CHAPTER XIV.

With reference to cork-healing and wound-fever the student may consult Shattock "On the Reparative processes which occur in Vegetable Tissues,"Journal of the Linnean Society, 1882, Vol. XIX., p. 1; and Shattock "On the Fall of Branchlets in the Aspen,"Journal of Botany, 1883, Vol. XXI., p. 306. Also Richards, "The Respiration of Wounded Plants,"Annals of Botany, Vol. X., 1896, p. 531; and "The Evolution of Heat by Wounded Plants,"Ann. of Bot., Vol. XI., 1897, p. 29.For details and figures respecting callus, see Sorauer,Physiol. of Plants, p. 175.In respect to the irritable movements referred to see Darwin,The Power of Movements in Plants, 1880, chapter III. The recent work of Nawaschin,Beobachtungen ueber den feineren Bau u. Umwandlungen von Plasmodiophora, Flora, Vol. LXXXVI., 1899, p. 404, should be read for details and literature concerning "Finger and Toe."

For details and figures respecting callus, see Sorauer,Physiol. of Plants, p. 175.

In respect to the irritable movements referred to see Darwin,The Power of Movements in Plants, 1880, chapter III. The recent work of Nawaschin,Beobachtungen ueber den feineren Bau u. Umwandlungen von Plasmodiophora, Flora, Vol. LXXXVI., 1899, p. 404, should be read for details and literature concerning "Finger and Toe."

Actions of poisons in small doses—Results of killing a few cells—Malformation—Enzymes—Secretions and excretions—Acids, poisons, etc.—Chemotactic phenomena—Parasitism—Epiphytes and endophytes—Symbiosis—Galls.

Physiological research has shown that the respiratory activity of cells may be increased by small doses of poisons, and even that growth may be accelerated by them—e.g.chloroform, ether—and, still more remarkable, that fermentative activity may be enhanced by minute doses of such powerful mineral poisons as mercuric chloride, iodine salts, etc., and that the cells may be gradually accustomed to larger doses without injury. Unfertilised eggs of insects have been started into growth by treatment with acids and those of frogs with mercury salts, and the germination of beans quickened by various poisonous alkaloids. In other words, graduated doses ofpoison may alter the physiological activity of living cells, inducing pathological phenomena, while larger doses kill them.

Now we know at least one parasitic fungus which poisons the cells of its host, and kills them, with similar symptoms to those resulting from excessive doses of the above-named toxic agents.Botrytishyphæ, living in the cell-walls of plants, but not entering the cells, excretes a poison which kills the protoplasm, and the fungus then feeds on the debris. Numerous other fungi form powerful poisons, but we do not know whether or how they employ them—e.g.Ergot.

It is obvious that if all the young cells of a root-tip or of the apex of a shoot, or those of a young leaf, are growing and dividing regularly, the killing of one or a few cells at one point on the side of the organ must result in irregularities—in malformation—of the adult organ. This has been proved experimentally by destroying a few cells with a needle. It can also be done by planting a minute mycelium ofBotrytislaterally on a young organ—e.g.a very young lily-bud. The fungus adheres to the surface, kills a few epidermis cells, and forms a foxy-red spot, which becomes concave as the dead cells lose water and dry. Since the rest of the bud goes on growing, however, while this dead point remains stationary, the latter gradually becomes the centre of a concavity, the growing tissues having grown round it: the bud is deformed. Numerous cases of malformed organs are explained in this way; a minute insect hasbitten or pierced the young tissue, or a fungus has killed a minute area, or a drop of acid condensed from fumes in the air is the lethal agent, and so forth. And even on a much larger scale we see the same kinds of agents at work. Wherever a patch of cells is killed whilst those around go on growing, there must result some deformation of the resulting organ, since had the injury been withheld the number and sizes of the cells now fixed in death would have increased and covered a larger area: they now serve to pull over to their side the still living and growing cells. The same results follow on any lateral wound: the killed spot of tissue serves as a point round which the continued growth of other parts of the organ turns. Hence the malformation is in these cases a secondary effect, and not, as in simple hypertrophy, a direct effect of the action of the cells involved in the injury.

There is another class of bodies secreted by fungi, however, which act directly on cells, viz. enzymes—that is, soluble bodies which are able to dissolve cellulose (cytases), starch (diastases), proteids (proteolytic enzymes), and other substances, by peculiar alterations in their constitution. It is by means of itscytasethatBotrytishyphae pierce the cellulose walls of plants, and no doubt in all cases where fungi pierce cell-walls it is by the solvent action of such a cytase, and similarly when haustoria penetrate into the cells. It is also by means of these starch-dissolving enzymes (diastases) and proteolyticenzymes, etc., that the hyphae inside the cells are enabled to make use of the starch, proteids, etc., they find there.

All living cells form materials, resulting from the activity of the protoplasm, which we may compare with the refuse or by-products formed in any great manufacturing industry: these by-products have to be got rid of if they are injurious or noisome (excretions), and if not—i.e.if they are capable of further use (secretions)—they have to be stored away till required. Some of the most prominent of these bodies excreted by fungi are, as we have seen, poisonous acids, such as oxalic acid, enzymes, and organic poisons, such as those in ergot. But similar enzymes, acids, poisons, etc., to those found in fungi are also found in the cells of other plants and animals; for only by means of their solvent actions can processes like digestion and assimilation of the starchy and other materials into the body-substance be accomplished, and we have seen that it is a general property of living cells to form acids, and other excretions and secretions.

Now we know very little about what may happen when an organism—say a fungus—secreting especially one kind of enzyme or poison or other active substance, comes into intimate contact with another—say a leaf-cell—which secretes predominantly others, but what we do know points to the certainty that various complications will occur.

