Disease in Plants

Laying of Wheat and other cereals is a particular case of etiolation. The seeds having been sown too thickly, the bases of the haulms, owing to the etiolation and consequent lack of carbohydrates, suffer from want of stiffening tissues, and the top-heavy plants fall over.

False etiolation depends on a similar abeyance of the chlorophyll, but in this case due to too low a temperature. It is often seen in Wheat and other monocotyledons when the young leaves unfold in cold weather in spring. The symptoms of "drawing" and tenderness are however absent.

Pallor due to too intense illumination must be kept sharply distinct from etiolation, the pale green or yellow hue being here due to the destruction of the chlorophyll by insolation, and the accessory symptoms of "drawing" are wanting.

Chlorosis is a form of pallor where the chlorophyll grains themselves are fully developed, but their green pigment remains in abeyance owing to a deficiency of iron in the soil, and can often be cured by adding traces of a ferrous salt. The distinction between Icterus, where the organs are only yellow, and Chlorosis proper, where they are nearly white cannot always be maintained. In the typical case only those organs whose cells are still young can become green on adding iron.

Yellowing or False Chlorosis may be experimentally induced by too much carbon-dioxide in the atmosphere. It also often ensues when the roots of plants in the open are waterlogged, owing to the stagnant water not only driving air from the root-hairs but accumulating dissolved substances which poison the plant. Trees frequently thus suffer from "wet feet" when their roots have penetrated down to a sodden impervious subsoil.

Yellowing accompanied by Wilting is a predominant symptom in most cases where transpiration is more active than root-absorption beyond a certain limit, as is well known in cases of prolonged drought. It may also be caused in evergreens by the foliage transpiring actively in bright January weather, for instance, while the ground is frozen and the chilled root-hairs cannot absorb.

In other cases similar appearances are traceable to insects devouring the roots, e.g. wireworms, and the malady is sometimes enhanced by their accumulations so fouling the wet soil that the roots die off, owing to want of oxygen and to the excess of carbon-dioxide and poisonous matters.

Yellowing may also result from the presence of poisonous or acid gases in the atmosphere or soil, such as chlorine, hydrochloric acid, sulphurous acid, etc., in the neighbourhood of chemical works, or from the escape of coal-gas in streets, etc., points of importance in connection with the use of fungicides and insecticides.

Yellowness is the prevailing symptom in many cases of fungus attack of the roots or collar of the plant, the resulting stoppage of transpiration being also sometimes supplemented by rotting of the roots, and the consequent deprival of oxygen and accumulation of foul gases. In other cases Fungi, and even Bacteria, have been found to have made their way into the principal vessels, the lumina of which they stop up, thus reducing the transpiration current.

Certain insects may also induce a general yellowing and wilting of plants by entering or destroying the tissues concerned in the transpiration—e.g. Oniscus, the Frit Fly, and Cecidomya, the Hessian Fly, which attack young winter wheat within the sheaths and cause the plants to turn yellow and wilt.

Albinism and Variegation are apparently due to causes totally different from any yet mentioned. Church's analyses have shown that albino leaves contain more water and less organic matter than green ones of the same plants, but not necessarily less ash constituents. The composition of the ash points to there being more potash and less lime in the white organs than in the green ones, and, speaking generally, the former are related to the latter much as young leaves are related to mature ones.

The whole matter is complicated by the behaviour of certain variegated plants—e.g. Ribbon grass, Calla, Abutilon, which are usually regarded as partial albinos.

Meyen showed long ago that such variegated plants, if grafted on green ones, may induce the development of variegated leaves on both scion and stock, and Morren and others have not only confirmed this but have also shown that variegation may be inherited through the seed. Nevertheless some care has to be taken with many of these variegations lest rich soil, bright light, and other favourable treatment favour the restitution of the green colour. These facts may be interpreted in various ways. Some disturbance of physiological functions of the roots, due to unfavourable conditions of soil, may be the cause; but Beijerinck has lately published some results which show that some of these albino diseases can be induced by inoculating normal plants with the juice of spotted ones even though such juice has been filtered through porcelain, and concludes that a "contagium fluidum vivum" of the nature of a transmissible enzyme is the agent which disturbs the physiology of the infected cells.

