plants, the more susceptible are they to the influence of environment with the introduction of disease, and this is confirmed, not only by personal observation, but by the experience of practical fruit-growers.
Again, cultivated fruits always tend to revert to the original form when the conditions of their high state of development are withdrawn. Moreover, such organs often show that this excessive development has obliterated, wholly or in part, those important functions connected with the reproductive processes which they were originally designed to fulfill. These are some of the evidences that all such monstrosities as our modern apples, pears, strawberries—in short, all our cultivated fruits—are in reality abnormal growths which we may designate as hypertrophied structures, and are therefore evidences of disease. In such cases, therefore, the questions of treatment are likely to become somewhat complicated, since, while maintaining a certain form of disease, we must exclude, prevent, and cure all others.
Diseases may be general in the system, or they may be localized, and this is a consideration of obvious importance when we bear in mind that, according as they are one or the other, they may be more or less destructive in their effects or be controlled with greater or less difficulty. When a disease involves the entire system, as in peach-yellows or pear-blight, it is often a matter of great difficulty to determine the controlling treatment; but in other cases, where the disorder is strictly of a local character, it may be a simple matter to remedy the trouble. In the case of those peculiar developments of the oak which give us the gall-nuts of commerce, or of similar abnormal developments in the tissues, we have instances of well-defined disease, but it is of a strictly local nature; the disturbance of functional activity does not extend beyond very narrow limits. It becomes, then, a simple matter to treat the case, because the part may be removed without inflicting injury upon other organs of the plant, and thus the knife is the sure remedy. Or, again, certain diseases may originate in the breaking of a limb or the fracture of a surface tissue. In such cases the disease will follow the injury and progress slowly, but it is often a simple and easy matter to prevent its introduction into the general system by properly caring for the wounded part in the first instance. Nature herself provides the means of warding off disease in just this way, and within certain limits her provisions are most effective. If a structure such as a vigorously-growing plant be injured, there at once appears a clear fluid, which gradually thickens into a mucilaginous substance, and finally becomes dry and hard. Under its early protection, a tissue of cork is formed over the wound as a healing and protective structure, impervious to air or water. Under it, the injured parts, now excluded from the air, are able to perfect the healing process by the formation of new tissue.
In plants, as in animals, diseases may be developed through a great variety of causes, but it is possible to bring them into a rude system of classification by means of which their consideration is greatly facilitated. The best arrangement of the kind which we have at present, one which answers very well, is that of Hartig, according to which diseases are developed through the action of—
1. Phenogamic plants.
Under the first head we have to deal with those plants, like the mistletoe and dodder, which grow upon others and draw their nourishment directly from them—hence are truly parasitic. Plants of this kind may contain a certain amount of chlorophyl, but usually possess no true roots; hence they are not only incapable of drawing nourishment directly from the soil, but they are also incapable of performing the assimilative functions by which materials for the formation of cellular structure are developed, in more than a limited manner if at all. Such plants, therefore, must depend entirely upon the already elaborated sap contained in their hosts, and, feeding exclusively upon this, the latter must suffer in a degree which is proportional to the development of the parasite. The tissues in which the latter feeds must thus become diseased, primarily through lack of nutrition, and so finally develop in an abnormal manner, as is seen to be the case in the often enormous knots which accompany the growth of the mistletoe upon the oak. Such excrescences often reach a diameter of three or more feet. A secondary feature of such diseases is then developed in the readiness with which such hypertrophies often yield to decay, or in the decay which is introduced into the various tissues of the host wherever the parasite penetrates. It is evident that diseases of this character may be, and usually are, of a strictly local nature, and, in the early stages at least, it is easy to remove both the disease and the cause by amputation. When local action has been long continued, however, the highly morbid condition of a limited portion of tissue may in time find sympathy in adjoining parts, and so by degrees the whole system become involved in a chronic disorder. We may thus remove the cause, but additional treatment will be essential to restore the system to its normal condition.
