CHAPTER XVI.
THE FACTORS OF AN EPIDEMIC.
_Ill.u.s.trations afforded by the potato disease--The larch disease--The phylloxera of the vine._
When we come to enquire into what circ.u.mstances 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 a dangerous one: any pest may become epidemic if the conditions favour it!
In 1844 and 1845 the potato disease a.s.sumed an epidemic character so appalling in its effects that it is no exaggeration to say that it const.i.tuted 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 conidioph.o.r.es of the _Phytophthora_--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 ma.s.s 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 where potatoes 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 superst.i.tions of earlier days. We must remember that prior to 1860 the life-history of _Phytophthora_ was 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 princ.i.p.al 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 acc.u.mulation of sugars in the tissues, and the partial etiolation of the latter implies less resistance to the entering hyphae, may be pa.s.sed over here, but in any case it is clear that we have several factors of the non-living environment here 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 pa.s.sage 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 is gone, and the tree pa.s.ses 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 alt.i.tude 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 factors of 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 ill.u.s.tration 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 that _Phylloxera_ has 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 quant.i.ty 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 has also been shown that hybrids can be obtained which are resistant.
But the most curious point of all is that _Phylloxera_ was 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 const.i.tution, almost immune from injury, or are least liable to attack ... the _Phylloxera_ finds 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 "const.i.tution" of the European vine that the want of resistance to _Phylloxera_ resides, 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 because _Phylloxera_ has spread there also. It must not be overlooked, however, that we are as yet very ignorant of all that is implied in the word "const.i.tution" as used above.
If we enquire further why the _Phylloxera_ epidemic 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.
NOTES TO CHAPTER XVI.
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 Gegenwartig herrschende Kartoffel Krankheit_, etc., Leipzic, 1861; and the pages of the _Gardeners" Chronicle_ from 1860-1900.
For the Larch disease he should consult Hartig, _Unters. aus der Foist. Botanischen Inst. Munchen_, B. I., 1880; and Willkomm, _Microscop. Feinde des Waldes_, B. II., 1868.
For _Phylloxera_ the literature is chiefly in the _Comptes Rendus_ and 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, and _Timber 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 "Ill.u.s.trations of 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.
CHAPTER XVII.
REMEDIAL MEASURES.
_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 some things he must avoid in dealing with certain plants, or otherwise they will not succeed; in other words they will succ.u.mb 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 of _Peronospora_ or _Pythium_, 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 princ.i.p.al 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._, qua.s.sia, 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 liquids are often serviceable, but great practical difficulties are apt to interfere with their use in the open field.
The princ.i.p.al liquids employed against fungi have been copper sulphate and other metallic compounds (Bordeaux mixture, Eau Celeste, etc.), various compounds of a.r.s.enic (_e.g._ "Paris green"), pota.s.sium 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 s.m.u.t is due to this practice, which has lately been adapted to potatoes, the principle being that the parasitic germs shall be killed while still adhering to the outside of the seeds, tubers, etc., before germination. "Finger and Toe" due to _Plasmodiophora_ has 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.
_Phylloxera_ has 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 ill.u.s.tration 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 ill.u.s.trates 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.