Koning, while confirming these results in the main, refers them to a micro-organism so small that it traverses the porcelain filter.

Upheaval of seedlings.—This is a common form of injury, resulting in death by drought and exposure, especially in seedling pines, wheat, etc., in soils exposed to alternate freezing and thawing during spring when there is no snow to protect the plants. The soil freezes during the night, and during the thaw next day water accumulates just below the surface. The freezing is then repeated, and, partly owing to the expansion of the forming ice and partly to the mechanical effect of the ice-crystals in the interstices, the surface of the soil is lifted and draws the roots with it. During the succeeding thaw the soil particles fall away from the lifted root-fibres, and frequent repetition of these processes results in such complete exposure of the roots to the full sun that the plantlet falls over and wilts.

Exposure of roots is also sometimes effected by winds displacing sandy soils liable to shifting in dry weather, and the resulting wilting of the plants thus exposed at their roots may be supplemented by damage due to the repeated impact of the wind-driven sharp grains of sand, which act like a sand-blast and erode the tissues.

In many of the cases given above the principal result is the weakening or destruction of the chlorophyll action. This means a loss of carbohydrates—sugars, starches, etc.—and in so far a starvation of the plant. The injurious effects are quantitative and cumulative: if large areas of foliage are concerned, or if the effect lasts a long time, the plant suffers from loss of food, and may die. In those cases where the effect is due to the cutting off of supplies at the roots, and where the yellowing is a secondary symptom, the disease is more general in character, and recovery is often impossible, because the loss of water cannot be compensated, and the results may be further complicated by the gradual penetration of poisonous matter into the cells. It is frequently necessary, though sometimes very difficult, to decide which is the primary and which secondary (or tertiary, etc.) symptoms in the order of their importance, and the diagnosis may be complicated by a number of accessory factors which it is impossible to treat generally.

Notes to Chapter XIX

The principal cases here described are dealt with in works on plant physiology, and in the works of Sorauer and Frank already referred to.

As regards damage due to uprooting of seedlings by frost, see Fisher, "Forest Protection" (Engl. ed. of Hess' Forstchutz), in Schlich's Manual of Forestry, Vol. IV., 1895, pp. 439-442.

On Albinism, see Church, "A Chemical Study of Vegetable Albinism," Journ. Chem. Soc., 1879, 1880, 1886.

Beijerinck's results are contained in his paper, "Ueber ein Contagium vivum fluidum," etc. (with English abstract), in Verhandl. d. Kon. Akad. v. Wetensch, te Amsterdam, 1898. Koning's paper is in Zeitschr. f. Pflanzenkrank., Vol. IX., 1899, p. 65. See also Nature, Oct. 11, 1900, p. 576.

CHAPTER XX.

SYMPTOMS OF DISEASE (Continued)

Spotted leaves—The colours of spots—White, yellow, brown, and black spots on leaves—Parti-coloured spots—The browning, etc., of leaves.

Discoloured spots or patches on the herbaceous parts of plants, especially leaves, furnish the prominent symptoms in a large class of diseases, due to many different causes, and although we cannot maintain this group of symptoms sharply apart from the last, as seen from the considerations on albinism, it is often well marked and of great diagnostic value. By far the greater number of spot-diseases are due to fungi, but this is by no means always the case. The most generally useful method of subdividing the classes, though artificial like all such classifications, will be according to the colour of the spots or flecks, which, moreover, are usually found on the leaves. It is necessary to note, however, that various conditions may modify the colour of spots on leaves. Many fungi, for instance, induce different coloured spots according to the age of the leaf or other organ attacked, or according to the species of host, the weather, etc. Moreover the spots due to these parasites are frequently yellow when young and some other colour, especially brown or black, when older.

Scale is the name given to the characteristic shield-like insects (Mytilaspis, Aspidiotus, etc.) which attach themselves to branches of Apples, Pears, Oranges, Camellias, and numerous other plants, and suck the juices. It is the female insect which has the body broadened out into the "scale," under which the young are brought up. Enormous damage has been done by some forms—e.g. the San José scale in the United States.

