Under one head it is thought best to bring together the discussions of the so-called enemies of plants, - the parasitic fungi and the depredating insects, together with the means of control. This composite article therefore comprises:

Disanthus cercidifolius. (X 1/3)

Fig. 1278. Disanthus cercidifolius. (X 1/3)

Page

Diseases due to parasitic fungi....

1021

Fungicides, or remedies for these diseases....

1027

Catalogue of diseases, with advice...........

1029

Insects and their depredations on plants......

1034

Insecticides and fumigation......

1042

Catalogue of insect depredators, with advice......

1047

Spraying........

1057

The reader now has before him a comprehensive survey of the subject. It is impossible, of course, to list all the plant diseases and all the insect pests in a compilation of this kind; but it is desired that the catalogues shall comprise the most important depredators of the leading horticultural plants. The reader should keep himself informed of the new knowledge and new practice by consulting current publications of the government and the experiment stations.

Diseases Of Plants

Disease in plants may be defined as any derangement or disorganization of the normal structure or physiological functions of the plant, as for example the formation of galls, cankers or distortions, rotting of plant parts, or disturbances in the sap system resulting in wilting, or in the nutritive processes resulting in such symptoms as dwarfing, chlorosis, and the like. Forms of plant diseases are shown in Figs. 1279-1292.

It is often very difficult to distinguish clearly between diseased conditions and abnormalities of other types. Bud-sports, doubling of blossoms, fasciations and many other similar abnormalities, while often the result of reaction to some pathogen, are not apparently always so and they are often spoken of as teratological phenomena. While the reaction of plants to insect attacks in the formation of galls, cankers, and so on, is to be regarded as symptom of disease, the injuries produced by the mere eating away of parts of leaf, stem or fruit are not usually so to be regarded. Even here, however, it is often difficult to draw a sharp line of demar-kation. While disease may usually be said to result in ultimate injury, there are apparently certain marked exceptions, as in the case of the root tubercles of legumes caused by the attacks of certain nitrogen-fixing parasitic bacteria. Here increased growth and crop-yield are generally held to result.

Diseases of plants are not something new or of recent development, as the grower is often inclined to think. The crops of the husbandman, from the earliest recorded history of his art, have been afflicted with diseases. In the historical writings of the Hebrews, the Bible, and in the writings of the Greeks and Romans, frequent mention is made of such diseases as rusts, smuts and mildews of grain and canker of trees. To be sure, the extensive and intensive crop-cultivation of modern times, together with the extraordinary worldwide transportation and exchange of crop-products, have greatly favored the distribution of plant pathogens (insects, fungi and bacteria), and afford them exceptional opportunities for destructive development. Nor are cultivated plants alone subject to disease. Disease epidemics among weeds and the wild flowers of the woods may be observed any season in localities in which weather conditions especially favor the causal organisms.

The study of the nature and control of plant diseases, however, is of recent development. The first man really to study plant diseases from the true modern economic point of view, that is, with the object of helping the grower to understand and combat or control diseases in his crops, was Julius Kuhn. This German, the son of a German land-owner and for many years himself the manager of a large agricultural estate, was the founder of an early German agricultural college. He interested himself, among other phases of agriculture, in plant diseases and their control and his book, "Die Krankheit der Kulturgewachse," published in 1858, is to be regarded as the first book of real economic importance on the subject of diseases in plants. In this remarkable volume is given a concise statement of the thoroughly digested and personally tested knowledge of his time, on the nature and control of plant diseases. He also describes a number of new methods, especially for seed treatment of cereals against smuts, which have become the foundation for many of our present-day practices.

Since Kuhn's day there have been remarkable developments in the control of plant diseases. The discovery of bordeaux mixture by the Frenchman Millar-det in 1882; the discovery of the formaldehyde treatment of seed for smut by the American plant pathologist, Arthur, in 1896; and the recent development of the use of lime-sulfur solutions and mixtures as a substitute for bordeaux in the spraying of apples and peaches, are but the most noteworthy of the many discoveries and developments in the remarkable growth of this economic science within the last half century.

The economic importance of plant diseases can scarcely be overestimated, as they constitute one of the chief losses in our agricultural resources. The loss from 5 to 25 per cent of many crops from diseases alone each year is so common as to be the general rule. The loss from potato diseases each season in the United States has been carefully estimated at not less than $36,000,000. Yet, it has been conclusively demonstrated by extensive experiments among potato-growers during a continuous period of ten years, that an annual average increase of over forty bushels per acre may be expected from spraying the crop with bordeaux mixture, from three to five times in the season at a total average cost of about $5 per acre. The loss from oat-smut commonly averages from 5 to 25 per cent of the crop, yet it may be absolutely prevented by seed treatment at almost insignificant cost. The loss from scab in the apple crop of New York State often totals not less than $3,000,000 and for the United States a corresponding loss of over $40,000,000. In 1900, the peach-growers of Georgia lost $5,000,000 by brown rot, while the average annual loss from the same disease in the entire United States is never less.