Part 11 (1/2)

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 that _Pangium 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 pota.s.sium 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 the _Gardeners' 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 which favour 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 quant.i.ties 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 compet.i.tors. 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 pest have been driven. True, we cannot always trace the tangled skein of inter-relations.h.i.+ps 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.

NOTES TO CHAPTER XVII.

The princ.i.p.al literature, now very voluminous, on this subject is contained in the publications of the U.S. Department of Agriculture from 1890 onwards. See especially _Bulletins_, Nos. 3, 6, and 9; _Farmers' Bulletin_, No. 91, 1899; and _The Journal of Mycology_ during the same period. See also Lodeman, _The Spraying of Plants_, London, 1896. A summary of the princ.i.p.al processes will be found in Ma.s.see, _Text-Book of Plant Diseases_, pp. 31-47.

With regard to the history of the subject, which still needs writing, the reader should not overlook Roberts, ”On the Therapeutical Action of Sulphur,” _St. George's Hospital Reports_, date unknown, but subsequent to the following: Berkeley, _Introduction to Cryptogamic Botany_, 1857, p. 277, with references. These are, I believe, with the references to steeping of wheat in De Bary, _Unters. uber d. Brandpilze_, Berlin, 1853, among the first attempts to utilise such remedies.

Further facts will be found in the pages of the _Gardeners'

Chronicle_, especially since 1890, and in _Zeitsch. f.

Pflanzen-krankheiten_ since 1891.

CHAPTER XVIII.

VARIATION AND DISEASE.

_Predisposition and immunity--Pathological conditions vary--Hardy varieties--”Disease-proof” varieties--Disease dodging--Thick skins--Indian wheats, etc. Cell-contents vary--Citrus, Cinchona, Almonds, etc. Double ideals in selection--Cultivation of pest and host-plant--Variations of fungi--Bacteria--Specialised races--Difficulties--Experiment only will solve the problems._

The numerous and often expensive failures in the application of any prophylactic treatment, have proved an acute stimulus to the research for other ways of combating the ravages of plant diseases. It is a matter of every-day experience that particular varieties of cultivated plants may suffer less from a given disease than others in the same district; also that one and the same species may suffer badly in one country and not in another--_e.g._ the Larch in the lowlands of Europe as contrasted with the same tree in its Alpine home, and the various species of American Vines in Europe.

These matters, in the hands of astute observers, are turning the attention of cultivators and experts to new aspects of the question of plant diseases, namely, the possible existence of immunity, and the breeding of disease-proof varieties; and the existence on the part of the host plant of predispositions to disease which may depend on some factors in the plant or in the environment over which it is possible to exercise control, or which, if known, can be avoided.

The matter is complicated by the recent demonstration of the fact that parasites also vary and can adapt themselves to altered conditions, as is shown by the history of the coffee-leaf disease (_Hemileia_) in Ceylon, and by Eriksson's results with Wheat-rusts (_Puccinia_) and various experiments with _Coleosporium_ and other Uredineae; but there are good grounds for concluding that hybridisation, grafting, and selection of varieties may do much towards the establishment of races which will resist particular diseases, as shown by Millardet's experiments with Vines, and the results obtained by Cobb and others with Wheat.

The great difficulty with so-called ”disease-proof varieties” is to test them under similar conditions in different countries, and for a sufficient period of time. A particular race of Wheat may behave very differently in Norfolk, Devons.h.i.+re, and Northumberland, and the recent introduction of the purely experimental method in this connection is a marked advance. However rough the experiments may of necessity have to be, it is only by such means that data can be gradually acc.u.mulated.

Having now obtained some insight into the factors concerned in disease, let us enquire further into the bearing of variation on these. It is evident that pathological conditions may vary; indeed they are themselves symptoms of variation, as we have seen. The history of all our cultivated plants shows abundantly that many of the variations obtained by breeding in our gardens, orchards, fields, etc., involve differences of response on the part of the plant to the very agencies which induce disease. Every year the florists' catalogues offer new ”hardy” varieties; but a hardy variety is simply, for our present purpose, one which succ.u.mbs less readily to frost, cutting winds, cold damp weather, and so forth. If anyone doubts that hardy varieties have been gradually bred by selection, I refer him to the evidence collected by De Candolle, Darwin, Wallace, Bailey and others. When we come to enquire into the causes of ”hardiness,” however, difficulties at once beset us. The adaptation may express itself in a difference in the time of flowering or leafing, the exigencies of the season being ”dodged,” as it were, in a manner which was impossible with the original stock, as appears to have occurred with Peaches in America; or it may be expressed in deeper rooting, as is said to be the case in some Apples, or in the acquirement of a more deciduous habit, or in actually increased resistance to low temperatures. In such cases we cannot trace what alterations have occurred in the cells and tissues concerned, though we may be sure that some changes do occur.

No experienced cultivator doubts that some varieties of Potato, Wheat, Vine, Chrysanthemum, etc., suffer more from epidemic diseases than others, and our yearly catalogues furnish us with plenty of promises of ”disease-proof” varieties. Here also we may imagine several ways in which a particular variety may resist or escape the epidemic attacks of fungi which in the same neighbourhood decimate other varieties. If we could breed a variety of the Larch which opened its buds later than the ordinary form in our northern plains, the probability of its escaping the Larch-disease would be increased in proportion to the shortness of the period of tender foliation described on p. 153. It has been claimed for certain varieties of Wheat that increased thickness of the cuticle and fewer stomata per square unit of surface have diminished the risk of infection by Rust fungi, and for certain varieties of Potato, that the thicker periderm of the tuber protects them against fungi in the soil.