Page:Popular Science Monthly Volume 42.djvu/663

Rh the disease its other common name of downy mildew. This fungus reproduces by means of minute particles called spores, corresponding in function to the seeds of higher plants. If one of these spores lights upon a moist leaf, its inner contents divide into a number of distinct particles, which soon escape through an opening in the spore wall; then each particle swims about in the film of water for a short time—resembling a little animal—when it becomes quiet and sends out a minute tube which penetrates the skin of the leaf. It then continues to develop inside the leaf, pushing about between the cells, and forming the mycelium or vegetative portion of the fungus. As there is little nourishment to be found between the cells, this mycelium develops minute processes, which push through the cell walls and absorb the cell contents. A small section of an affected leaf, greatly magnified, is represented at Fig. 5, the unshaded double-walled spaces showing the leaf cells, the shaded part between the walls the mycelium of the fungus, and the projections a, a, the processes or suckers that penetrate the cells. When these vegetative portions of the fungus have developed in the leaf to a certain extent, they send out through the breathing pores or stomata their fruiting branches, which bear upon their tips the small oval spores (Fig. 6). These fruiting branches form the so-called mildew on the plant, and, as they only appear under certain atmospheric conditions, the mycelium may exist in the vine for some time before this outward manifestation of its presence is seen. On this account a whole vineyard sometimes appears to be invaded by the mildew in a single night.

From the above description it will be readily seen that this fungus can not be successfully combated after it has established itself within the tissues of the host. To prevent its injuries one must also prevent its ingress to the plant. Fortunately, this can be done by coating the green parts of the vine with some substance having a destructive effect upon the spores of fungi. The salts of copper have such an effect, and in consequence they have come into general use as fungicides. They were first experimented with on a large scale in the vineyards of France, and gave such satisfactory results that they were adopted in a practical way by many growers. In America this use for them has hardly been