Disease in Plants

Notes to Chapter IX

The bibliography here quoted will be found in Berkeley, "Vegetable Pathology," Gardener's Chronicle, 1854, p. 4; Plowright, British Uredineæ and Ustilagineæ, 1889; Eriksson and Henning, Die Getreideroste, Stockholm, 1896; De Bary, Comparative Morphology and Biology of the Fungi, etc., 1887; Frank, Die Krankheiten der Pflanzen, 1895-96, and scattered in the works referred to in them and in the text.

CHAPTER X.

HEALTH AND DISEASE

Variation—Disease—Comparison to a top. Health—Extinction of species—Natural demise. Examples of complex interactions in health—Interference, and tendencies to ill-health.

When we come to enquire into the causes of disease, it appears at first an obvious and easy plan to subdivide them into groups of factors which interfere with the normal physiology of the plant. Scientific experience shows, however, that the easy and the obvious are here, as elsewhere in nature, only apparent, for disease, like health, is an extremely complex phenomenon, involving many reactions and interactions between the plant and its environment. If we agree that a living plant in a state of health is not a fixed and unaltering thing, but is ever varying and undergoing adaptive changes as its life works out its labyrinthine course through the vicissitudes of the also ever-varying environment, then we cannot escape the conviction that a diseased plant, so long as it lives, is also varying in response to the environment. The principal difference between the two cases is, that whereas the normal healthy plant varies more or less regularly and rhythmically about a mean, the diseased one is tending to vary too suddenly or too far in some particular directions from the mean; the healthy plant may, for our present purposes, be roughly likened to a properly balanced top spinning regularly and well, whereas the diseased one is lurching here, or wobbling there, to the great danger of its stability. For we must recognise at the outset that disease is but variation in directions dangerous to the life of the plant. Health consists in variation also, but not in such dangerous grooves. That the passage from health to disease is gradual and ill-defined in many cases will readily be seen. In fact we cannot completely define disease. Mere abnormality of form, colour, size, etc., is not necessarily a sign of disease, in the usual sense of the word, otherwise the striking variations of our cultivated plants would suggest gloomy thoughts indeed, whereas we have reason to believe that many cultivated varieties are more healthy—in the sense of resisting dangerous exigencies of the environment—than the stocks they came from. Strictly speaking, no two buds on a fruit-tree are alike, and the shoots they produce vary in position, exposure, number, and vigour of leaves, and so forth. The minute variations here referred to are not seen by the ordinary observer, but those who bud, graft and multiply by cuttings on a large scale know that such bud-variations are important, quite apart from more extensive "sports" which occasionally occur.

On the other hand, we have reason to believe that many species have died out gradually as the environment altered. These plants died because they did not vary sufficiently, or did not vary in the right directions; they became diseased with respect to the then prevailing conditions of normal physiology or health.

Disease, therefore, may be said to be variation of functions in directions, or to extents, which threaten the life of the plant, the normal in all cases being the state of the plant characteristic of the species.

Even now, however, we have not obtained a complete definition, because, since all plants die sooner or later, we have not excluded the natural demise of the individual or its parts, and no one would call the autumnal fall of leaves, or the withering of an annual after flowering, death from disease. Clearly then the idea of disease implies danger of premature death, and probably this is as near as we shall get to a satisfactory definition. Since this matter is of primary importance for our present theme, I will add the following instances for consideration.

A plant in perfect health and in the fullest exercise of all its functions, has its roots in a soil which is suitably warmed and aerated, contains the right quantities of water which dissolve just the proper proportions of all the essential mineral salts, but nothing poisonous, while the soil itself has a texture such that the roots and root-hairs can extend and do their utmost in absorbing.

The leaves above are exposed to just the right intensity of light, in air which is not too dry, and is of suitable temperature and composition, containing no poisonous exhalations, etc.; and as the foliage is gently moved by the breeze, it manufactures carbohydrates at the optimum rate in the chlorophyll, and the so-called "elaborated sap" containing the dissolved organic food-supplies is prepared in the tissues in maximum quantities and of just the right degrees of concentration and quality for use in the buds, stem, roots, etc., for which it is destined as they draw on the supplies.

