Fig. 44. Work of black-rot of the grape.
Since the disease is carried through the winter in mummied fruits and diseased wood, the desirability of destroying these mummied grapes and the leaves and prunings of infected vines as soon as possible is apparent. This treatment, however, is not sufficient, and the disease can be effectually controlled only by thorough spraying with bordeaux mixture (4–4–50). The first application should be made just before the grape blossoms; the second, shortly after blossoming. The amount of material applied matters less than evenness in distribution and fineness of the spray as applied. In rainy seasons, perhaps a third or a fourth application should be made in regions where the disease is serious; the third is made when the berries are the size of a pea; the fourth, as the berries become large enough to touch each other.
Downy-mildew.
Downy-mildew (Plasmopara viticola) rivals black-rot for first place among fungous diseases of the grape. It is found in all grape regions east of the Rocky Mountains but does most harm in northern localities. Like black-rot, downy-mildew attacks all the tender growing parts of the vine, but is chiefly found on the foliage and is usually less destructive than black-rot. As first seen on the foliage, the work of the fungus appears as greenish-yellow, irregular spots upon the upper surface which later become reddish-brown. At the same time on the under surface of the leaf, a thin, white downy growth puts forth. The spores of the fungus are produced on this downy growth, and under favorable conditions are distributed by wind and water to all tender parts of the vine, where they germinate and begin their work of destruction. The fruit is attacked when partly grown, as shown in [Fig. 45], becoming covered with the gray down of the fungus, the "gray-rot" of the grape-grower. If the berries escape the disease until half grown, the fungus causes a brownish-purple spot that soon covers the whole grape, giving the disease at this stage the name of "brown-rot." Besides the summer-spores, another form of reproductive bodies is produced in the winter to carry the fungus through the resting period.
Fig. 45. Grapes attacked by downy-mildew.
Downy-mildew, like black-rot, spreads most rapidly and does most injury in hot, wet weather. As with practically all diseases of the grape, much can be accomplished in the way of control of the disease by destroying infested leaves, shoots and berries which contain the winter spores, but these sanitary measures are not sufficiently effective and vineyards must be sprayed as recommended for black-rot, except that the first application should be made before the blossom-buds appear.