In the second class of causes we have the cryptogamic parasites, or, more properly, the saprophytes, to contend with. These plants, like the parasites proper, are incapable of providing their own nourishment from the soil and air, and so must depend for their growth upon already-formed organic matter. But this is not all: it is characteristic of their growth that they live upon organic matter which is in an active state of decomposition, and it will thus be easy to see that they are not far removed from being the cause of the decomposition in bodies which have already ceased to live. In their action upon non-vitalized matter, it is quite possible that they are the active promoters of disorganization; but the case is somewhat different with the living organism. Here the growth of the saprophyte has to contend with the vitality of the host, and, so long as this latter is normally maintained, it is most probable that the intruder will fail to gain sufficient hold to exert any appreciable injury. But the struggle continues, and if, by reason of accident or peculiar conditions of environment, the vitality of the host be reduced below certain limits, then the saprophyte or parasite, as the case may be, at once exerts a preponderating influence which is highly deleterious. Or, again, if the plant be diseased through the operation of other causes, then the fungus can exert its influence to produce secondary features of an already disordered condition. These views find confirmation in the general action of fungi upon tissues. It is observed that they are more or less abundant in the rough outer bark and on the surface of most plants; but, though they are present, their growth is limited, and confined to those tissues which are either dead or of very low vitality, while the plant suffers in no wise from their presence. Let the plant be injured or diseased, however, and at once the parasite gains a firmer hold, the tenacity of which will increase continually until remedial measures are applied. Thus, we often find the breakage of a limb to be but the open door by which rot is introduced into the interior tissues. We may consider, however, that with most of the mycelial forms of fungi their action is more or less localized, as in the smut of corn (Ustilago maydis), or the disease called cedar-apples (Gymnosporangium Sabinæ), or the curl of the peach-leaf (Exoascus deformans). So far as they are localized, therefore, their treatment is a simple matter, since it only involves cautery or removal of the affected part. Owing, however, to their peculiar habits of growth, and the insidious rapidity with which the spores may be disseminated, they may cause a disease of the general system when the conditions of the latter are favorable. But it is not such an easy matter to dispose of all these organisms. In the animal, it is now well demonstrated that disease may be directly produced by the action of certain schizomycetes, such as the micrococci and allied germs; and it is even claimed by some that they have a corresponding pathogenic function in the vegetable organism. These latter views, however, rest upon insufficient evidence at present; but, in considering certain diseases of plants at least, analogy would dictate measures of caution in formulating an opinion which wholly disregards the importance of these minute structures as pathogenic agents. Whether actually the cause of disease, or only of secondary features, in either case they are most difficult elements to deal with.
The third class embraces a variety of causes which may be directly controllable by man or not. Injuries may be inflicted by insects, as so generally occurs in the formation of galls upon leaves; in the punctures which various boring insects, as the scolytus and ægeria make for the deposition of their eggs; and, more especially, as in the subsequent action of the larvæ. There are, also, injuries which may be inflicted by animals and man, either by accident or design, and which permit the operation of fungoid growths with the development of secondary features. All these are of a strictly local nature, and the question whether or not the entire system will be involved in disorder must largely depend upon the extent and nature of the injury in the first instance.
The treatment may or may not be difficult. Where insect action is strictly local, as in galls, the amputation of the parts is sufficient; but, where the injury is inflicted by boring larvæ, the grub must first be destroyed, and this requires certain knowledge of the habits of these insects in the different stages of development. In the case of the scolytid borers the treatment is especially difficult, as the beetles are very small, and hard to destroy; but it is an interesting fact that the ovipositing of these insects is in itself indicative of an already diseased condition, so that the surest and best remedy is a complete destruction of the plant or tree, together with the borers, by fire.