Between these assimilating organs, the leaves, and the absorbing roots, we have in the stem the wood, with its vessels adapted in quantity and calibre to convey the water containing dissolved salts from the absorbing roots to the leaves (to say nothing of other parts) and, separated from this wood by the cambium, we find the sieve-tubes and cortical tissues in suitable quantity conveying the "elaborated sap"—the solutions of organic food-materials from the leaves down to the roots, up to the buds, and elsewhere. Joining these cortical and wood tissues are adapted series of medullary rays which, apart from other connections, bring about the necessary interchanges of water and "elaborated sap" with the cambium, the formative tissue which has to be fed and served by them, and which by its growth supplies new vessels and sieve-tubes, etc., to carry the continually increasing quantities of water and food substances as the roots and leaves increase in number and area, and thus enables this ideally correlated system to go on working at maximum energy.

Now suppose the same plant with its roots in an unsuitable soil—too dry or too poor in mineral supplies, for instance—the transpiring leaves above cannot obtain sufficient water and salts to supply their needs, but we will suppose hypothetically that they still assimilate under the same ideal conditions as before. The supplies now coming to the cambium are diminished, since the want of water and minerals compels the leaves to put aside any excess of carbohydrates (e.g. as stored starch-grains), and the plastic materials which do pass to the cambium so deficient in water cannot be directly utilised, and a starvation period sets in. Consequently the cambium forms less wood, and this will contain fewer and smaller vessels, and so reduce the conducting passages: fewer sieve-tubes also are constructed, and the paths of the water current and food supplies narrowed, which of course reacts on the tissues everywhere. The reserve substances may slowly be dissolved and distributed, however, and considerable quantities be passed in course of time into the roots, which, as opportunity offers, gradually employ them in making new roots, and if the disturbance has not gone too far and the conditions do not become unfavourable, an increased root-supply may by its larger absorbing area gradually establish the former state of equilibrium of functions. But this at the expense of the plant, which is smaller, has fewer leaves and narrower water channels, etc., than a plant not thus checked, and it may take a long time to make up for the loss of time and stature thus incurred. Indeed if the plant is an annual no recovery at all may occur, the reserves passing into fruit and seeds instead of slowly supplying the roots as described.

If it be asked, can such a condition of affairs as that described really occur, we have only to think of a transplanted specimen with its roots maimed and put into unsuitable soil, or of plants in the open with feeding roots gnawed by an insect, etc., or of a tree hitherto in equilibrium with its fellows in a plantation suddenly set free by thinning and so forth.

Now take the case where the roots are maintaining their maximum functional activity, but the leaves—owing to want of light, too much moisture or too low a temperature of the air—are functionally depressed. Here we get a state of over-saturation with water set up, the tissues are turgid to bursting point, what supplies do traverse the sieve-tubes, cortex, etc., do so slowly and are excessively diluted, and the cambium again forms less wood, but the lumina of the vessels are larger and the lignification less complete. Growth in length is excessive, but more leaves are formed, though they are apt to be abnormally thin and may be small. Little or no reserves are stored anywhere, and the watery tissues contain dangerously diffusible substances which may render them an easy prey to parasitic fungi. Here again, however, if the disturbance of equilibrium has not gone too far, and if the season permits, the new leaves may come into full activity and the situation be saved by transpiration and assimilation gradually increasing and restoring the equilibrium. But, as before, the plant has suffered, and shows the effect in its weak shoots, retarded flowering, and other ways.

Such plight as is here described may actually be attained in greenhouses where over-watering is the fault, and even in the open it is not uncommon in rainy summers, or in plantations where dominant trees get the upper hand and partially shade more slowly growing species, or in fields where rank grass is allowed to overwhelm crops of lower stature.

