The first important records of Black Rot come from Missouri in 1861, although the disease had been known many years prior to this date. About that time (1861) there was a rise in the grape industry near St. Louis, which, especially in 1860 to 1864, was accompanied by epiphytotics of Black Rot. In 1885 the disease was first recognized in France, into which country the pathogene had, at some previous date, been introduced from America. In 1886 there was very little of the disease, owing to the dry weather; however, in 1887 it became serious in many localities new to the trouble. The vineyardists abroad were desirous of obtaining the best varieties and so they went not only to the wild grapes for cuttings, but imported them from America. They were further interested in getting possession of varieties resistant to the phylloxera, and consequently the introduction of the Black Rot organism was comparatively easy. So serious had the grape disease situation become in France by 1887, that the noted Viala was detailed by the French government to visit America in the interest of French viticulture. Since this date many papers dealing with the disease have appeared in various languages, but more particularly in French and English.
At present the Black Rot disease is practically omnipresent. Its geographical range in Europe is approximately coincident with that of the grape; for the pathogene, having reached France, spread rapidly into Germany, Italy and Asia Minor.
In the United States it may be found in all grape-growing regions except in the state of California, where the conditions are highly unfavorable to it. In that state it is almost unknown, but its place is taken by the Powdery Mildew fungus.
Wherever found, this disease is of great importance because of the injury it produces to all of the parts above ground. There may result serious loss by the reduction in the amount of fruit; and the leaves may be spotted to such an extent that they die. The very general range of the trouble makes it a constant foe, especially wherever susceptible varieties occur. All authorities are agreed that Black Rot is the most serious fungous disease with which grape-growers generally have to contend. The amount of loss occurring each year varies with the season, there being an abundance of it in wet seasons and much less in dry ones. The disease is more troublesome in warm, humid regions than cool, dry ones. In North Carolina the heavy losses come within a week after blossoming-time, while in New York the greatest injury is incurred when the berry is one-half to two-thirds grown. In many sections of the country the destructive nature of this disease has been responsible for the abandonment of grape-growing. In 1906 Michigan vineyardists lost 30 to 40 per cent of their crops on account of Black Rot; in Ohio in 1905 the loss is put at 30 per cent of the crop, meaning a loss of $28,500. The losses in many regions are practically total; this has been true in several states. A case is on record for 1906 where a large vineyard of 200 acres in New York did not yield enough grapes to pay for operating-expenses; the reduced yield here was due to the action of the Black Rot pathogene.
As a rule all the green parts of the vine may be affected with Black Rot. The old portions and the fruit of rotundifolia varieties, like the Scuppernong, in southern United States are rarely attacked.
The name Black Rot, which is in use in America and Europe, applies to the condition of affected grapes (Fig. 59). When the berries are about one-half grown, the disease shows its first symptoms in the form of a blanching. This is soon replaced by a whitening of the diseased area, which is now more evident. A brownish line appears at the margin, and there results, on account of a light-colored circular disk with an encircling darker band, a bird's-eye effect. Some grape-culturists confuse this stage with the true bird's-eye, or anthracnose, described on page 250. The spot increases in size rapidly and its surface becomes sunken. In a few hours after the beginning of the development of the lesion, numerous minute brown specks appear on the light-colored center. These are the fruiting bodies of the Black Rot organism, which very shortly become so numerous as to give the spot a blackish aspect. Occasionally the extent of the lesion is halted, and in a few days there is formed a thin black superficial crust on the side of the berry. The usual course of development, however, is for the berry to become entirely involved. After about a week the berry is a hard and shriveled mummy (Fig. 59).
Fig. 59. - Black Rot on grape berries.
Although the berries, of all susceptible parts of the vine, are most severely affected, the disease may first be observed on the leaves (Fig. 60). In June or early in July Black Rot appears on the leaves in the form of reddish brown, somewhat circular spots. Close observations show that the first evidence of the trouble on the leaves is a slight blanching of the affected tissue. The small veinlets form the margin of the spot so that, while the shape of the lesion tends to be circular, the outline is finely cren-ulate (Fig. 60). Soon the center of the affected area becomes ashen-gray, but at this stage the margin is still brown. By the time the spot is just visible to the unaided eye the margin appears as a black line. Two or more spots may coalesce to form large irregular areas. The fruiting bodies of the patho - gene protrude through the cuticle of the leaf. These structures are black and are arranged promiscuously or in more or less concentric rings (Fig. 60).
Fig. 60. - Black Rot lesions on grape leaf.
The Black Rot lesions occur as small dark depressions or cankers on the stems, tendrils, peduncles, petioles and leaf-veins. On the canes the spots rarely extend more than a quarter of the distance around, but the tendrils and leaf-petioles may be nearly or wholly girdled. The affected areas are small, one - twelfth of an inch to one inch long, and are elliptical or considerably elongated in form.