Where injuries are inflicted by man, proper attention in caring for the injured part will prevent the introduction of disease. Nature provides means for the healing of injuries produced in this way, and in many cases it is possible for very extensive injuries to be healed without any aid beyond Nature's own efforts. Grape-vines and other vigorously growing plants often exhibit a most remarkable recuperative power. One of the most notable instances of this kind was brought to my attention in 1874. During the early spring the bark of a weeping-willow was removed from the base of the trunk, making a complete girdle for a distance of eighteen inches from the ground. In some places the cambium tissue was not fully destroyed, and this materially aided in the healing process. From the upper part of the girdle, or, more properly, from the lower portion of the uninjured bark, a new growth was rapidly formed and pushed downward, soon taking the form of aerial roots. In one or two instances these became more or less connected with the trunk over the girdled portion, but most of them remained distinct, and all finally penetrated the soil, with which they established a normal connection. In another instance, when removing some young squashes from vines under experiment, the former were separated by a knife, but left in place for collection at a later time. One, however, was overlooked at the time of collection, and, when the final harvest was made, it was discovered firmly united to the stem from which it was originally separated, and had attained considerable size. Upon careful examination of the parts, both externally and under the microscope, it appeared that—1. When the cut was made the squash was not displaced, and the cut surfaces immediately came together again. 2. As determined by a "fault" in a crack of the epidermis, the squash rotated in position as the cut was made, thus accomplishing a displacement of nearly one quarter of an inch on the surface of a stem three quarters of an inch in diameter. 3. The healing was complete in the interior of the stem, but the line of section was plainly visible under the microscope. 4. The union of the epidermis and tissues immediately below was not accomplished, and there was thus left, by shrinkage of the parts, a groove which extended completely around the stem and demonstrated the completeness of the section in the first instance.
In these examples, therefore, we have illustrations of the inherent tendency of all plants to overcome disease and injury through the operation of vitality.
In soil influences we have to contend with conditions which are not always so easy to control, and, when once they have produced their effect upon the plant, the diseased condition is a somewhat difficult matter to correctly diagnose and treat. Soil influences operate in a variety of ways; it may be through excessive humidity, as determined by the stagnation of water through imperfect drainage, or the natural condition and position of the water-table, or it may be through the mechanical condition. While these conditions may not actually cause disease, they will certainly promote it when once developed, and we therefore find a certain part of remedial measures to consist in thorough drainage and cultivation. But more than this, we find in special or general exhaustion of the soil a fruitful source of disease. Lands which have been cropped for a long period become at least specially exhausted, and in such case usually in the direction of that food-element most essential to the growth of the plant which has brought about the exhaustion. There is thus developed a debilitated condition of the entire system, by means of which the normal functions are impaired, and this in itself constitutes a disease. But the debilitated state permits the operation of other forms of plant-life which would otherwise be unable to develop readily, and also allows certain abnormal physiological and chemical changes to occur, all of which promote secondary features and thus bring about complication. This, it seems tolerably certain, is the case in peach-yellows, and may also prove to be the case in other diseases such as pear-blight. Diseases developed in this way, however, are most difficult to treat, because the entire system is involved. Remedial measures must therefore be directed toward—1. Removing the cause; 2. Building up the general system; 3. Restoring to a normal condition the disordered organic function. In the case of peach-yellows, the results of chemical analysis, as well as the changes produced by special treatment, show that in all probability the specific is chlorine as contained in muriate of potash, while a general toning of the system may be accomplished by the judicious application of a complete food as determined from the ash composition.
Atmospheric conditions are largely, if not wholly, beyond the control of man. They include, of course, the varying conditions of heat and moisture, and are thus either highly stimulating and favor the excessive growth of weak structure and parasites, on the one hand, or they are depressing and cause a stagnation of vital activity, and thus injure the plants, as through excessive drought; while this, in turn, leads to the development of parasites, which would not otherwise gain a firm hold. We can not expect to modify the conditions which produce these results; we can only hope to so prepare the plant, by judicious treatment, that it will suffer no material injury from the peculiar meteorological conditions in which it is placed. With this in view, we would doubtless find it wise to apply strong food, which will retard the vegetative process, and tend to the more solid maturity of the parts already formed. Nor must we neglect the importance of a judicious course of irrigation during drought. Doubtless the time will come when every man who depends upon the growth of plants for his living will recognize a well-devised system of irrigation, which may be applied to all his orchards and cultivated fields when necessary, as an indispensable part of the machinery which a successful business demands.