Now it will be evident that either of these typical cases of temporary disturbance of functional equilibrium may be carried too far: in the first case the plant may wilt and wither, in the second it may rupture and rot, to take these eventualities only. And yet it is difficult to call these indispositions diseases: they are rather examples of extreme departures from the normal standard of health, just on the borderland between health and disease. A step further, as it were, and disease supervenes: certain tissues die from want of water, and a necrotic area is formed, or the cortex bursts and a wound is formed in another way, or some fungus gets a hold, and so on. These abnormal states are particularly apt to predispose the plant to disease—insects revel in such semi-wilted leaves and shoots crammed with reserves, and fungi in the water-logged leaves of the second case, while a cold dry wind is peculiarly fatal to such tissues.

Notes to Chapter X

The reader may consult Hartig, Diseases of Trees, Eng. ed., 1894, Introduction; Sorauer, Pflanzen Krankheiten, pp. 1-12, and Frank, Die Krankheiten der Pflanzen, B. 1, p. 5, for definitions of disease.

CHAPTER XI.

CAUSES OF DISEASE

A. External causes—I. Non-living environment: soil, atmosphere, temperature—II. Living environment: plants, animals—Complex interactions—Predisposing causes—No one factor works alone—Tangled problems of natural selection involved. B. So-called internal causes.

It is customary to classify the causes of disease in plants into two principal groups—(1) those due to the action of the non-living environment—soil, atmosphere, physical conditions such as temperature, light, etc.; and (2) those brought about by the activities of living organisms—plants and animals of various species. Before passing to further subdivisions under these two heads, however, it is necessary to observe that no disease can be efficiently caused by an organism alone, since its powers for injury as a parasite, or otherwise, are affected by its non-living environment as well as by the host-plant. For instance, the spores of a parasitic fungus which would infect and rapidly destroy a potato plant in moist warm weather may be showered on to such a plant with impunity if the air remains dry and cool—or on to a cabbage under any circumstances as far as we know.

Again, probably no one factor of the non-living environment ever suffices to induce a disease, possibly because no such thing as only one change at a time ever occurs. For instance, it is difficult to say, when a soil becomes sodden with water, whether the excess of water and dissolved matters, the want of air displaced by the water, the lowering of the temperature, or the accumulation of foul products, etc., is the principal factor in causing the damage which results, and we have to determine by the balance of experimental evidence which is the dominant factor in all such cases.

The study of aetiology of disease is in fact only a particular case of that of aetiology in general. Plants at high altitudes in the Alps acquire very different characteristics from the same species in the plains. Is this due to the low temperature, the rarer atmosphere, the more intense illumination, the changes in moisture, etc., etc.? The question is more difficult than it appears at first sight, and we must remember that, complex as are the factors working on the host, they are equally complex in their actions on a parasite attacking the host, whence the resulting disease becomes indeed a tangled problem of natural selection.

Finally it remains to say a few words about a numerous class of cases where no external cause of disease can be discovered. It was formerly the custom to group such cases of "Internal Causes" by themselves, but apart from the fact that many of these mysterious diseases have subsequently been shown to be due to the action of external agencies, the whole question of internal causes resolves itself into one of relations between the plant and its surroundings, and it becomes evident that no inherited or internal disease can be regarded as explained until we know the external causes which have so modified the structure and working of the living cells as to make them abnormal in their reactions to other parts of the plant. "Internal causes" of disease, therefore, is a phrase expressing our ignorance, but somewhat more emphatically than usual. If this is clearly understood there seems no reason against its employment for the time being in the artificial scheme of classification we require. With regard to external causes due to the non-living environment, excess or deficiency of materials in the soil, water, or atmosphere plays an important part, and—since we may neglect purely aquatic plants—it is customary to speak of diseases due to unsuitable soils or to injurious atmospheric influences. For instance, any deficiency in the supplies of the necessary mineral salts (compounds of calcium, magnesium, potassium with sulphuric, nitric and phosphoric acids, etc.) leads to pathological changes, as also does the lack of the necessary traces of iron. But it is equally true that the presence of such ingredients in excess or in combinations unsuited to the plants also leads to disaster, as also does the presence of minerals or other compounds which poison the root-hairs—e.g. products of decomposition, soluble salts of copper and other poisons. That these matters are bound up with the whole question of manuring and of proper soil-analyses will be evident.

