Page:Popular Science Monthly Volume 78.djvu/479

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LANTS are subject to disease. As in the human being these diseases decrease vigor and productiveness of the organism or cause death. An attack upon valuable plant products such as ripe fruit, tubers and root crops, and mature timber may result in depreciation in value or even entire loss of the product. The rot of apples upon the tree or in the bin, the common blue mold seen upon lemons and oranges, the wide-spread blight of pear and apple twigs (Fig, 1) are familiar examples of plant disease.

In the early years of American history these afflictions were regarded as natural and inevitable, but during the last three or four decades scientific study has shown that rots, blights, molds, mildews, rusts, smuts, etc., are true diseases; that they do not constitute any part of the normal life stages of the plant affected. That they are caused by living parasites and, moreover, that they are often preventable.

Plant diseases have increased largely in number and destructiveness during recent years. This is due in part to the migration of disease from county to county and state to state; in part to the cultivation of weak or susceptible varieties and in part to long continued cropping in a given region, thus affording opportunity for the plant pests, which may at first have been weak and unimportant, to become thoroughly established and aggressive.

The asparagus rust, which has in some states nearly prohibited asparagus culture, offers an excellent example of migration in its progressive westward march across our continent. This invasion seems to have occupied the years between 1896 and 1902 since the rust was first noted at New Jersey in 1896, South Carolina 1897, Michigan 1898, Illinois 1899, Dakota, Nebraska and Texas 1900, California 1901 or 1902.

The destructive pear blight, our worst pear disease, made a similar journey, starting from the neighborhood of the Hudson valley, near the beginning of the last century, reached the Rockies sometime after 1886 and arrived in California about 1895.

Diseases have come to America from Europe. Grape anthracnose, cabbage club root, potato wart (Fig. 2), are noteworthy examples; similarly,