The conditions which produce disease in plants, as well as the direct and secondary effects of their operation, are likely to be more or less complicated, and thus render a direct course of diagnosis and treatment correspondingly hard to reach and apply; but we can hardly form a correct estimate of these difficulties by analogy with a disordered condition of the animal. We have, at the outset, structures of widely different organization, which not only depend upon very different conditions of nutrition, but which are placed in widely different conditions of environment other than this. On the one hand, we have forms which, once developed, occupy a definite position, and their relations to environment—soil conditions, food-supply, etc.—are in a measure fixed. On the other hand, we have more highly organized bodies, which are continually changing their location, and they are thus brought into new relationships, to which they must adapt themselves, and this is liable to complicate the phases of disease already present. I think it will appear, however, that—at least in many cases, especially where nutrition is chiefly involved—we must apply the same general principles in the one case as in the other.
It was shown, not long since, by my friend Dr. Goessman, that in certain cases of disease the normal and abnormal conditions are correlated to the presence of relatively greater and less quantities of certain food-elements. This was demonstrated by chemical analysis of the diseased wood or fruit, the naturally healthy structure, and, again, he diseased structure after being restored by a course of treatment which involved an application of the elements supposed to be wanting. In the case of the peach-yellows, concerning which we have the fullest data, he found the potash to increase in the healthy and decrease in the diseased; while the lime decreased in the healthy and increased in the diseased; and furthermore that, under treatment, the appearance of greater or less quantities of potash was reciprocal with similar changes in the lime present. The following analyses will show this relation:
CRAWFORD'S EARLY PEACH.
It has further been shown by myself that in some cases of disease, notably the one just referred to, there are important modifications of cellular structure and cell-contents as typical of the pathological condition. The growth becomes depauperate as a whole, and also in the various anatomical elements of the structure. At the same time the foliage assumes an abnormal color, and fails to perform its functions in the assimilative process. Yet, again, with reference to the storage of assimilated material, there is often an abnormal accumulation of such elaborated food in parts where it should not appear, except in limited quantity.
All these conditions in the case of peach-yellows are coincident with the development of the peculiar chemical conditions as noted above; and it is further a most interesting fact that, while the excess of lime and want of potash occur together with depauperate structure, loss of color in the foliage, and excessive storage of starch, an increase of the potash and decrease of lime occur simultaneously with a disappearance of these various abnormal conditions.
We are now led to inquire as to the proper course to pursue in making a diagnosis.
When the disease is strictly localized, as when produced by injuries, or by the action of parasites proper; when the local disturbance is of sufficiently recent origin to render it improbable that the general system has become involved—then the diagnosis is in most cases a simple matter, and chiefly involves the correct recognition of the cause of the disturbance—i.e., the name and character of the insect or parasite, or the particular means by which the injury was first inflicted.
When the disease involves the entire system, and the conditions become more complicated, then the difficulty increases. A correct and complete diagnosis can then be made only when we consider—
1. The chemical composition in health and disease.
(b) Color and general condition of the bark.
We can not hope to correctly determine the nature of diseases by seeking new light upon strictly botanical grounds alone—e. g., by assuming that they originate more or less directly in fungoid growths. Nor can we hope to get at the origin and cure of these disorders from a purely chemical stand-point. The two lines of inquiry must be followed together until they merge in one harmonious result. In such manner alone may we hope in the future to solve the difficult problems now awaiting the patient student, to whom they will bring abundant reward.
These thoughts are offered as a mere outline of the direction which such considerations in vegetable pathology are now taking, and of the form they have already assumed.