5. After an interval of from ten to fourteen days from the fourth spraying.
[107] 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.
[108] Powdery mildew (Uncinula necator (Schw.) Burr.) is caused by a fungus which lives on the surface of the leaves. It subsists by means of sucker-like organs which penetrate the walls of the surface layer of cells. The vegetative portion of the parasite consists of fine white filaments which spread over the surface of the leaves, shoots and fruit. In the summer these filaments send up short, irregular stalks upon which large numbers of barrel-shaped spores are produced in chain-like arrangement. These are the summer spores of the fungus. They are borne in greatest quantity on the upper surfaces of the leaves and give the leaf a gray, powdery appearance—hence the name, powdery mildew. Affected leaves finally become light brown and often fall. Diseased fruits are gray in color, scurfy, become specked with brown, fail to develop and often burst on one side thereby showing the seeds. The winter or resting spores are borne in sacs, in the latter part of the season. The spore sacs, in their turn, are borne in small, black, spherical spore cases, each furnished with a number of slender appendages having curled tips. The powdery mildew, unlike most other fungus troubles of the grape, is most prevalent in hot dry weather. The disease is combatted by dusting with flowers of sulphur or by spraying with bordeaux mixture as for black-rot.
[109] Anthracnose (Sphaceloma ampelinum De By.).—This disease attacks any of the tender portions of the growing vine. When the leaves are affected dark spots are first formed on their surface. As the disease advances these spots enlarge, and irregular cracks are often formed through the dead tissue. Frequently many of these small cracks run together, forming a long irregular slit through the leaf. Similar marks are formed on the tender shoots, though they are not so noticeable. When the fruit is attacked the disease is sometimes called bird’s-eye rot. Circular spots are formed on the surface of the berry. The spots may be of different colors and usually have a dark border; as the spots enlarge and eat in, a seed is often exposed in the center. In rotting the tissue becomes hard and wrinkled. Sometimes the disease girdles the stem of a fruit cluster, cutting off the supply of sap from the grapes beyond the diseased line and causing them to shrivel and die.
Anthracnose does not spread as rapidly as some other vineyard diseases, neither does it yield as readily to treatment. When a vineyard is badly infested with anthracnose, it requires prompt attention and a careful treatment to control the disease. It is not satisfactorily controlled by bordeaux mixture alone. It is suggested that in addition to such treatment with this mixture as is given for black-rot the plan be followed which is advocated by certain European authorities, of applying a warm saturated solution of copperas (iron sulphate) in spring when the buds are swelling but before they begin to open. One per ct. or more of sulphuric acid may be added to the solution before it is applied. This solution must be handled with care as it is very caustic. It is applied with swabs or if the acid is not used it may be sprayed. It is essential that the work be done thoroughly, covering all the surface of the canes.
[110] Chlorosis or yellow leaf.—The name is applied to a grape disease in which the foliage turns yellow, later becoming brown. It is common in several parts of the State but more particularly in the Central Lakes district. Chlorosis is more likely to appear in wet seasons. Some varieties, as the Diamond, are much more susceptible than others. In some seasons portions of the leaves may become yellow but eventually regain their normal color so that at the close of the season the vine appears to be in a healthy condition. In other instances the yellow color extends over the entire leaf; brown, dead patches appear; the leaf curls and eventually drops from the vine. If the vine loses its leaves two or three seasons in succession it is likely to die. One striking peculiarity of the disease is the fact that a badly diseased vine may appear by the side of a perfectly healthy vine of the same variety.
The cause of chlorosis, as given by foreign investigators, is the presence of a large amount of lime in the soil which prevents the roots from taking up an amount of iron sufficient for satisfactory growth. Their experiments seem to show that the difficulty may be overcome by applying a small amount of sulphate of iron around affected plants. But since there are a number of good American varieties that are not subject to chlorosis, perhaps the better method to pursue is to plant only such varieties as are known to be free from this trouble.
The standard varieties given in the following list are, so far as we know, practically exempt from chlorosis: Moore Early, Concord, Winchell, Delaware, Worden, Niagara, Catawba, Vergennes and Agawam.
[111] Tarr, R. S., The Physical Geography of New York State: 4. 1902.
[112] See Story of the Vine, E. R. Emerson: 198. 1902.