Black Rot is produced by the fungus Guignardia Bidwellii. The trouble begins in the spring, when both ascospores and pycnospores are liberated from their hibernating quarters. The former spores are discharged with considerable force into the air for some distance. Subsequently they are carried by air currents to a variety of places. Some of the spores perchance fall on the fruits, leaves or shoots, where Black Rot lesions may ultimately be produced. The pycnospores ooze out of the pycnidia which have hibernated on old canes, tendrils, leaf-petioles or mummies. They are washed by rains to the above-ground susceptible parts, where they may finally induce Black Rot areas. Either of these spores - ascospores or pycnospores - then, may start the trouble in the spring. With the presence of moisture they germinate and the resulting germtubes penetrate the vine. For some time the fungus is developing a mycelium within the attacked organ, and it may be many days before there is any visible evidence of disease. The length of this period varies with the weather conditions, and with the part affected. When it is hot and dry the period is materially reduced, and with cool weather it is lengthened. In tender, juicy fruits this period is shorter than in stems and leaves. On fruits the period is from 8 to 18 days; on leaves, 10 to 21 days.
The diseased berries show mycelium both between and within their cells. Eventually the cells are killed, they collapse and the surface becomes sunken. Similar action takes place in the leaves, canes and other attacked organs. Soon there follows the development of fruit - bodies, the pycnidia. The mycelium forms a gnarl just beneath the epidermis, and in a few hours a crop of pycnospores is matured. These are imbedded in a mass of gelatinous matter, and when moistened a marked swelling of the mass occurs and the pycnospores are forced out in a coil. They are subsequently scattered to other leaves, fruits and woody parts, where new lesions are developed. This process is repeated many times through the summer. With the advent of autumn the fungus ceases vegetative activities and prepares for winter. Some of the pycnidia live over until spring, as already stated. In other cases, generally in August, sporeless pycnidia appear on mummies; in place of spores the cavity of this body is filled with a whitish cellular tissue. In such a state the structure is sometimes referred to as a pycnosclerotium, and as such the fungus passes the winter. In the spring the solid interior of the pycnosclerotium is replaced by asci which eject most of their spores in June. Some of the ascospores are not discharged until later, and may even be found as late as October. With two kinds of spores always present in a vineyard of affected plants it is not difficult to understand how unsprayed, susceptible varieties may be ruined in a favorable season of considerable rain.
Black Rot has long been kept in check by the application of bordeaux mixture. It is essential to successful protection of the vines that the spraying should be made before rains and that the work be thoroughly done. Apply bordeaux mixture 5-5 - 50 as follows: (1) when the second or third leaf is showing; (2) before the blossoms open; (3) after the blossoms have fallen; (4) about 10 to 14 days later; (5) again in 10 to 14 days.
In spraying and in the employment of other measures against Black Rot the following matters should be kept clearly in mind:
(1) The disease is of long - standing the world over and is well known from experience to be destructive. In regions where it occurs practically every year to a greater or less extent, the application of the above scheduled sprayings should not be neglected.
(2) All parts above the ground are likely to be affected. Spray every part accordingly. Watch for the disease in June.
(3) The fruit is rotted, the leaves are spotted, and the woody parts are cankered as a result of the action of the mycelium of G. Bidwellii.
(4) The inoculum comes from (a) old fallen mummies, (6) dingers, (c) tendrils and canes. Get rid of these sources of trouble as far as possible. Destroy mummies by carrying them out at picking time. Plow the vineyard in the spring: this is commendable both as good viticultural practice and in order to bury fallen mummies. It is not believed a profitable practice to collect and burn dingers. Nor are sanitary measures especially applicable to cankered tendrils and canes. The grower must depend upon bordeaux mixture for the protection of these parts.
Reddick, D. The black rot disease of grapes. Cornell Univ. Agr.
Exp. Sta, Bul. 293: 289-364. 1911. (Extensive bibliography.) Shear, C. L., and Miles, G. F., and Hawkins, L. A. The control of Black Rot of the grape. U. S. Agr. Dept. Pl Indus. Bur. Bul. 155:
1 - 42. 1909. Scribner, F. L., and Viala, P. Black rot (Laestadia Bidwellii). U. S.
Agr. Dept. Bot. Div. Veg. Path. Sec. Bul. 7: 1-29. 1888. Reddick, D., and Wilson, C. S. The black rot of the grape, and its control. Cornell Univ. Agr. Exp. Sta. Bul. 253: 367-388. 1908. Wilson, C. S., and Reddick, D. The black rot of the grape and its control. Cornell Univ. Agr. Exp. Sta. Bul. 266: 391-411. 1909. Edson, A. W. The black rot of grapes in North Carolina and its treatment. North Carolina Agr. Exp. Sta. Bul. 185: 133 - 156. 1903.