Note 107 — Chapter III · THE VITICULTURE OF NEW YORK

Downy mildew (Plasmopara viticola (B. & C.) Berl. & De Toni) is a troublesome fungus attacking all of the tender growing parts of the grape. It does most damage to the leaves, upon the upper surface of which it produces greenish-yellow spots of irregular outline. At the same time a loose white downy growth appears on the under side of the leaves. This growth consists of short filaments bearing spores, the summer spores, which are carried by the elements to other growing parts of the plant, thus spreading the disease. Affected berries, if young, first show a brown spot, and become covered with the gray down which distinguishes the fungus. On older berries the fungus causes a brown-purple spot which spreads until it takes in the whole berry, which then becomes soft and often falls, or they may become hard and persist. At this stage the disease is commonly known as “brown rot”. The winter, or resting, spores are produced in the tissue of fruit and leaves and with a thick protective covering. The winter spores are dark, almost black, in color. Downy mildew spreads and does most damage in hot wet weather. Spraying with bordeaux mixture as indicated for black-rot will keep downy mildew in check.