DISEASES OF PLANTS form a subject of study interesting equally in its scientific and its economic or practical relations, but in regard to the most important parts of which much obscurity and uncertainty still exists. Enough, indeed, is known to show that, as might have been expected, an analogy subsists between the kinds of disease to which plants are subject, and those of animals, both in their nature and their causes, yet with wide differences, according to the difference. between vegetable and animal life. Plants, like animals, are liable to suffer from unsuitable external circumstances, as of temperature, drought, or moisture, etc. : they are liable, like animals, to suffer from deficiency of food, from excess of it, or from being compelled to subsist on improper kinds of it, or too exclusively on some particular kind. They often suffer much from vegetable parasites, chiefly fungi, and from multitudes of minute animals, which, with out eating them up, destroy organs essential to their health, or prey upon their juices. The constitutions of plants are accommodated to particular temperatures, and they neither flourish when the temperature is for any considerable time much above or much below certain limits, very different, however, for different species. Light is of the greatest importance to vegetable life, and a want or deficiency of it speedily pro duces an unhealthy condition, the proper chemical changes not taking place in the juices of the plant; and this unhealthy condition, sometimes very extensively produced in gloomy seasons, when the deficiency of light is accompanied with excess of moisture both in the air and in the soil, renders plants very liable to the attacks both of vege table parasites and of minute animal tribes.
Excess of nutriment, causing an extreme rapidity of growth, sometimes produces an unhealthy condition in particular parts of plants, in which a greater amount of tissue is developed iu a single season than can be thoroughly matured.—Manures, injudiciously and unsuitably applied, are often productive of Putrescent matter coming in contact with the roots of many plants, is very injurious to them, and causes canker.— Contagion, as a cause of disease in plants, if not fully demonstrated, is rendered highly probable by such facts as the memorable prevalence of the potato disease, and the rapid spread of the vine disease (Mium); nor does the existence of particular fungi in the diseased plants materially affect this probability.
With regard to the diseases of plants generally, little has been hitherto found practi cable in the way of cure, and prevention is the object chiefly aimed at in all investiga tions of their nature and causes.
Some of the most important diseases of plants are noticed under particular heads; and some-of the most destructive parasitic fungi are described in their proper places in this work.