Preventable 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 existenice.
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 preventable 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 Preventable 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 Celeste, 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.