Page:Popular Science Monthly Volume 43.djvu/88

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78
THE POPULAR SCIENCE MONTHLY.

kind of plant as the apple tree upon which it thrives. It is not confined to the fruit, but grows luxuriantly upon the foliage, causing it to become blotched with the brown patches and otherwise destroyed. The mold consists of fine, cobwebby threads, which penetrate the leaf and rob it of nourishment, and after a time form patches upon the surfaces, where innumerable spores of a dark color are produced.

The apples are first attacked by the scab fungus when they are quite small, probably while the tree is in blossom, or shortly after. At that time the surface of the young fruit is tender and has no well-developed skin, which, when the fruit nears maturity, might be so tough as to prevent the entrance of the scab mold. This, therefore, is a defect that does not come upon the fruit after harvest, and usually does not spread much after the apples are in the barrel.

The knowledge of the fact that the scab is due to a mold that begins to infest the fruit in early summer has led to experiments

PSM V43 D088 Apple scab.jpg
Fig. 2.—Apple Scar.

in spraying the trees during the growing season with the Bordeaux mixture and other fungicides, with marked success in checking its ravages. Trees sprayed three or four times in May or June have borne abundant fruit comparatively free from scab, while unsprayed trees otherwise alike yielded a scant amount of distorted, scabby, withered apples. Fig. 2 shows an apple that is a fair illustration of the working of the scab fungus.

One of the most interesting things in connection with the study of the decays of apples is the relation which one mold bears to another. There are several very common kinds of molds.