Ills of the American Bunch Grapes

Alvin J. Braun.

The American bunch grapes and muscadine grapes are grown almost exclusively in the eastern United States because the climate there is unfavorable for the commercial production of most of the European varieties. The American bunch grapes are either the selections from natural crosses or hybrids produced by crossing native species with the vinifera, or European, grape.

The principal eastern grape-producing States are New York, Ohio, Michigan, and Pennsylvania. In New York State, which produces about one-third of the grapes grown in the Eastern States, the value of the grape crop and grape products was about 13 million dollars in 1952.

The bunch grapes are used mainly for unfermented grape juice, wine, jelly, and fresh fruit. The Concord has been the leading variety for a long time, but interest has grown in varieties suitable for making champagnes and high-quality wines. For that purpose the Catawba and Delaware varieties are most generally grown now. The introduction of promising French hybrids may change preferences as to varieties in the next few decades.

Diseases have been largely responsible for limiting the areas of commercial production. Black rot was primarily the cause of the abandonment of large acreages in the East in the early 1900's. Downy mildew,powdery mildew, dead arm, anthrax nose, and crown gall have made extensive control measures necessary in some localities.

Diseases of the American bunch grapes generally are harder to control in the Southern States, presumably because of the prevailing high temperatures, high humidity, abundant rainfall, and longer growing season. The weather conditions greatly influence the occurrence and severity of the diseases, even in a locality. Consequently the danger always exists that diseases that have caused only minor loss may develop in epidemic proportions. As a general rule, however, each variety in any given region has its own specific diseases for which control measures must be considered.

The selection of a vineyard site with adequate air drainage is highly important in reducing the possibility of serious disease losses. The removal of badly affected parts of the vine at the time of pruning and general vineyard sanitation make attempts to control disease more effective. Practices that make conditions less favorable for the development of the overwintering stages and spore dissemination may be considered as measures to supplement the recommended spray program.

In vineyards where disease losses are apt to be serious when no spraying is done, the benefits from properly applied treatments are greater than their cost. If the losses are sporadic and minor, it is doubtful whether the expense of the full schedule of treatments is returned through increased yields. The decision to apply control measures, therefore, depends on a knowledge of the history of disease losses on a particular variety in a given locality.

Although dusting for disease control has been considered less effective than spraying, experiments in New York vineyards from 1946 through 1952 indicate that dusting with especially adapted dust applicators might be entirely satisfactory against some diseases if the applications are properly tuned. In several of the tests, good to excellent control of black rot, downy mildew, and powdery mildew was also obtained with concentrate sprays (using 25 gallons to an acre) where an air blast was employed to carry the fine mist to the vines. The investigations were still in the developmental stage in 1953.

BLACK ROT, caused by Guignardia bidwellii, is the most widely distributed and the most destructive disease of grapes east of the Rocky Mountains. It is especially prevalent in the more humid sections and is particularly destructive in the Southern States and along the Gulf coast. Environmental conditions in the Pacific coast grape-producing areas apparently are not conducive to the establishment of black rot. It does not seem possible that an occasional source of infection would not have been carried into the area on early imports of grape stock from the East. Black rot is probably indigenous to North America. All vinifera varieties and most of the widely grown American-type bunch grapes are susceptible to it.

Among the more- susceptible varieties commonly grown in the East are Catawba, Concord, Dutchess, and Niagara. Part or complete resistance to black rot is found in Campbell Early, Clinton, Delaware, Diamond, Eaton, Elvira, Fredonia, Ives, Missouri Riesling, Moore Early, Portland, Worden.

The fungus is capable of infecting all green parts of the vines, including leaf blades, petioles, tendrils, shoots, blossoms, and fruits. The fruit infections are the most damaging. In most instances they represent the only damage of economic importance to the crop. Most infection occurs on the younger developing plant parts, but fruit infection may continue until harvest, especially in vineyards where early-season infections are numerous and where weather conditions favorable for spore dispersal and germination occur late in the season.

The foliage infections appear as tiny, more or less circular spots. The spots are reddish brown and are usually encircled by a ring of yellow tissue. Through the coalescence of many spots, large areas of the leaf may become affected. Pycnidia develop in the spots, and within a short time pycnidiospores are formed. The lesions on the shoots, petioles, and tendrils are usually longer than those formed on the leaves.