For instance, if certain bacteria which prefer analkaline medium, and yeasts which prefer an acid environment are mixed in a saccharine solution, it depends on the reaction of the liquid which organism gains the upper hand: if the liquid is acid the yeast may dominate the bacteria; if alkaline it may be suppressed by them.

That a parasite may be prevented from successfully attacking a particular plant is shown by the failure ofCuscutato establish its haustoria in poisonous plants such asEuphorbia,Aloe, etc., and it has been pointed out that poisonous secretions in the cells of the plant protect them against the penetration of fungi. This cannot be taken as meaning that any poison protects against any parasite, however, forEuphorbiais itself subject to attacks of Uredineae, andPangium edule, which contains prussic acid and is extremely poisonous to most animals, is eaten with avidity by several insects, while nematode worms can live in its tissues. This is no more remarkable, however, than the fact thatFontaria, a myriapod, secretes prussic acid in its own tissues, or than that certain glands of the stomach secrete free hydrochloric acid, andDoliumforms sulphuric acid in its glands.

There is yet a further point to notice here. It has been proved that certain substances formed in plant-cells, not necessarily nutritive, attract the hyphae of parasitic fungi or repel them, according to the kind and degree of concentration. So clear has this proof been made that it was possible in experiments conducted apart from a host plant,to make the hyphae on one side of an artificial membrane—e.g.collodion—penetrate it by placing one of these attractive (chemotropic) substances in suitable proportions on the other side. The hyphae dissolved holes in the membrane by means of enzymes and plunged into the attractive substance on the other side.

The foregoing sketch gives us a glimpse into the causes at work in parasitism.

Suppose a fungus on the outside of the epidermis of a young organ—say a leaf. It may be unable to penetrate into the plant, and finding no suitable food outside it dies: or it may be satisfied with the traces of organic matter on the epidermis and then lives the life of a saprophyte. Or it may be able to establish a hold-fast on the tender epidermal surface, but without entering the cells, and irritate the developing organ by contact stimulation, inducing slight abnormalities; if in its further, purely superficial growth such an epiphyte covers large areas of the leaf, and especially if the hyphae are dark coloured—e.g.Dematiumand other "Sooty Moulds"—injury may be done to the leaf owing to the shading action which deprives the chlorophyll below of its full supply of solar energy. Some epiphytes, however, are able to fix their hyphae to the epidermis by sending minute peg-like projections into the cuticle—Trichosphaeria,Herpotrichia—while others send haustoria right through the outer epidermal walls—e.g.Erysiphe—and thus supplement mere contact-irritation and shading byactual absorption from the external cells. Here the fungus is a parasitic epiphyte.

A stage further is attained in those fungi which enter the stomata and live in the intercellular spaces—e.g.many Uredineae andPhytophthora—and many such intercellular endophytes increase their attack on the cells by piercing their walls with minute (Cystopus) or large and branched (Peronospora) haustoria, or even eventually pierce the cells and traverse them bodily (Pythium). In all these cases it is clear that conflicts must occur between poison and antidote, acid and alkali, attractive and repellent substances, enzyme and enzyme, etc., as was hinted at above; and the same must take place when the parasite is endophytic and intracellular from the first, as in Chytridiaceae, etc., the zoospores of which pierce the outer cell-walls and forthwith grow into the cells. There are also fungi which, while able to pierce the outer cell-walls, and grow forward in the thickness of the wall itself, cannot enter the living cells themselves—e.g.Botrytis. In the example mentioned, the fungus excretes a poison, oxalic acid, which soaks into and kills the cells next its point of attack: into these dead cells it then extends, and, invigorated by feeding on them, extends into other cell-walls and excretes more poison, and so on.

On the basis of the foregoing it seems possible to sketch a general view of the nature of parasitism. In order that a fungus may enter the cells it must be able to overcome not only the resistance of thecell-walls, but that of the living protoplasm also: if it cannot do the latter it must remain outside, as a mere epiphyte, or at most an intercellular endophyte. If it can do neither it must either content itself with a saprophytic existence or fail, so far as that particular host-plant is concerned. Its inability to enter may be due to there being no chemotropic attraction, or to its incapacity to dissolve the cell-walls, or to the existence in the cell of some antagonistic substance which neutralises its acid secretions, destroys its enzymes or poisons, or is even directly poisonous to it.

Moreover when once inside it does not follow that it can kill the cell. The protoplasm of the latter may have been unable to prevent the fungus enemy from breaking through its first line of defence—the cell-wall, but it may be quite capable of maintaining the fight at close quarters, and we see signs of the progress of the struggle in hypertrophy, accumulation of stores, and other changes in the invaded cells and their contents.

Finally, the invested or invaded cell may so adapt itself to the demands of the invader that a sort of arrangement is arrived at by which life in common—Symbiosis—is established, each organism doing something for the other and each taking something from the other. In this latter case, which is often realised—e.g.lichens, leguminous plants and the organisms in their root-nodules, mycorrhiza, etc.—we leave the domain of disease, which supervenes indeed if the other symbiont is lacking.

Some interesting facts bearing on the matters here under discussion, have been obtained from the study ofGalls, the curious outgrowths found on many plants and due to the action of insects.

A typical gall exhibits three distinct and characteristic layers of tissue surrounding the hollow chamber in which the larva of the insect lies, viz., an outer layer of soft cells forming a parenchyma covered with an epidermis, and frequently also with a layer of cork; an inner stratum consisting of very thin-walled delicate cells filled with protoplasmic and reserve food-materials on which the larva feeds; and between the two a more or less definite layer of thick-walled sclerenchyma cells which serve as a protection against accidents to the larva as the outer layer shrivels or rots, or if it is exposed to the attack of marauders. This layer may be absent from galls which have a short life only. Vascular bundles run into the outer layer from the leaf-veins or the stele of the shoot, etc. Such galls abound in tannin, and are frequently of use in the arts on this account: they also contain starch, and proteid substances and crystals of calcium oxalate. When the larva has consumed the stores of food material and reached the adult stage it eats its way out and escapes.