The superficial resemblances of the patches of eggs of some Lepidoptera to Aecidia and other fungi may be noted in passing—e.g. Bombyx neustria on Apple twigs, Aporia Crataegi.

White or greyish spots are the common symptom marking the presence of many Peronosporeae and Erysipheae in or on leaves, e.g. Peronospora Trifoliorum, P. parasitica on Crucifers, etc., and Sphaerotheca on Hops; also Septoria piricola, Cystopus, Entyloma Ranunculi, etc.

White spots are also caused by insects such as Tetranychus (red spider) on Clover and other plants.

Yellow, or Orange-coloured Spots. In cases where these occur on leaves, and in the case of grasses, etc., on the leaf sheaths as well, they commonly indicate the presence of Uredineae, and sections under the microscope will show the mycelium in the tissues beneath. Species of Uromyces, Puccinia, etc., in the Uredo state have the spots powdery with spores; Aecidia show the characteristic "cluster cups," and so forth. These spots are often slightly pustular, and in some cases markedly so.

Other fungi also induce yellow spots on leaves—e.g. Phyllosticta on Beans, Exoascus on Poplars, Clasterosporium on Apricot leaves, Synchytrium Succisae on Centaurea, etc.

Yellow spots are also a frequent symptom of the presence of Aphides, of Red Spider, etc. Thus the minute golden yellow spots sometimes crowded on Oak leaves are due to Phylloxera punctures.

Yellow patches are formed on the large leaves of Arisarum by a species of parasitic Alga, Phyllosiphon, which lives in the mesophyll. Many tropical leaves are spotted yellow by epiphytic Algae—e.g. Cephaleuros.

It must be noticed that many fungi produce yellow spots or flecks in the earlier stages, which turn brown or black as the fructifications appear, e.g. Dilophia graminis, Rhytisma acerinum.

The yellow-spotted leaves of Farfugium grande (Senecio Kaempferi) are so like those of Petasites attacked with Aecidium in its early stages, that an expert might be deceived until the microscopic analysis was completed.

Red spots, varying from rusty or foxy red to bright crimson, are the symptomatic accompaniment of several fungi, the former often characterising the teleutospore or aecidium stage of Uredineae—e.g. Aecidium Grossulariae—the latter sometimes indicating the presence of Chytridiaceae.

Red spots are also caused by Gloeosporium Fragariae on Strawberry leaves, Polystigma rubrum on Plums.

Crimson spots on Apple and Pear leaves are also due to Phytoptus: they turn brown later.

Brown spots or flecks, varying in hue from dull slaty brown to deep red browns, are a common symptom of Fungus and Insect diseases, the colour often indicating the death of the tissues, rather than any special peculiarity of the action of the parasite. Good examples are furnished by the Potato-disease, and by Peronospora viticola, Sphaerella vitis and other disease-fungi of the Grape Vine. The teleutospore stage of many Uredineae also occurs in deep brown spots.

Black spots and flecks are exceedingly common symptoms of the presence of fungi, e.g. Fusicladium on Apples and Pears, and the pycnidial and ascus stages of many Ascomycetes—e.g. Phyllachora graminis. The teleutospore stages of species of Puccinia, Phragmidium, etc., are also so deep in colour as to appear almost black.

Scab on Pears is due to the presence of Fusicladium, which indurates the outer skin of the fruit causing it to crack under pressure from within, and to dry up, the deep brown to black patches of fungus persisting on the dead surface.

Black spots on grasses and sedges are caused by Ustilagineae, and are commonest in the grain, the soot-like powdery spores (Smut) being very characteristic. Ustilago longissima induces black streaks on the leaves. Many of these fungi cause distortions or pustules on leaves and other organs.

Brown and black leaf spots are frequently furnished with concentric contours arranged round a paler or other coloured central point—e.g. Cercospora on Beans, Ascochyta on Peas.

Brown spots with bright red margins are formed in young Beans by Gloeosporium.

