Disease in plants

by H. Marshall Ward

Many attempts have been made to classify diseases on a basis which assumes the essential distinction of the above cases, and we read of diseases of the various organs—root-diseases, stem-diseases, leaf-diseases, and so forth; or of the various tissues—timber-diseases, diseases of the cambium, of the bark, of the parenchyma, and so on. Furthermore, attempts have been made to speak of general functional disease, of diseases of the respiratory organs, of the absorptive organs, and so forth, as opposed to local lesions.

Critical examination, however, shows that no such distinctions can be consistently maintained, partly because the organs and functions of plants are not so sharply marked off as they are in animals, the diseases of which have suggested the above classification, and partly because all disease originates in the cells and tissues, and it is a matter of detail only that in some cases—e.g. severe freezing or drought of seedlings, or when some ingredient is wanting in the soil—the diseased condition affects practically every cell alike from the first, while in others it spreads more or less rapidly from some one spot.

Even the distinction into physiological diseases versus parasitic diseases cannot be maintained from the standpoint of the nature of the disease itself. All disease is physiological in so far as it consists in disturbance of normal physiological function, for pathology is merely abnormal physiology, no matter how it is brought about. This is not saying that no importance is to be attached to the mode in which disease is incurred or induced: it is merely insisting on the truth that the disease itself consists in the living cell-substance—the protoplasm—not working normally as it does in health, and this, whether want of water, minerals, or organic food be the cause, or whether the presence of some poison or mechanical irritant be the disturbing agent, as also whether such want or irritation be due to some defect in soil or air, or to the ravages of a fungus or an insect.

This being understood I need not dwell on the common fallacy of confounding the fungus, insect, soil or other agent with the disease itself, or of making the same blunder in confusing symptoms with maladies. In this sense, wheat rust is not a disease: it is a symptom which betrays the presence of a disease-inducing fungus, the Rust fungus. Similarly, chlorosis is not a disease: it is a symptom of imperfect chlorophyll action, and the best proof of the truth of both statements is that in both cases the fundamental disease-action is the starvation of the cell-protoplasm of carbohydrates and other essential food matters—in the one case because the fungus steals the carbohydrates as fast as the leaves can make them, in the second because the leaf is unable to make them.

The foundation of a knowledge of disease in plants therefore centres in the understanding of the pathology of living cells.

If a suitable mass of living cells is neatly cut with a sharp razor the first perceptible change is one of colour: the white "flesh" of a potato or an apple, for instance, turns brown as the air enters the cut cells, and the microscope shows that this browning affects cell-walls and contents alike. The cut cells also die forthwith; and the oxygen of the air combining with some of their constituents forms the brown colouring matter which soaks into the cell-walls. The uninjured cells below them grow longer, pushing up the dead débris, and divide across by walls parallel to the plane of the wound, and so form series of tabular cells with thin walls, which also soon turn brown and die, the cell-walls meanwhile undergoing changes which convert them into cork. The living cells deeper down are now shut off from the outer world by a skin, of several layers, of cork-cells, which prevent the further free access of air or moisture. During the period of active cell-division which initiates the cork, the temperature of the growing cells rises: a sort of fever (wound-fever) is induced, evidently owing to the active respiration of the growing cells.

This healing by cork occurs in any tissue of living cells exposed by a cut—leaf-tissue, young stem or root, fruit, cambium, etc.; and the same applies to any other kind of cutting or tearing injury—such as a prick with a needle or the proboscis of an insect, a stripping, or even a bruise.

Such healing is prepared for and carried out very thoroughly in the case of falling leaves and cast branches, the plane of separation being covered by a cicatrix of cork.

If the cell-tissue under the wound is actually growing at the time, however, a further process is observed when the wound-cork has been formed. The uninjured cells below go on growing outwards more vigorously than ever, the pressure of the overlying tissues taken off by the cut having been removed, and, lifting up the cork-layer as they do so, they rapidly divide into a juicy mass of thin-walled cells which is of a cushion-like nature and is termed a Callus. This callus is at first a homogeneous tissue of cells which are all alike capable of growing and dividing, but in course of time it undergoes changes in different parts which result in the formation of tracheids, vessels, fibres and other tissue-elements, and even organs, just as the embryonic tissues of the growing points, cambium, etc., of the healthy plant give origin to new growths. Such wound-wood, however, is apt to differ considerably in the arrangement, constitution and hardness of its parts as compared with normal wood, and its peculiar density and cross-graining are often conspicuous.

If instead of a simple tissue, the cut or other wound lays bare a complex mass such as wood, the resultant changes are essentially the same to start with. The living cells bordering the wound form cork, and then those deeper down grow out and form a callus. The exposure of the wood however, entails alterations in its non-living elements also. The lignified walls of tracheids, fibres, etc., turn brown to a considerable depth, and this browning seems to be—like all such discolorations in wounds—due to oxidation changes in the tannins and other bodies present: the process is probably similar to what occurs in humification and in the conversion of sap-wood into heart-wood in trees. Such wood is not merely dead, but it is also incapable of conveying water in the lumina of its elements, which slowly fill with similarly dark-coloured, impervious masses of materials termed "wound-gum," the nature of which is obscure, but which slowly undergoes further changes into resin-like substances.