The growth of such a gall is preceded by the laying of an egg on or in the embryonic tissue of a leaf, stem, or other young part, and it is interesting to note that only organs in the meristematic stage can form galls, and that it isby no means necessary that the tissues should be wounded. Moreover, the egg as such is incapable of stimulating the plant tissues, but when it hatches, the resulting larva, beginning to feed on the cells, irritates the tissues and rapid growth and cell-division occur, as in the case of other wounds or of fungus attacks. The actual wound made by the ovipositor heals up at once. It is evident from numerous recent researches that these true galls are not due to any poisonous or irritating liquid injected by the parent, but that the stimulus to the tissue formation is similar to that exerted by a wound. The young gall is in fact a callus enclosing the living larva, and it is the continued irritation of the latter which keeps up the stimulation. The final shape and constitution of the gall depend on mutual reactions—not as yet explained in detail—between the species of plant and the species of gall-insect concerned, as may readily be seen from the extraordinary variations in size, shape, colouring, hairiness and other structural peculiarities of the galls on one species of, for instance, the common oak. From what we have learnt about fungus parasites, however, there can be little doubt that reactions between the cells and the larva of the insect occur, resembling those which take place between the cells and the hyphae of the fungus, and this is borne out by the study of other hypertrophies due to animals;e.g.Nematode worms in roots, and the remarkable galls—the simplest known—onVaucheria, caused by the entrance into this alga of a speciesofNotommata, which induces a different gall on each of the various species of its host plants.

It must be concluded that the formation of theVaucheriagall is induced by the mechanical irritation which the Rotifer causes in the protoplasm. These galls are comparable to the hypertrophies inPiloboluscaused by the presence ofPleotrachelus.

Attempts to induce the development of galls artificially by injecting formic, acetic and other vegetable acids, poisons and other substances into the tissues have, however, failed, and even the substances contained in the insect or gall itself only produced negative results. Nothing further was obtained than slight callus formations in some cases. Nor have experimenters succeeded in obtaining more than slight distortions by fixing insects on the growing leaves in such positions that they could scratch the epidermis.

We must therefore conclude that very complex interactions between the plant and insect are here concerned, among which may be the infiltration of some liquid from larva to plant—many of these gall larvae are strongly scented, and Kustenmacher says that fluids excreted by the larva are absorbed by the gall-tissue apparently as nutriment. This would point to the symbiotic character of galls and their guests.

With regard to the action of poisons in small doses see further Johannsen,Das Aether-Verfahren beim Fruhtreiben,Jena, 1900, and, forBotrytis, see Marshall Ward, "A Lily Disease,"Annals of Botany, Vol. II., 1889, p. 388.The subject of enzymes has been exhaustively treated by Green,The Soluble Ferments and Fermentations, Cambridge, 1899, to which the reader is referred for literature. I have taken the statements regardingFontariaandDoliumfrom Kassowitz,Allgemeine Biologie, p. 182. The two most important works on chemotactic phenomena are Pfeffer, "Uber Chemotaktische Bewegungen," etc.,Unters. aus dem Bot. Inst. zu Tubingen, B. II., p. 582, and Miyoshi, "Die Durchbohrung von Membranen durch Pilzfaden,"Pringsh. Jahrb. f. Wiss. Bot., B. XXVIII., 1895, p. 269, and from these the further literature can be traced. As regards the nature of parasitism see Marshall Ward, "On Some Relations between Host and Parasite," etc., being the Croonian Lecture delivered before the Royal Society,Proc. Roy. Soc., Vol. 47, p. 393. On Symbiosis, see Marshall Ward, "Symbiosis,"Annals of Botany, 1899, Vol. XIII., p. 549, where the literature is collected. For a general account of galls the reader may consult Kerner,The Natural History of Plants, Eng. ed., 1895, Vol. II., pp. 527-554, and Adler,Alternating Generations, A Biological Study of Oak Galls, etc., 1894.

The subject of enzymes has been exhaustively treated by Green,The Soluble Ferments and Fermentations, Cambridge, 1899, to which the reader is referred for literature. I have taken the statements regardingFontariaandDoliumfrom Kassowitz,Allgemeine Biologie, p. 182. The two most important works on chemotactic phenomena are Pfeffer, "Uber Chemotaktische Bewegungen," etc.,Unters. aus dem Bot. Inst. zu Tubingen, B. II., p. 582, and Miyoshi, "Die Durchbohrung von Membranen durch Pilzfaden,"Pringsh. Jahrb. f. Wiss. Bot., B. XXVIII., 1895, p. 269, and from these the further literature can be traced. As regards the nature of parasitism see Marshall Ward, "On Some Relations between Host and Parasite," etc., being the Croonian Lecture delivered before the Royal Society,Proc. Roy. Soc., Vol. 47, p. 393. On Symbiosis, see Marshall Ward, "Symbiosis,"Annals of Botany, 1899, Vol. XIII., p. 549, where the literature is collected. For a general account of galls the reader may consult Kerner,The Natural History of Plants, Eng. ed., 1895, Vol. II., pp. 527-554, and Adler,Alternating Generations, A Biological Study of Oak Galls, etc., 1894.

Dissemination of fungi by the aid of snails, rabbits, bees, and insects—Man—Distribution in soil, on clothes, through the post, etc.—Worms, wind—Puffing of spores—Creeping of mycelia—Lurking parasites—Spread of insects and other animals—Losses due to epidemics.