Species of Fumago, Herpotrichia, etc., may cover the entire surface of the leaf with sooty patches, or even weave the leaves together as if with black spider-webs.

Mal nero of the Vine is a particular case of black spotting and streaking of the leaves for which no satisfactory explanation is as yet to hand. As with Chestnuts, Walnuts, and other plants containing much tannin, the dark spots appear to be due to this substance, but whether the predisposing cause is a lack of some ingredients in the soil, or some temperature reaction, or fungi at the roots, is as yet unknown. The most recent explanation puts the disease down to the action of bacteria, but the results obtained by different workers lead to uncertainty.

The "dying back" of leaves, especially of grasses, from the tip, is usually accompanied by a succession of colours—yellow, red, brown, to black—and is a common symptom of parching from summer drought; and spots of similar colours, frequently commencing at the margins of leaves, are characteristic symptoms of the injurious action of acid gases in the air.

Brown and blackish spots on Pears are caused by a species of Thrips.

In many cases the minute spots of Rust-fungi on one and the same leaf are bright orange yellow (uredo), deep brown, or almost purple-black (teleutospores), foxy-red brown (older uredospores), or dead slaty black where the old teleutospores have died off—e.g. Uromyces Fabae on Beans, U. Pisi on Peas, etc.

Parti-coloured leaves.—The leaves sometimes start shrivelling with red edges, while yellow, red, and finally brown and black blotches appear on the lamina, from no known cause—e.g. Vines. In other cases similar mimicry of the autumnal colouring of leaves results from the action of acid gases.

Burning is a common name for all cases where the leaves turn red or red-brown in hot, dry weather, and many varieties are distinguished in different countries and on different plants, because species react dissimilarly. The primary cause is usually want of water—drought.

Foxy leaves are a common sign of drought on hot soils, and the disease may usually be recognised by the gradual extension of the drying and fox-red colour proceeding from the older to the younger leaves, and from base to apex—e.g. Hops.

Coppery leaves.—The leaves of the Hop, etc., may show yellow spots and gradually turn red-brown—copper-coloured—as they dry; the damage is due to Tetranychus, the so-called Red Spider. These cases must of course be carefully distinguished from the normal copper-brown of certain varieties of Beech, Beet, Coleus, etc.

Silver-leaf.—The leaves of Plum, Apple, and other fruit trees often obtain a peculiar silvery appearance in hot summers, the cause of which is unknown.

Discolorations in the form of confluent yellow and orange patches, etc., resembling variegations, are not infrequently due to the ravages of Red Spider and mites—e.g. on Kidney Beans.

Sun-spots.—Yellow spots, which may turn brown or black according to the species of plant affected and the intensity of the action, are often caused by the focussing of the solar rays by lens-like thickenings due to inequalities in the glass of greenhouses, or by drops of water on them or on other leaves, e.g. Palms, Dracaena, etc. The action is that of a burning glass, and extends throughout the leaf-tissues. Young grapes, etc., may also be injured in this way. Water-drops on the glass can only act long enough to produce such injuries if the atmosphere is saturated. The old idea that a drop on a leaf can thus focus the sun's rays into the tissues beneath is not tenable.

Here again we see that the disease-agencies concerned in producing the symptoms described in this chapter, agree for the most part in so far that the principal effect is generally the disturbance of chlorophyll action in the spots or flecks on the leaves, and the rendering useless of these areas so far as providing further food-supplies is concerned. The effects may be due merely to the shading action of a parasite—e.g. epiphytic fungi—or to actual destruction of the tissues invaded—e.g. by endophytic fungi—or the tissues may be burnt, poisoned, etc. In so far the results are again quantitative and cumulative, and the amount of damage depends on the number and size of the spots or other areas affected, and the proportion of foliage involved, as well as the length of time the injurious action is at work. But, again, it must be remembered that several symptoms may co-exist, and matters may be complicated by the spread of the destructive agent, or its consequences, to other parts, and in some cases we are quite uninformed as to the true nature of the disease.

