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.