Another essential factor is the nature and quantity of organic materials in the soil, whether leaf-mould and decomposing vegetable remains, stable manures, or other animal matters, all of which affect different species very differently, and produce very different results in different soils. It is necessary to apprehend in this connection what has been stated above: that soil is not a mere dead structureless medium, and that the root-hairs of ordinary plants cannot deal with large quantities of putrefying organic matter: that a good soil must abound in useful bacteria and fungi to render such substances available—and in very various ways—and that it must be open and aerated, of proper temperature and suitably supplied with water, and so forth, or disaster will result. Here, again, then we are brought into close contact with all that is known of fermentation, nitrification, and the various biological changes going on in soil, and the application of such knowledge to the practice of manuring and tillage in all its forms.

In view of the above remarks, the danger of "over-feeding," in this sense, has a real meaning for horticulturists, though it must not be forgotten that no substance is really a food until it is assimilable into the protoplasm: manures, etc., are food-materials, not food. The futility of mere chemical analyses to prove what a plant requires is now well known, and it is only on the basis of long and carefully conducted experiments that we can ever discover what a particular plant in a particular soil, situation, and climate requires for healthy development. Again, the quantity of water in soil may be too great or too small for given species, and this either on the average for the year, or during critical periods only; and it is obviously important whether the excess or deficiency is due to improper supplies of water, the depth or shallowness of the soil, its retentive powers, or the nature of the sub-soil and so on, again bringing the whole matter into connection with our understanding of the physical constitution and structure of soils, and the nature of soil-drainage.

For instance, a common way of killing ferns is to keep the roots and soil wet and the air and fronds dry, whereas the natural habitats provide for wet and shaded fronds and well-drained soil.

It may be noted here that in most cases where gardeners speak of plants being killed under the "drip" of trees—e.g. Beech, the injury is due, not to the effects of water but to the shade: the loss of light is so great that the shaded plants die of inanition because their leaves are not able to provide sufficient carbohydrates.

Closely bound up with this is the question of the gases in soils. Apart from the disastrous effects of poisons—e.g. coal gas escaping from pipes under pavements in towns, etc., diseased conditions often result from deficiency of oxygen at the root-hairs, due to imperfect aeration of soils, brought about by stagnant water, excess of animal matter, and so forth.

Unsuitable constitution of the atmosphere is also a fruitful source of disease, though its effects are commoner in closed stoves and greenhouses than in the open. Nevertheless the continual exhalation of sulphurous fumes, chlorine, and other poisonous gases in the neighbourhood of manufacturing centres or of large smoky towns, volcanoes, etc., play their part in injuring plants; and excessive moisture in the form of mist, rain, etc., is also important. All these matters bring us at once into the region of physiology, and only an intelligent appreciation of what is known about the action of the atmosphere on the soil and the plant will save the peasantry of a country from a hopeless mysticism but little removed from that of the Middle Ages, when blights and other evils were vaguely referred to the river-mists, thunder clouds, and easterly winds.

If we summarise the above as the material factors of the environment, we may classify another set of external non-living causes of disease as the non-material factors. Such are principally the following:

The space at the disposal of plants greatly affects their welfare. The crowding of roots in the soil and of foliage in the air, resulting in the loss of light to the leaves, involves deficiency of all the materials referred to above—minerals, organic materials, gases, and water—and no better illustration of the intense struggle for existence among these apparently passive and motionless beings, plants, can be given than an over-crowded seedbed or plantation. If left to themselves such over-stocked areas exhibit to the keen eye of the trained observer all the phases of starvation, weakness, wounding, rot, and, so to speak, brutal dominance of the stronger over the weaker which it is the object of cultivation to prevent. Here, then, we are brought face to face with the true significance of thinning and weeding out, pruning, and similar processes.