Notes to Chapter XX

Further information regarding these "leaf-diseases" will be found in special works dealing with the fungi and insects which cause them. In addition to works already quoted, the reader may also be referred for Fungi to Massee, A Textbook of Plant-diseases caused by Cryptogamic Parasites, London, 1899; or Prillieux, Les Maladies des Plantes Agricoles, 1895. See also Marshall Ward, Coffee-leaf Disease, Sessional Papers, XVII., Ceylon, 1881, and Journ. Linn. Soc., Vol. XIX., 1882, p. 299.

The question of "Sun-spots" has been dealt with by Jönnson in Zeitschr. f. Pflanzenkrankh., 1892, p. 358.

CHAPTER XXI.

ARTIFICIAL WOUNDS

The nature of wounds and of healing processes—Knife wounds—Simple cuts—Stripping—Cuttings—Branch-stumps and pruning—Stool-stumps—Ringing—Bruises.

Wounds.—All the parts of plants are exposed to the danger of wounds, from mechanical causes such as wind, falling stones or trees, hail, etc., or from the bites of animals such as rabbits, worms, and insects, and although such injuries are rarely in themselves dangerous, they open the way to other agencies—water, fungi, etc., which may work great havoc; or the loss of the destroyed or removed tissues is felt in diminished nutrition, restriction of the assimilative area, or in some other way.

We have seen that living cells die when cut, bruised, or torn; and that the cells next below in a layer of active tissue are stimulated by the exposure to increased growth and division, and at once produce a layer of cork, the impervious walls of which again protect the living cells beneath. This is found to occur in all cell-tissues provided the cells are still living, and it matters not whether the wound occurs in the mesophyll of a leaf, the storage parenchyma of a Potato-tuber, the cortex of a root or stem, or in the fleshy parts of a young fruit, the normal effect of the wound is in all cases to call forth an elongation of the uninjured cells beneath, in a direction at right angles to the plane of the injured surface, which cells then divide by successive walls across their axis of growth: the layers of cells thus cut off are then converted into cork, by the suberisation of their walls. Further changes may then go on beneath the protective layer of wound-cork thus produced, and these changes vary according to the nature of the cells beneath: the cambium forms new wood, the medullary rays similar rays, cortex new cortex, and so on.

Knife-wounds.—Artificial cuts in stems are easily recognised and soon heal up unless disturbed. Several cases, differing in complexity, are to be distinguished. The simplest is that of a longitudinal, oblique, or horizontal short cut in which the point of the knife severs all the tissues of the stem down to the wood. The first effect usually observed is that the wound gapes, especially if longitudinal, because the cortex, tightly stretched on the wood cylinder, contracts elastically. This exposes the living cortex, phloem and cambium to the air, and such tissues at once behave as already described above: the cells actually cut die, those next below grow out under the released pressure, and these give rise to cells which become cork. As the growth and cell-division continue in the cells below this thin elastic cork-layer, they form a soft herbaceous cushion or callus looking like a thickened lip to each margin of the cut. Each lip soon meets its opposite neighbour, and the wound is closed over, a slight projection with a median axial depression alone appearing on the surface. The depression contains the trapped-in callus-cork squeezed more and more in the plane of the cut as the two lips of callus press one against the other, and sections across the stem and perpendicular to the axis of the cut show that this thin cork, like a bit of brown paper, alone intervenes between the cambium, phloem and cortex respectively of each lip, as each layer attempts to bridge over the interval. If the healing proceeds normally, these layers, each pressing against the trapped cork-film, and growing more and more in thickness, shear the cork-layer and tear its cells asunder, and very soon we find odd cells of the cambium of one lip meeting cambium cells of the other, phloem meeting phloem, and cortex cortex, and the normal thickening of the now fused layers previously separated by the knife goes on as if nothing had happened, the only external sign of the wound being a slight ridge-like elevation, and, internally, traces of the dead cells and cork trapped here and there beneath the ridge. When the conjoined cambium resumes the development of a continuous layer of xylem and phloem, no further trace of the injury is observable, unless a speck of dead cells remains buried beneath the new wood, and indicates the line where the knife point killed the former cambium and scored the surface of the wood in making the wound.