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-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.

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, Devonshire, 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.

That certain thick-skinned Apples, Tomatoes, and Plums pack and store better than those with a more tender epidermis seems proved--that is to say, they suffer less from fungi which gain access through bruises and other wounds; but it cannot be said that any convincing proof is yet to hand explaining in detail why some races of wheat resist Rust, or why the roots of American Vines suffer less from _Phylloxera_ than others.

One of the most extraordinary cases known to me in this connection is the unconscious selection on the part of native Indian cultivators, perfectly ignorant of the principles involved, of spring and autumn forms of Rice, Wheat, Castor Oil, Sugar Cane, Cotton, and other crops.

"It has been estimated that Bengal alone possesses as many as 10,000 recognisable forms of rice." Now there is not the slightest ground for doubt that these have been unconsciously bred from the semi-aquatic native species during the many centuries of Indian agriculture, and nevertheless they have, among other peculiar races, some hill-breeds which they cultivate on dry soils and without direct inundation. That is to say, they possess tropical and temperate races differing far more than our spring and summer wheats.

Something has been gained, then, if we can show that there is nothing absurd or hopeless in the search for disease-proof or resistant races, and I think this can be done. We must not forget that the ideal usually set before himself by a breeder of plants has. .h.i.therto been almost exclusively some standard of size, form, colouring, and so forth, of the flower, or of taste and texture of the fruit, tuber, etc., though experiments with _Cinchona_, with brewery yeasts, and other plants remind us that variations in other directions have been attended to also.

Now it is obvious that in breeding sour limes and sweet oranges the cultivator is selecting, and intensifying by selection, very different metabolic processes in the cell: he can test the results of these, and so the selection proceeds.

The question is, Could he select at the same time those variations in cell activity which express themselves in properties of the flower, fruit, foliage, etc., he desires, as well as such variations as aid the cells in repelling fungi, insects, or exigencies of the non-living environment?

That more or less disease-proof varieties could be selected if that object alone were kept in view can hardly be doubted; plenty of examples exist already which show that the necessary variations to work upon exist in just those secretions of protoplasm, etc., which we have seen are concerned in repelling or attracting parasites.

The Sweet Almond has lost the power of producing amygdalin and prussic acid in its cells; Cinchona plants vary immensely in the quant.i.ty of quinine formed, and in European hot-houses may even form none at all; some varieties of Maize have sugar and dextrine instead of starch in their endosperms, or coloured instead of clear sap in the aleurone layer, and recent researches prove that they can transmit these peculiarities to hybrid offspring; non-poisonous bacteria have frequently been got from poisonous species simply by cultivation under special conditions, and pigmented forms can be bred into non-pigmented races.

But we see that the difficulty of selection is increased in the case postulated above, because two ideals are to be worked up to, and they may conceivably be incompatible. Not necessarily so, however, for breeders have solved such problems before in obtaining early _and_ heavy cropping races of potatoes, wheat, etc., sweet _and_ large grapes, strawberries, etc., hardy _and_ brilliant flowers, and so forth.

There is, however, another aspect of this question of variability in organisms in this connection to be considered. Ever since cultivation began man has probably been cultivating not only the crops he desires, but also the pests which infest them, and if variation of his chosen plants occurs--and no one will deny that--surely variation of the fungi and insects which live on them also takes place. That this is so can be demonstrated, though, since it is not part of my theme to go into the question of peculiarities of species and races of parasites, the subject must here be pa.s.sed over with a few remarks only.

Recent researches have shown not only that fungi vary immensely in form and morphological characters according to the amount and kind of food-materials put at their disposal, thus bringing the whole question of polymorphism into the domain of experimental physiology, but that their capacities for infection, spore formation, etc., are also capable of variation and are dependent on the quality and quant.i.ty of food supplies, water, as well as on the temperature, illumination, and other factors of the environment. This is true of parasites as well as of saprophytes. _Botrytis_ forms conidia only in darkness and in moist air.

