Part 10 (1/2)
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 relations.h.i.+ps 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.