The dissemination of plant diseases is a subject which has been far too much neglected, but our knowledge of it is slowly increasing. The spores of fungi such as Rusts and Erysipheae are often carried from plant to plant by snails; those of root-destroying and tree-killing Polyporei by rabbits, rats, and other mammals which rub their fur against the hymenophores. Bees have been shown to carry the spores ofSclerotiniaand infect the stigmas of Bilberries, etc., with them; and flies convey the conidia of Ergot from grain to grain. Insects, indeed, of all kinds are great disseminators of disease—as witness also the part played bymosquitoes in transferring the malaria parasite to man—and beetles, bees, flies, etc., of all sorts probably play more active parts in this work than has yet been proved, since they not only carry spores attached like pollen to their hairy bodies, but in many cases in their alimentary canal, to be spread later in the dung.

The part played by man in conveying fungi from plant to plant counts for much. Not only do gardeners and farm labourers carry spores on their boots and clothes as they pass from infected to non-infected areas, but carted soil and manure are frequently infested with spores of Smuts,Fusarium,Polyporus, and the sclerotia or rhizomorphs ofSclerotinia,Agaricus melleus,Dematophora, etc. Man also sends diseases through the post, and by rail and ship, by spores or mycelia attached to seedlings, bulbs, fruits, flowers, etc., as shown in several cases of potato, vine, hollyhock, lily, and hyacinth diseases. Every time a carpenter saws a piece of fresh timber with the saw which has been used previously for cutting wood attacked with dry rot, he risks infecting it with the fungus. Similarly in pruning: every cut with a knife which the gardener has used on infected branches may infect the tree.

Cuttings made with a soil-contaminated knife and stuck into ordinary soil in dirty boxes covered with equally dirty glass, present every chance for infection by soil organisms; bacteria and fungi obtain access to the vessels, and derive plenty of food from the juices, and the wonder is not thatso many cuttings "damp off," but that any are raised at all under ordinary conditions.

That worms bring buried spores to the surface can hardly be doubted after Pasteur's experiments with Anthrax, and the principle of Darwin's discoveries of the important bearing of the habits of earthworms on this subject, and that the soil attached to the feet of ducks and other birds teems with small seeds, applies to fungi also. Wind is also responsible for distributing fungus-spores over wide areas, as may be easily proved by fixing a glass slide smeared with glycerine in the course of a breeze passing over an infected area.

But although the fungi are, generally speaking, passive in regard to their distribution, such is by no means always the case. Apart from the fact that some forms attract insects by means of honey dew (Ergot), or by sweet odours (Spermogonia,Sclerotinia), the zoospores ofPythium,Phytophthora, etc., are motile, and although they cannot move far in the films of water in which they travel, nevertheless in a wet potato field, with the wind flapping the leaves one against the other, some dissemination of importance must be actively brought about, and similarly with the amoebae ofPlasmodiophorain the soil.

The shooting of ascospores into the air by certain species ofPeziza, from the discs of which the spores may be seen to puff out in clouds, affords further evidence that fungi cannot be regarded as entirely passive in respect to distribution of their spores. But when we come tocertain of the soil fungi—e.g.Agaricus melleus,Dematophora, etc.—the active creeping forward by growth in the soil of their rhizomorphs and mycelial strands afford examples of active spreading of considerable importance in the vineyard and forest, since they pass from root to root and from tree to tree and may infect the entire area in course of time.

Not the least significant mode of dissemination is that by which what I have termed "lurking parasites" are spread: such are fungi which attach themselves to the seeds, fruits, tubers, etc., of other plants and so obtain all the advantages of being carried and sown with the latter—e.g.Ustilagineae and Uredineae which adhere to grain,Verticillium,Nectria, etc., in potatoes and other plants.

The spread of diseases due to animals, especially insects, is of course more active, in consequence of the motility of the distributing agents. This is most marked in the winged species, of which locusts, beetles, moths and butterflies, flies and wasps furnish well-known examples; and is not inconsiderable in the case of wingless and merely creeping species. It is noteworthy that many forms wingless in the parasitic stage are winged at certain periods,e.g.the females ofPhylloxera.

That man also spreads insect pests is well known and acted upon, as witness the phylloxera laws—which, however, it is to be feared too often only illustrate once more the adage concerning the shutting of the stable door after the horse has gone.

It would be tedious to attempt anything like a complete account of the estimates of loss in different countries, due to the ravages of insects and fungi, but the following examples should surely serve to convince anyone of the magnitude of these losses and of the economic importance of the whole question, and the reader may be referred to the special literature for further details.

The coffee leaf-disease of Ceylon, due to the fungusHemileia, is estimated to have cost that Colony considerably over £1,000,000 per annum for several years. One estimate puts the loss in ten years at from £12,000,000 to £15,000,000. The hop-aphis is estimated to have cost Kent £2,700,000 in the year 1882. In 1874 the Agricultural Commissioner of the United States estimated the annual loss, due to the ravages of insects on cotton alone, to amount to £5,000,000; and in 1882 the annual loss to the United States due to insects, calculated for all kinds of agricultural produce, was put at the appalling figure of from £40,000,000 to £60,000,000 sterling. In India, the annual loss due to wheat-rust alone has recently been estimated at 4,000,000 to 20,000,000 rupees, and one insect alone is said to have cost the cotton planters a quarter of the crop—valued at seven crores of rupees—in bad years. Similarly, in Australia the annual loss from wheat-rust has been put at from £2,000,000 to £3,000,000. In 1891 the loss in Prussia alone from grain-rusts wasofficially estimated at over £20,000,000 sterling. Need more be said? Even allowing for considerable exaggerations in such estimates it is clear that the damage to crops in any country soon amounts to sums which even at low rates of interest would easily yield incomes capable of supporting the best equipped laboratories and staffs for investigations directed to the explanation of the phenomena in detail, the sole basis on which intelligent preventive and therapeutic measures can be based. But it is far from likely that the estimates are exaggerated. The planting and agricultural communities are as a rule opposed to the publication of statistics—or at least have been so in various countries and at different times—and if we knew the damage done to all crops even in our own Empire, the results would probably astonish us far more than the above figures have done.

