With the nineteenth century, and the founding of the modern theories of nutrition by Ingenhousz, Priestley, and De Saussure, we find a new era started. As the discoveries of the microscopists continued to build up our knowledge of the anatomy of plants and began to elucidate the biology of the fungi and other cryptogams, while the chemists and physiologists laid the foundations of our modern science of plant life, it gradually became possible to tabulate and classify plant diseases, and discuss their symptoms and causes in a more scientific manner. Even in 1833, however, Turpin, and a far better observer, Unger, regarded parasitic fungi as due to diseased outgrowths of chlorophyll-corpuscles and parenchyma cells, views shared by Meyen (1837) and Schleiden (1846). We may pass over the various treatises of Wiegmann (1839), Meyen (1841), Raspail (1846), Kühn (1859), and a number of other works of the period, merely referring with emphasis to Berkeley's admirable papers in the Gardener's Chronicle (1854) for a summary of what was then known. All these works antedate De Bary's Morphologie und Physiologie der Pilze, etc. (1866), in which he brought together the results of his researches during the decade, proving the real nature of parasitic diseases and infection as worked out by experiments between 1853 and 1863.
This work put the whole subject of parasitic diseases of plants and animals on a new footing, and paved the way for the modern treatment of plant pathology as elaborated in the treatises of Frank (1880 and 1895), Sorauer (1886), Kirchner (1890), and others, to which the reader is referred for further details. I will merely quote the following passage from Raspail's Histoire Naturelle de la Santé et de la Maladie, 1846 (vol. ii., p. 176), in illustration of the views entertained by high authorities just prior to De Bary's work: "L'insecte qui produit les erineum, uredo, æcidium, xyloma, puccinia, n'est donc plus pour nous un insecte inconnu, mais un acarus (grise), un aphis (puceron) ou un thrips, qui produit au printemps une déviation, etc."
And this view, that fungi already well known to mycologists were called forth by the punctures of insects, was regarded as not out of harmony with the idea that the fungus itself was an abnormal outgrowth of the tissues of the host.
The proper study of plant pathology presupposes and involves a knowledge of the physiology of plants, of the normal relations of the latter to their environment, and of the biology of those animals and plants (principally insects and fungi) which are parasitic on them. It is of the first importance to understand that a disease is a condition of abnormal physiology, and that the boundary lines between health and ill-health are vague and difficult to define. As with the study of the diseases of man and other animals, so with those of plants, the practice resolves itself into the accurate observation and interpretation of symptoms (Diagnosis) on the one hand, and of causes (Aetiology) on the other, before any conclusions of value can be drawn as to preventive or remedial measures (Therapeutics). In plants, however, symptoms of disease are apt to exhibit themselves in a very general manner, or at any rate it may be that our perceptions of them differentiate symptoms due to very different reactions imperfectly, probably because the organisation of the plant is less specialised than that of animals. The turning yellow and premature falling of leaves, for instance, is a frequent symptom of disease; but it may be due to a long series of different causes of ill-health—e.g. drought, too high or too low a temperature, light of insufficient or of excessive intensity, a superfluity of water at the roots, the presence in the tissues of parasitic fungi, or that of worms or insects at the roots or elsewhere, poisonous gases in the air, soil, etc., and so forth. Consequently the science of plant pathology is much concerned with the direct action of external causes, which are probably less obscure than in the case of animals, though by no means always obvious. Such considerations at any rate seem to account for the fact that most authorities on plant pathology base their classification on the causes of disease, there being few noteworthy exceptions.
Notes to Chapter IX.
The bibliography here quoted will be found in Berkeley, "Vegetable Pathology," Gardener's Chronicle, 1854, p. 4; Plowright, British Uredineæ and Ustilagineæ, 1889; Eriksson and Henning, Die Getreideroste, Stockholm, 1896; De Bary, Comparative Morphology and Biology of the Fungi, etc., 1887; Frank, Die Krankheiten der Pflanzen, 1895-96, and scattered in the works referred to in them and in the text.