On the fruits the lesions first appear as minute tan-colored spots. The lesions rapidly increase in size. After a few days the entire berry is rotted. Pycnidia develop, and the berry soon turns black, shrivels, and becomes mummified. Pycnidiospores produced on the affected parts are exuded in moist weather. They infect other susceptible parts of the vine when moisture and temperature favorable for spore germination occur. Late in the season immature perithecia develop in the mummies. They develop asci during the winter. In the spring they discharge mature ascospores capable of infecting the developing leaves and shoots. There is evidence that early-season infections might also result from pycnidiospores developed in the lesions on overwintered canes, tendrils, or other parts.

When periods of rainfall or persisting, dew are frequent before bloom, infections may become numerous on the vegetative parts of the vine. Then 25 percent or more of the berries may become infected in a single infection period during or shortly after the bloom period. As moisture is needed for the expulsion and germination of the spores, the disease is more destructive in seasons of heavy rainfall.

Black rot can be effectively controlled by three or four applications of ferbam (1 1/2 pounds in 100 gallons of water) or bordeaux mixture (4 pounds each of copper sulfate and hydrated spray lime in 100 gallons of water). In vineyards where conditions favor the early-season development of lesions on the vegetative parts, the first application should be made when the new shoots are 18 to 24 inches long. Applications immediately before bloom and immediately after bloom are especially important in preventing infection of the developing berries. Under most conditions an additional application 7 to 14 days after bloom, depending on the weather, is required to protect the rapidly enlarging berries from infection.

Investigations at the New York State Agricultural Experiment Station from 1945 through 1951 indicate that certain modifications of the previously recommended control practices are advisable: Four properly timed applications to alternate sides of the row were as effective as three applications applied to both sides. The experiments also demonstrated that thorough coverage of the berries with the fungicide is not essential.

Ferbam, besides being more effective than bordeaux mixture or any of the other fungicides tested, caused no injury to the vines and under certain conditions stimulated growth of the vines and the yield. On the other hand, bordeaux frequently injured the foliage so that vine growth and yield were lower. Three ferbam sprays applied at a reduced concentration of 1 100 gave excellent control in several trials under conditions where 50 percent or more of the berries rotted in the check plot.

The addition of various spreaders and stickers to the sprays did not increase the effectiveness of ferbam. When the spray coverage in the tests was less thorough, the addition of the adjuvants actually reduced the effectiveness of ferbam. Ferbam 1 1/2 100 is generally recommended for the control of black rot under average conditions when 100 to 150 gallons of spray are applied. When a lower application rate is employed or in places where black rot is unusually difficult to control, more might be necessary.

DOWNY MILDEW, caused by Plasmopara viticola, is common in most eastern grape-growing areas. The fungus thrives best during cool, moist weather. Therefore it is most destructive in the northern areas.

Downy mildew may cause a rot of the bunches on most of the American bunch varieties commonly grown in the Eastern States. Downy mildew fruit rot has seldom been observed on Concord and Delaware. Foliage infection, sometimes resulting in partial or complete defoliation, occurs in most varieties. The foliage of Delaware and some vinifera hybrids is extremely susceptible to infection; in cool, moist weather, complete defoliation may occur before the crop ripens. Early defoliation may retard ripening to the extent of making the crop of little or no commercial value and renders the vine more subject to winter injury.

When the weather is favorable, primary infection usually occurs just before bloom, but in some seasons in the Northern States the first infections may not become evident until 3 or 4 weeks after bloom.

New infections first become apparent on the upper surface of leaves as water-soaked places, which soon become light-yellow, roughly circular spots. The spots merge with the surrounding green tissue without any distinct line between diseased and healthy tissues. The spots enlarge and usually form irregular lesions. The centers of the lesions become brown in a few days. A downy white growth of branching conidiophores develops on the under surface of the leaf lesions in periods of frequent rainfall and high humidity. The conidia spread the disease to other parts of the vine through the summer whenever weather conditions favorable for spore dispersal and germination prevail.

Oospores, the overwintering stage of the fungus, are produced in the intercellular spaces of the affected parts. These spores remain dormant until the following spring, when they are set free by the disintegration of the diseased tissues in which they were formed. Only oospores that have been subjected to freezing temperatures can germinate. The oospores germinate by the production of short unbranched promycelia, which bear large sporangia. Each sporangium releases swarm-spores capable of producing the primary infections on the vines when moisture and temperature are favorable.