On the dissemination of fungi, the reader will find Fulton, "Dispersal of the Spores of Fungi by the Agency of Insects,"Ann. Bot., Vol. III., 1889, p. 207, and Sturgis, "On Some Aspects of Vegetable Pathology and the Conditions which Influence the Dissemination of Plant Diseases,"Botanical Gazette, Vol. XXV., 1898, p. 187, both useful papers. Further information will be found in Zopf,Die Pilze, Breslau, 1890, pp. 79-95 and 228, and Wagner, "Ueber die Verbreitung der Pilze durch Schnecken," inZeitschr. f. Pflanzen Krankh., 1896, p. 144. The estimates as to losses due to epidemics are taken from Watt,Agricultural Ledger, Calcutta, 1895, p. 71; Balfour,The Agricultural Pests of India, London, 1887, pp. 13-15;Eriksson and Henning,Die Getreideroste; the publications of the U.S. Department of Agriculture,The Kew Bulletin, and elsewhere. The reader will find further examples in Massee,Text-Book of Plant Diseases, 1899, pp. 47-51. Both these subjects are well worth further attention, and I know of no complete account of them.

Illustrations afforded by the potato disease—The larch disease—The phylloxera of the vine.

When we come to enquire into what circumstances bring about those severe and apparently sudden attacks on our crops, orchards, gardens, and forests by hosts of some particular parasite, bringing about all the dreaded features of an epidemic disease, we soon discover the existence of a series of complex problems of intertwined relationships between one organism and another, and between both and the non-living environment, which fully justify the caution already given against concluding that any cause of disease can be a single agent working alone.

The statement of prophecy that a particular insect or fungus need not be feared, because it is found to do so little harm in particular cases or districts examined, will thus be seen to be adangerous one: any pest may become epidemic if the conditions favour it!

In 1844 and 1845 the potato disease assumed an epidemic character so appalling in its effects that it is no exaggeration to say that it constituted a national disaster in several countries. It was stated at the time that this disease had been known for some time in Belgium, in Canada and the United States, in Ireland, in the Isle of Thanet, and in other parts of the world. Similar, but less devastating epidemics have occurred in various years since. It was generally noticed during such epidemics that the plants themselves were full of foliage, surcharged with moisture, and of a luxuriant green colour promising abundant crops. The now well-known spots, at first pale and then brown and fringed with a whitish mould-like growth—the conidiophores of thePhytophthora—were observed during the dull cloudy and wet weather, cooler than usual, when the atmosphere was saturated for days together, in July and August. The actual amount of rain does not appear to have been excessive, but most observers seem to agree that dull weather with moist air had succeeded a warm forcing period of growth. So rapidly did the disease run its course that in a few days nearly all the plants were a rotting blackened mass in the fields, and the potatoes dug up afterwards were either already rotten or soon became so in the stores. Further experience has confirmed this, and we now know that the epidemic is very apt to appear in any region wherepotatoes are grown on a large scale, in dull moist weather, especially in fields exposed to mists, heavy dews, etc., about July and August, when the foliage is full and turgid. Similarly on heavy wet soils, unless the season is remarkably open and dry; but also on dry light soils in rainy seasons. So evident was this that many believed that the mists and dew brought the disease—harking back to the superstitions of earlier days. We must remember that prior to 1860 the life-history ofPhytophthorawas not known. Since De Bary's proof of the germination of the zoospores and of the infection of the leaves, the course of the hyphae in them and in the haulms, the origin of the conidia, etc., and the confirmation by numerous competent observers of the true fungus nature of this disease, we are now in a position to understand the principal factors of the various epidemics of potato disease.

It is not merely that the potato-fields afford plenty of food for the fungus, and that the dull weather causes the tissues to be surcharged with moisture, owing to diminished transpiration, but the mists and dew—to say nothing of actual rain and the flapping of wet leaves—favour the germination and spread of the zoospores throughout the field. Whether the dull light also favours the accumulation of sugars in the tissues, and the partial etiolation of the latter implies less resistance to the entering hyphae, may be passed over here, but in any case it is clear that we have several factors of the non-living environmenthere favouring the parasite and not improving the chances of the host, even if they do not directly disfavour it.

As another instance I will take the Larch-disease, which is due to the ravages of a Peziza (Dasyscypha Willkommii) the hyphae of which obtain access by wounds to the sieve-tubes and cambium of the stem, and gradually kill them over a larger and larger area and so ring the tree, with the symptoms of canker described below.

Now the Larch fungus is also to be found on trees in their Alpine home, but there it does very little damage and never becomes epidemic except in certain sheltered regions near lakes and in other damp situations. How then are we to explain the extensive ravages of the Larch disease over the whole of Europe during the latter half of this century? The extensive planting, providing large supplies for the fungus, does not suffice to explain it, because there are large areas of pure Larch in the Alps which do not suffer.

In its mountain home the Larch loses its leaves in September and remains quiescent through the intensely cold winter, until May. Then come the short spring and rapid passage to summer, and the Larch buds open with remarkable celerity when they do begin—i.e.when the roots are thoroughly awakened to activity. Hence the tender period of young foliage is reduced to a minimum, and any agencies which can only injure the young leaves and shoots in the tender stage must do their work in a few days, or the opportunity isgone, and the tree passes forthwith into its summer state.

