Part 20 (1/2)
[Ill.u.s.tration: 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.
_Powdery-mildew._
Less troublesome than downy-mildew in the East, powdery-mildew (_Uncinula necator_), unless checked, is capable of destroying the entire crop of European grapes on the Pacific slope. In the East it sometimes causes great loss on the several varieties known as ”Rogers hybrids” and, curiously enough, is often a rather serious disease of the Concord. The disease is caused by a superficial fungus which pa.s.ses the winter on fallen leaves and also on the canes. The spores begin to germinate a few weeks after the grape blossoms, but the disease is not often found until the grapes are nearly half grown. The fine white filaments of the fungus, which const.i.tute the vegetative portion of the parasite, then attack the leaves, shoots and fruit, sending up short irregular branches on which great numbers of spores are borne. These give the upper surface of the leaf a gray, powdery appearance, hence the name. Eventually the diseased leaves become light brown and if the disease is severe, soon fall. Infected berries take on a gray, scurfy appearance, speckled with brown, are checked in growth and often burst on one side, exposing the seeds. The berries, however, do not become soft and shrunken as when attacked by the downy-mildew. The disease pa.s.ses the winter in resting-spores produced late in the growing season. Powdery-mildew differs from other fungous diseases of the grape in being more prevalent in hot, dry seasons than in cold, wet ones.
In eastern America powdery-mildew is controlled by the treatment recommended for black-rot. When black-rot is not prevalent, two sprays with bordeaux mixture are recommended; the first in early July and the second about two weeks later. On the Pacific coast, however, powdery-mildew or ”odium” as it is often called there, the name coming from Europe, is more cheaply and more successfully combated by dusting with flowers of sulfur. Dusting is often done by hand or with perforated cans but this is wasteful and uncertain, and any one of several sulfur-sprayers may be used which does the work better.
_Anthracnose._
Another widespread disease is anthracnose (_Sphaceloma ampelinum_), called ”birds-eye-rot” because of the peculiar spots produced on the affected fruits, which attacks leaves, shoots and fruits of the vine.
It first appears on the leaves in small, irregular, dark brown sunken spots with a dark margin. Later it appears on the fruits, having much the same appearance though the spots are usually larger and more sunken, the disease being most characteristic on the fruit, however.
Frequently two or more spots unite and so cover the greater part of the berry. The fruits become hard, more or less wrinkled, and the diseased area often ruptures, exposing the seed, much as with powdery-mildew. The spores of the fungus are produced in great numbers on diseased areas during the growing season and are borne on thread-like filaments which live throughout the winter in the tissues of the vine and are ready for new growth in the spring. Winter-spores have not yet been discovered.
Anthracnose is widely distributed in eastern America but seldom causes great or general loss, most of the commercial grapes being relatively immune to the disease. A few sorts rather commonly grown in home vineyards, as Diamond, Brighton and Agawam, suffer most from anthracnose. Spraying with bordeaux mixture, as recommended for black-rot, is usually sufficient to keep the disease in check.
_Dead-arm disease._
A troublesome disease of recent appearance is now doing considerable damage in the Chautauqua grape-belt along the sh.o.r.es of Lake Erie, being most common on the Concord. From the fact that it is usually found on one arm of the vine it is called ”dead-arm disease”
(_Cryptosporella viticola_.) The disease is caused by a fungus which pa.s.ses the winter in small, black fruiting bodies in the dead parts of the vine. Early in the spring the fungus spreads by means of spores to the young shoots and later in the season attacks mature berries, producing small, black, oblong spots of black-rot. Sooner or later, if the diseased shoot is not cut off, the fungus spreads to the arms or trunk of the vine, producing a slow, dry rot which eventually kills the affected part. Fortunately, the presence of the disease is quickly detected by small yellowish leaves, much crimped about the margin.
The fungus is easily controlled by marking the diseased arms when the first symptoms appear and cutting these off at pruning time. If the vine is much mutilated by such pruning, usually suckers can be brought up from beneath the surface of the ground to renew the vine. The applications of bordeaux mixture recommended for black-rot are valuable in preventing the dead-arm disease. The disease is largely prevented by renewing the old wood of the vine as soon as the trunk begins to show a gnarled appearance.