Klebahn found that a _Puccinia_ growing on _Digraphis_ infected _Polygonatum_ readily and completely, _Convallaria_ imperfectly, whereas if sown on _Majanthemum_ it only just infected the plant and then remained sterile, while it refused to infect _Paris_ at all. Magnus has shown that _Peronospora parasitica_ can only infect meristematic tissues, and that when it co-exists with _Cystopus_ on _Capsella_, as is usually the case, it enters the latter plant by infecting the gall-like pustules of hypertrophied tissue induced by that parasite. Numerous parasitic fungi can only penetrate particular parts of plants. For instance, the _Ustilago_ of wheat can only infect the young seedling, and grows for weeks as a barren mycelium, only becoming a dominant fungus in the endosperm. Numerous other examples could be given, but these suffice to show some of the ways in which the nature of the food substratum supplied by the host affects the fungus. It is obvious that if the nature of this food changes, the fungus is also affected, and no doubt this is the princ.i.p.al reason why Rust-fungi, for instance, vary so much in their vigour and reproductive power on different wheats and gra.s.ses, though the other factors of the environment must also be of influence on them as well as on the hosts.

But--and this is the second point--modern research is also showing that the various species of Rust-fungi have split up into different varieties or specialised races, according to the particular host plants they inhabit. For instance there are special varieties or races of the particular species known as _Puccinia graminis_, the wheat rust, each of which grows well on various kinds of grain and gra.s.ses but refuses to infect others. Thus, the variety which infects Wheat refuses to infect Barley or Oats, while that variety which grows on Rye will not take on Wheat and so forth. Now it is important to notice that these specialised races are indistinguishable one from another by their visible microscopic characters: they are all botanically of the species _Puccinia graminis_ which forms its aecida on the Barberry. We must therefore conclude that we have here the same phenomenon as that met with in culture-races of bacteria which, having been fed for several generations on media rich in proteids, refuse to grow on media rich in carbohydrates, or when attenuated races are developed by culture under special conditions.

Now since such physiological races as I have described are by no means confined to _Puccinia_ but are also known in _Melampsora_, _Gymnosporangium_ and other fungi, we must conclude from this and from what we know of variation in plants and animals generally, that variation and adaptation are common among parasites, insects as well as fungi.

These considerations will serve to show moreover that the question of breeding disease-proof varieties of our cultivated plants is complicated by the danger of our breeding at the same time adapted races of their pests. It appears at first sight extremely improbable that we should escape the danger by breeding from those specimens of our plants which have best survived a fungus epidemic. Still, it must not be forgotten that "hardy varieties," and races adapted to other exigencies of the non-living environment, have been bred by selection--and nevertheless this variable non-living environment is always with us. The matter is therefore simply and solely one of experiment, and the retort that a disease-resisting variety of any particular plant has not yet been raised is no more valid than the objection that a true blue primrose has not yet been obtained: whether the same remark can be made with regard to any hope of a _disease-proof_ plant may be another matter, but in any case it must be made more cautiously in the light of our present experience.

NOTES TO CHAPTER XVIII.

The reader will find more on this subject in Bailey's _Survival of the Unlike_ and the literature quoted in the notes to Chapter VIII.

For varieties of Indian Wheats, etc., see Watt, _Agricultural Ledger_, Calcutta, 1895.

For a discussion on so-called "Disease-proof Wheats" consult Eriksson & Henning, _Die Getreideroste_.

Magnus' paper is in the _Berichte der Deutschen bot.

Gesellsch._, 1894, p. 39.

Concerning physiological races and adapted varieties of _Puccinia_, etc., see Eriksson, "A General View of the Princ.i.p.al Results of Swedish Research into Grain Rust,"

_Botanical Gazette_, vol. 25, 1898, p. 26.

For an account of Wheat-rust see Marshall Ward, "Ill.u.s.trations of the Structure and Life-history of _Puccinia graminis_, etc.," _Ann. of Bot._, 1888, Vol. II., p. 215.