In the plains, on the contrary, the Larch begins to open at varying dates from March to May, and during the tardy spring encounters all kinds of vicissitudes in the way of frosts and cold winds following on warm days which have started the root-action—for we must bear in mind that the roots are more easily awakened after our warmer winters than is safe for the tree.

It amounts to this, therefore, that in the plains the long continued period of foliation allows insects, frost, winds, etc., some six weeks or two months in which to injure the slowly sprouting tender shoots, whereas in the mountain heights they have only a fortnight or so in which to do such damage. That the lower altitude and longer summer are not in themselves inimical to Larch is proved by the splendid growths made by the trees first planted a century ago. Then came the epidemic of Larch-disease: the fungus, which is merely endemic—i.e.obtains a livelihood here and there on odd trees, or groups of trees in warmer or damper nooks—in the Alps, was favoured by the more numerous points of attack afforded to its spores by injuries due to insects—Coleophora,Chermes, etc.—and frost wounds, as well as by the longer periods of moist dull weather, and the longer season of foliation. Moreover, as time went on almost every consignment of young Larch-trees sent abroad was already infected. Here again, then, we find the factorsof an epidemic consisting in events which favour the reproduction and spread of a fungus more than they do the well-being of the host.

As a third illustration I will take the case of an insect epidemic. In 1863 a disease was observed on vines in the South of France which frightened the growers as they realised its destructive effects: the roots decayed and the leaves turned yellow and died before the grapes ripened, and such vines threw out fewer and feebler shoots the following year, and often none at all afterwards. In 1865 the disease was evidently becoming epidemic near Bordeaux, and in 1868 it was shown to be due to an insect,Phylloxera, the female of which lays its eggs on the roots, where they hatch. The louse-like offspring sticks its proboscis into the tissues as far as the central cylinder. The irritated pericycle and cortex then grow and form nodules of soft juicy root-tissue at which the insect continues to suck. Rapid reproduction results in the majority of the young rootlets being thus attacked, and since they cannot form their normal periderm and harden off properly they rot, and admit fungi and other evils, in consequence of which the vine suffers also in the parts above ground.

Evidence that the general damage is due to the diminished root-action is found in the peculiarly dry poor wood formed in the "canes" of diseased plants.

By 1877 the epidemic had spread to the northern limits of the French vineyards, and by 1888 half the vines in the country were attacked,and the yield of wine reduced from half a million hectolitres to 50,000 only. Meanwhile the disease had spread to Italy, Germany, Madeira, Portugal, and even to the Cape, though not in epidemic form as in the Bordeaux centre whence it spread.

Now it appears thatPhylloxerahas long been in the habit of doing damage to vines in America, where, however, it attacks the leaves, on which it makes pocket-like galls, rather than the roots. Moreover, there are species and varieties of American vines which, even when planted in Europe, do not suffer at all from this insect at the roots, either because the rootlets do not push out at the same season as those of the European form, or because they form wood more rapidly and completely, or secrete resinous and other matters distasteful to the insect in greater quantity and are thus capable of healing the wounds, or in some other way they do not respond to the attack or suit the insect. In any case the attack on the leaf rather than the root seems to be the exception in European vineyards and the rule in American species, and we appear to be face to face with a problem of specific predisposition to this particular malady. That the resistant properties of the vines of America—not all, only particular species and varieties are thus "immune"—can be utilised has been proved by European growers; and not only so, for Millardet and others have shown that the European vine grafted on to these resistant stocks suffer less than when on their own roots. It hasalso been shown that hybrids can be obtained which are resistant.

But the most curious point of all is thatPhylloxerawas itself a native of America, and came thence to Europe. It had played its part with certain fungi in ruining all the attempts to introduce the European vine into America many years ago. A recent authority on the evolution of American fruits writes as follows:

"All the most amenable types of grapes had long since perished in the struggle for existence, and the types which now persist are necessarily those which are, from their very make-up or constitution, almost immune from injury, or are least liable to attack . . . thePhylloxerafinds tough rations on the hard, cord-like roots of any of our eastern species of grapes. But an unnaturalised and unsophisticated foreigner, being unused to the enemy and undefended, falls a ready victim; or if the enemy is transported to a foreign country the same thing occurs."

Further proof that it is in the "constitution" of the European vine that the want of resistance toPhylloxeraresides, is furnished by the fact that in California and the Pacific states the European vine was introduced with more success, but is now suffering badly becausePhylloxerahas spread there also. It must not be overlooked, however, that we are as yet very ignorant of all that is implied in the word "constitution" as used above.

If we enquire further why thePhylloxeraepidemic was so much worse in the Southern vineyards than in the more Northern ones of Germany, the opinion seems to prevail that the warmer climates favour the insect. Further, it appears that, in Italy, the vines in loose open soil, provided it is equally rich in mineral food-materials and offers no disadvantages as regards drainage, suffer less than those in closer soils, the reasons alleged being that the young roots can push out more rapidly and widely, and so obtain holdfasts with greater distances between them.