_Sh.e.l.ling._
In eastern America, especially in the Chautauqua grape-belt, grape-growers not infrequently lose a large part of the crop by the premature falling of the grapes from the stems. The trouble is an ancient one and is designated as ”sh.e.l.ling” or ”rattling.” This premature dropping usually begins at the end of a cl.u.s.ter, and cl.u.s.ters farthest from the trunk are earliest affected. When vineyards suffer badly from this sh.e.l.ling, the vines often take on a sickly appearance, the foliage falling off in color and the outer margins of the leaves drying up more or less. The fallen fruit has an insipid taste and is, of course, worthless even if it could be harvested.
The cause of the trouble is not known. Grapes may ”rattle” on high land or low land, on poor soil or rich soil, on heavy or light soil. A vineyard may be affected one year and not the next. Grape-growers usually attribute the trouble to faulty nutrition, but applications of fertilizers have not proved a preventive. Old and well-established vineyards seem freer from the trouble than new and poorly established plantings. The most reasonable theory as to the cause of sh.e.l.ling is that it comes from faulty nutrition of the vine, but the conditions so affecting the nutrition are not yet satisfactorily determined.
_Diseases of minor importance._
Ripe-rot or bitter-rot (_Glomerella rufomaculans_) is a disease due to the same fungus causing the bitter-rot of the apple. As the name indicates, the disease usually appears on the fruit at ripening time and under favorable conditions continues after the grapes are picked.
It may also attack the leaves and stems. The first indication of the fungus is the appearance of reddish-brown spots which spread and eventually cover the whole fruit. The berries do not shrivel, but the rotted surface becomes dotted with pustules in which the spores are borne. It is hard to tell how much damage this disease does, but it is not usually great and the late applications of bordeaux mixture for black-rot or powdery-mildew are very effective in controlling it.
Crown-gall, now known to be a bacterial disease which causes knots or galls on the roots of various wild and cultivated plants, sometimes attacks grape roots or even the vines above ground. Occasionally, the disease is rather serious, but it is not often to be reckoned with in the vineyard regions of America. Fungicides are useless in combating the disease and all that can be done is to exercise great care in planting infected stock. It is doubtful whether crown-gall ever seriously injures vines in northern regions, although it may occasionally do so in the South.
In California there is a somewhat mysterious disease known as ”Anaheim disease,” because of its having first made its appearance in the vicinity of Anaheim. As near as can be learned, the disease first appeared in 1884 and then spread rapidly from forty to fifty miles from the point where it began its ravages, causing direct and indirect loss of many millions of dollars, and leading to the abandonment of grape-growing in some parts of southern California. Fortunately, in recent years the Anaheim disease is less aggressive but still does more or less damage. The nature and the treatment of this disease are not as yet fully determined, although several experimenters are studying the trouble. Californians whose vineyards suffer from this disease should apply to the experiment station at Berkeley for the latest information in regard to it.
Coulure is another trouble of the vine in California of which little is yet known, either as to cause or treatment. The term signifies the failure of the fruit to set or to remain on the cl.u.s.ters. The trouble occurs in varying degrees from the loss of a few berries to the complete sh.e.l.ling of the fruit from the stem. It is worse in some localities than others and in some varieties than others. Various causes have been a.s.signed to the disease, chief of which, and most probable, are unfavorable climatic conditions.
CONTROL OF INSECTS AND DISEASES
From the number of insects and diseases found on the grape, it would seem that, literally, ”pestilence walketh in darkness and destruction wasteth at noonday” in the vineyards of the country. But not many of the ills that grape-flesh is heir to are ever found in one region, and the vineyard is seldom attacked by many diseases or insects in a single season. There was a time, as we have said before, when grape-growers were so beset by pests which they could not control, that viticulture was one of the most uncertain fields in agriculture.
But one brilliant discovery after another has brought the pests of the grape under the hand of man until now there are but few that need cause much expense in treatment or worry as to the outcome.
Plants cannot be attacked by diseases unless infection is permitted.