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 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 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 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 liquids are 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 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 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 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 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 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 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 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 principal 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 principal processes will be found in Massee, 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. über 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.