Unsuitable temperature is one of the commonest of all sources of disease, for every plant is adapted to certain ranges of temperature, and best adapted to a given optimum somewhere between the maximum and minimum temperature for each function. Consequently any serious departure from the mean may bring about physiological disturbances of the nature of disease, and this in very various ways, as exemplified by the results of frost, sun-scorching, drought, hail-storms, forest fires, and so forth.

As a predisposing factor to disease abnormal temperature effects play a great part. Many wound-fungi gain their entrance through frost-cracks, bruises due to hailstones, or into tissues chilled below the normal.

No less remarkable are the diseases primarily due to insufficient or improper exposure to light, which affects the chlorophyll-apparatus and the process of carbon-assimilation and through these the whole well-being of the plant. Every plant is adapted to certain ranges of light intensity, and most cultivators know how impossible it is to grow shade plants in fully exposed situations, and how easily plants which live in open sunny situations are "drawn" and killed by shade. It is equally important to have the right kind of light, as disastrous experiences with greenhouses glazed with glass which cut off certain rays of light have taught. Here, again, it is important to notice that the optimum intensity or quality of light may differ for different functions and organs of the plant, as is shown by many adaptations on the part of species growing in natural situations—e.g. bud protection, orientation of leaves, etc.—and it may be taken as a rule that etiolated plants are peculiarly susceptible to other diseases.

As regards other factors of the inorganic environment, disasters which come within the scope of our subject may be brought about by many agencies, the mechanical effects of snow and hail, wind, avalanches, etc., the effects of lightning, and so forth, being a few of them.

Notes to Chapter XI

For other detailed classifications of the causes of disease the reader is referred to the works of Sorauer and of Frank referred to in the last chapter. Also Kirchner, Pflanzen Krankheiten, Stuttgart, 1890.

Of more historical importance are the older classifications of Berkeley, Gardeners' Chronicle, 1854, and Re, Gardeners' Chronicle, 1849-50. These latter are interesting as showing the very different views held by the earlier workers, and comparison of these with the modern views helps to mark the progress of physiology during the half century which has intervened.

CHAPTER XII.

CAUSES OF DISEASE. THE LIVING ENVIRONMENT

Causes due to animals—Vertebrata—Wounds, etc.—Invertebrata—Insects, etc.—Plants as causes of disease—Phanerogams, weeds, etc.—Cryptogams, fungi—Epidemics, etc.

Passing now to those causes of disease which are connected with the living environment, we may obviously divide them into two groups of agents, animals and plants.

Among animals, the various vertebrata, including man, are especially responsible for the larger kinds of wounds and wholesale destructive processes due to breakage, stripping of leaves and bark, cutting and biting, and so forth. Cattle, rabbits, rats and mice, squirrels and birds of various kinds stand out prominently as enemies to trees and other plants, to which they do immense injury in various ways by their horns, teeth, claws, and beaks; and the damage which an ignorant gardener or forester can do with his ill-guided footsteps, axe, spade, and knife can only be appreciated by one who knows the habits of plants.

It is among the invertebrata, however, especially insects and worms, that the most striking agents of disease in plants are to be found, for, with the exception of certain rodents—and we may logically include also human invasions—vertebrate animals do not often appear in such numbers as to bring about the epidemics and scourges only too commonly caused by insect pests.

Insects injure plants in very various ways. Some, such as locusts, simply devour all before them; others, e.g. caterpillars, destroy the leaves and bring about all the phenomena of defoliation. Others, again, eat the buds—e.g. Grapholitha; or the roots—e.g. wire-worms, and so maim the plant that its foliage and assimilation suffer, or its roots become too scanty to supply the transpiration current. Many aphides, etc., puncture the leaves, suck out the sap, and produce deformations and arrest of leaf-surface, as well as actual loss of substance, and when numerous such insects induce all the evils of defoliation. Others, such as the leaf-miners, tunnel into the leaves, with similar results on a smaller scale.