The student may obtain further information on the history of the Potato disease by consulting the following: Berkeley, "Observations, Botanical and Physiological, on the Potato Murrain,"Journal of the Horticultural Society, Vol. I., 1846, p. 9; De Bary,Die Gegenwärtig herrschende Kartoffel Krankheit, etc., Leipzic, 1861; and the pages of theGardeners' Chroniclefrom 1860-1900.For the Larch disease he should consult Hartig,Unters. aus der Foist. Botanischen Inst. München, B. I., 1880; and Willkomm,Microscop. Feinde des Waldes, B. II., 1868.ForPhylloxerathe literature is chiefly in theComptes Rendusand other French publications since 1875, and in the Reports of the U.S. Dept. of Agriculture.For a summary of the facts concerning the life-histories of the parasites referred to above, see Frank,Krankheiten der Pflanzen, and Marshall Ward,Diseases of Plants, p. 59, andTimber and Some of its Diseases, London, 1889, chapter X.Also Marshall Ward, "On some Relations between Host and Parasite in certain epidemic Diseases of Plants,"Proc. Roy. Soc., Vol. XLVII., 1890, pp. 393-443; and "Illustrationsof the Structure and Life-history of Phytophthora infestans,"Quart. Journ. Microsc. Soc., Vol. XXVII., 1887, p. 413; also Marshall Ward, "Researches on the Life-history of Hemileia vastratrix,"Journ. Linn. Soc., Vol. XIX., 1882, p. 299; and "On the Morphology of Hemileia vastatrix,"Quart. Journ. Microsc. Soc., 1881, Vol. XXI., p. 1.

For the Larch disease he should consult Hartig,Unters. aus der Foist. Botanischen Inst. München, B. I., 1880; and Willkomm,Microscop. Feinde des Waldes, B. II., 1868.

ForPhylloxerathe literature is chiefly in theComptes Rendusand other French publications since 1875, and in the Reports of the U.S. Dept. of Agriculture.

For a summary of the facts concerning the life-histories of the parasites referred to above, see Frank,Krankheiten der Pflanzen, and Marshall Ward,Diseases of Plants, p. 59, andTimber and Some of its Diseases, London, 1889, chapter X.

Also Marshall Ward, "On some Relations between Host and Parasite in certain epidemic Diseases of Plants,"Proc. Roy. Soc., Vol. XLVII., 1890, pp. 393-443; and "Illustrationsof the Structure and Life-history of Phytophthora infestans,"Quart. Journ. Microsc. Soc., Vol. XXVII., 1887, p. 413; also Marshall Ward, "Researches on the Life-history of Hemileia vastratrix,"Journ. Linn. Soc., Vol. XIX., 1882, p. 299; and "On the Morphology of Hemileia vastatrix,"Quart. Journ. Microsc. Soc., 1881, Vol. XXI., p. 1.

Preventible diseases—The principles of therapeutics—Powders and their application—Spraying with liquids—Nature of chemicals employed—Employment of epidemics and natural checks—The struggle for existence.

It may be said that in no connection is the proverb "Prevention is better than cure" more applicable than with this subject, and undoubtedly the best utilitarian argument that can be used in favour of a thorough study of the causes of disease is that only by understanding these causes is there any hope of avoiding the exposure of crops, garden plants, forest trees, etc., to the attacks of preventible diseases. Moreover, only an intelligent appreciation of the causes of a disease will enable the cultivator to take steps to mitigate their effects when once the damage has begun its course. Every cultivator learns by experience or by precept that there are somethings he must avoid in dealing with certain plants, or otherwise they will not succeed; in other words they will succumb to diseased conditions and die. It is partly owing to the want of systematisation of this knowledge, and its extension in other directions, that such extraordinary blunders are made in ignorant practice, and trees for instance are planted in low-lying frost beds which would succeed in slightly higher situations, or seeds subject to damping-off are sown in beds rife with the spores ofPeronosporaorPythium, and so forth.

Many diseases, however, are not preventible in the present state of our knowledge, or prevailing conditions are such that the risk must be run of endemic diseases gradually becoming epidemic, and thus the natural desire for some means of checking the ravages of some pest or another has led to innumerable trials to minimise the effects by prophylactic measures. The procedure almost invariably followed where parasites are concerned, consists in either dusting the plants with some chemical in the form of a powder, or spraying it with a liquid, or occasionally in enveloping the plant in some gas, in each case poisonous to the insect- or fungus-pest. The principal rules to be observed are: (1) the poison employed must be sufficiently strong or concentrated to kill the parasite, but not sufficiently powerful to injure the host; (2) it must be applied at the right period, as suggested by a knowledge of the life-history of the fungus or insect in question.

Obviously it is of no use to apply such topical remedies to a parasite while it is spending the greater part of its life inside the tissues of the host. Further, questions of expense of the materials employed and of the labour of applying them help to limit the adoption of such measures.

Among the various kinds of powders employed, finely divided sulphur, or a mixture of sulphur and lime, have been used with success in some cases—e.g.against Hop mildew and other epiphytic Erysipheae, and against red spider, aphides, etc., the gaseous sulphur dioxide evolved being the efficacious agent. In other cases pyrethrum or tobacco powder, wood ashes, etc., have been employed against insects. Such powders are applied by hand or by means of bellows, and are very easily manipulated in most cases, though, like all such applications, the dangers of concentration at particular spots owing to uneven distribution, or of dilution and washing off by rain, have to be incurred.

Far more numerous are the various liquids which have been employed for washing, spraying, or steeping the affected parts of diseased plants. Water alone, or aqueous decoctions or emulsions of various kinds—e.g., quassia, tobacco, soap, or aloes, have been widely employed against insects such as green fly, red spider, etc. In greenhouses, where the leaves can be washed by hand or thoroughly syringed, and the concentration and time of action thoroughly controlled, such liquidsare often serviceable, but great practical difficulties are apt to interfere with their use in the open field.

The principal liquids employed against fungi have been copper sulphate and other metallic compounds (Bordeaux mixture, Eau Céleste, etc.), various compounds of arsenic (e.g."Paris green"), potassium sulphite, permanganate, etc., and emulsions of carbolic acid, petroleum, and such like antiseptics, for the exact composition of which the special treatises must be consulted. Some of these, especially Bordeaux mixture, have been experimented with on a very large scale, especially in America, and various forms of spraying machines have been introduced for dealing with large areas.

