PATHOLOGY OF PLANTS Plant pathology (or Phytopathology) is that branch of bio logical science which treats of the diseases of plants. While there can be no doubt that diseases of plants, and in particular of culti vated plants, have manifested themselves from time immemorial, it is only within comparatively recent years that their study has been taken up in an organized and scientific manner. The appli cation of the scientific method generally to the study of practical agricultural problems is a development scarcely older than the 19th century.
xxviii., 22, I Kings viii., 37) under such terms as "blasting" and "mildew." The precise nature of the diseases referred to cannot now be determined but there is little doubt that some of the epi demic diseases of cereals, such as rust or mildew, are indicated. The ravages of insects are frequently noted, such as for instance plagues of locusts, caterpillars and canker-worms.
In Greek literature we find references to plant diseases in the writings of Aristotle, and more particularly in those of his pupil Theophrastus, the father of botany. The latter describes diseases of a number of cultivated crops, such as various legumes, cereals, fig, olive, etc., and observes that varieties of the same crop may show different degrees of resistance to disease. This is probably the first mention of the idea of "resistant varieties" which figures so largely in present day writings on the subject. Among Roman writers the younger Pliny, who flourished in the 1st century of our era, devotes some attention to the subject and makes a num ber of well-founded observations regarding the incidence of cer tain plant diseases.
The middle ages were for plant pathology, as for all other sci ences, dark ages and we have to come down to the Renaissance, with its general quickening of interest in all intellectual pursuits, to find evidence of further progress. The 17th and ISth centuries wit
nessed considerable activity in the study of plant diseases. The new studies proceeded in the main from two different directions.
The practical gardener or agriculturalist was chiefly concerned in describing the various diseases to which his crops were subject and in attempting to evolve methods of treatment. On the other hand, the student of human pathology was interested in the dis eases of plants from the possible analogies to his own problems which such a study might offer. The whole science of botany had in the old days a distinct medical bias. It is not surprising, therefore, that the study of plant diseases has been to a large extent modelled upon the older science of human pathology, the terminology of which has been freely borrowed.
The elucidation of the real nature of plant disease, from the point of view of the causal agent, is mainly dependent upon the perfection of the microscope. But even the extended powers of observation which the use of that instrument brought to bear on the subject did not at once lead to a correct appreciation of the true nature of plant disease. Various structures were observed and accurately figured in detail as being associated with the dis eased parts, but they were interpreted as portions of the plant which had become changed by the disease and not as belonging to a distinct organism which was responsible for the disease. The first incontestible proof that a specific plant disease was due to the invasion of the tissues of the plant by a definite parasitic organ ism was furnished by the German scientist, De Bary, whose pio neer researches prepared the way for a host of other workers, at first chiefly in Germany, and later in all civilized countries. The loth century has witnessed an extraordinary activity in the study of problems of plant disease, most strikingly, perhaps, in the U.S.A. The economic importance of the subject, as a branch of scientific agriculture, has become increasingly recognized and the prosecution of plant disease researches is being more and more taken up by Government departments in all countries possessing a progressive agriculture.