It is clear that these spraying operations are more particularly adapted to field crops such as Turnips, Hops, Vines, Potatoes, and to garden and greenhouse plants than to woods and plantations; as a rule they cannot be applied to forest trees—though they have been used in orchards—or to roots, seeds, and other parts in the soil, and many special forms of treatment have been devised for particular cases of these kinds.

One of the oldest of these is the steeping of grain in solutions of copper, or in hot water, just before sowing, and the practical eradication of Bunt and, partially, of Smut is due to this practice, which has lately been adapted to potatoes, the principle being that the parasitic germs shallbe killed while still adhering to the outside of the seeds, tubers, etc., before germination. "Finger and Toe" due toPlasmodiophorahas been successfully dealt with by the application of lime, but we do not know whether the effect is owing to indirect actions in the soil, to direct actions on the plasmodia, or to the increased production of root-hairs induced by liming.

Phylloxerahas been treated by plunging into the soil near the roots small blocks of some slowly-soluble medium, such as gelatine, impregnated with carbon-bisulphide, the volatile fumes of which kill the insect, and even more drastic remedies have been tried along similar lines. In America orchard trees infested with insects or fungi have been covered one by one with light tents, and the vapours of prussic acid, burning sulphur, and other poisons allowed to act inside the tent. In all such cases it must be remembered that uncontrolled ignorance of the properties of poisons on the part of the operator may lead to disaster, and the same applies to the much easier treatment of greenhouses, and cases where poisoned food is laid about for insects or vermin.

Attempts, not altogether unsuccessful on the small scale, have also been made to introduce epidemic diseases among rats, mice, and locusts and other insects, by inoculating some of them with parasitic bacteria or fungi (Empusa,Isaria, etc.), and then allowing them to run loose in the hope that they will communicate the disease to their fellows.The introduction of lady-birds into districts infested with Coccideae and similar pests which they devour, is also recorded as successful, as also the importation of birds into forests plagued with caterpillars. It must not be over-looked, however, that man's interference with the existing balance of events in the natural struggle for existence is occasionally disastrous, as witness the results of importing rabbits into Australia, goats into the Canary Islands, and sparrows in various countries. Darwin's well-known illustration of the inter-relations between clover, bees, field-mice, and cats (Orig. of Species, 6th ed., 1876, p. 57), which shows the astounding probability of the dependence of such a plant on the number of cats in the neighbourhood, well illustrates the situation.

Mere mention must be made of other special treatments.

Caterpillars and larger animals are often picked by hand or their natural enemies—e.g.birds, are encouraged in forests. Locusts are caught in nets, trenches, etc., and buried. Woodlice, slugs, etc., are often trapped by laying attractive food such as carrots and overhauling the traps daily: similarly with earwigs. Rings of tar round tree stems have been employed to prevent caterpillars creeping up them.

American Blight has been treated by rapidly flaming the stems. Syringing with hot water has also been employed for vines affected with mildew, mealy bug, etc.

With regard to the alleged immunity from devouring insects of certain poisonous plants, it has been pointed out thatPangium edule, which abounds in prussic acid, is infested with a grub, and ivy is occasionally eaten by caterpillars.

Another point as regards insect pests is the well-known destructive effect of a cold, wet spring on the young larvae. The use of cyanide of potassium requires especial care, but has been described as easily carried out with success in greenhouses.

It seems probable that lady-birds, the larvae of wasp-flies and lace-wings, and ichneumon-flies as well as wrens can keep down aphides.

For an example of the treatment of a complex case of "chlorosis" with mineral manures, the reader may consult theGardeners' Chronicle, 1899 (July), p. 405. Many similar cases have been recorded, but it should not be overlooked that very complex inter-relations are here involved.

Charlock has been successfully dealt with by applying 5 lbs. of copper sulphate in 25 gallons of water to each acre of land while the weeds are young.

In all these cases the guiding idea is derived from accurate knowledge of the habits of the insect, fungus, or pest concerned, and obviously the procedure must be timed accordingly. It is a particular case of the struggle for existence, where man steps in as a third and (so to speak) unexpected living agent.

It is clear from our study of the factors of an epidemic that one of the primary conditions whichfavour the spread of any disease is provided by growing any crop continuously in "pure culture" over large areas. This is sufficiently exemplified by the disastrous spread of such diseases as Wheat-rust, Larch-disease, Potato-disease, Phylloxera, Hop-disease, Sugar-cane disease, Coffee-leaf disease, and numerous other maladies which have now become historic in agricultural, planting, and forest annals. Providing the favourite food-supply in large quantities is not the only factor of an epidemic, but it is a most important one in that it not only facilitates the growth and reproduction of a pest, but affords it every opportunity of spreading rapidly and widely.

Moreover, Nature herself shows us that such pests are kept in check in her domain by the struggle for existence entailed by innumerable barriers and competitors. As matter of experience also it is found that rotation of crops, planting forests of mixed species, and breaking up large areas of cultivation into plantations, fields, etc., of different species afford natural and often efficient checks to the ravages of fungus and insect pests. Over and over again it has been found that a fungus or an insect which is merely endemic so long as it is isolated in the forest, where its host is separated from other plants of the same species by other plants which it cannot attack, becomes epidemic when let loose on the continuous acres so beloved of the planter. And the same reasoning applies to the success of such pests on open areas from which the birds or other enemies of the pesthave been driven. True, we cannot always trace the tangled skein of inter-relationships between one organism and another in Nature: the recognition of the principle of natural selection and the struggle for existence is too recent, and our studies of natural history as yet too imperfect to lay all the factors clear, but no observant and thoughtful man can avoid the truth of the general principle here laid down. The history of all great planting enterprises teaches us that he who undertakes to cultivate any plant continuously in open culture over large areas must run the risk of epidemics.

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