This disease was first observed in America in 1834. Some time prior to 1878 the pathogene was carried to Europe on American stock, for in September of that year it was first recorded in France. During the five preceding years, French growers had been warned against such importations from the United States on account of the Downy Mildew of grapes. At that time European vineyardists were eager to use the American root stock on which to graft French vines as a remedy for the grape phylloxera, another pest of American origin which had already been so destructive to French viticulture. But in spite of the admonitions of the French scientists against importations from America, the Downy Mildew finally reached the European vineyards. From France the mildew-pathogene spread throughout Europe, where it is now a very troublesome foe. In the United States the disease is very generally known, being more destructive to European varieties than to domestic forms. No variety is notably resistant under all conditions. Both smooth and pubescent, wild and cultivated, sorts are affected. The Downy Mildew also occurs on five - leaved ivy, a close relative of the grape.
The greatest losses to American viticulturists from this disease are incurred in northern United States, where in some localities it has been estimated that 25 to 75 per cent of the crop is destroyed. Losses in Europe have been enormous. The disease is destructive not only to the berries but to practically all young or green portions of the vine. The nature of the losses is as follows: (1) shelling; rotting and mummification of the fruit; (2) spotting of the foliage, often resulting in premature defoliation; and (3) sometimes a dwarfing and killing of canes and leaves.
On the upper surface of the leaf the first signs of Downy Mildew are in the form of small greenish yellow indefinite spots, the margins of which gradually merge into the darker green of the leaf. In a short time there appears within the spot a network of small, reddish brown lines, the discolored smaller veins. These lines become more pronounced until the whole diseased portion is brown, dry and brittle, and eventually cracked. These symptoms apply particularly to susceptible leaves. On the foliage of the more resistant vines there occurs only a slight change in color; within this area are found numerous brown punctations, which do not unite to form a dead, brown spot in the leaf-tissue. The disease on the lower surface of the leaf is at first similar to that on the upper, but is very soon covered by a downy, white growth (Fig. 61) which is most pronounced just beneath the greenish yellow portion mentioned above. On the brown portion the whitish felt, the mildew - pathogene, soon disappears. The amount of mildew on a leaf depends on weather conditions and on the variety. The lower portion of a vine may be denuded of its foliage.
Fig. 61. - Downy Mildew on lower surface of grape leaf.
The canes, leaf-petioles and tendrils are subject to attack, especially in wild varieties. At first the affected portion has a water - soaked, shiny appearance, which is attended by a local swelling of the tissue. The lesion is bare at first, but soon the mildew covers it. In older cases the affected area becomes brown and dead, and a depression results. In cases of severe attack the cane is dwarfed, the leaves remain small, and the cane may die (Fig. 62).
The fruit (Fig. 63) and flowers of wild varieties commonly show the Downy Mildew. The attack may come at any time from flowering to maturation. The first sign of the trouble on the fruit is a hardening of the berry, together with a change from its normal color to a grayish blue lead - color. It is during this stage that the mildew appears. In later stages the berry withers, turns brown or red, and finally shrivels into a mummy (Fig. 63). Diseased fruits shell very easily.
Fig. 62. - Downy Mildew on grape - shoot.
The Downy Mildew pathogene is a fungus, Plasmopara Viticola. It has both a sexual and an asexual stage, the former represented by oospores, the latter by conidia. It is generally agreed that the oospores carry the fungus through the winter in the old fallen leaves. In the spring these oospores germinate, forming a conidiophore, on which is borne another spore, a conidium. The conidium in turn, at maturity, germinates, but instead of forming a germtube, its contents break up into six or eight extremely small naked spores. Each of these has two cilia which give it the power of motion; on account of this motile character these small naked bodies have been named swarmspores. The production of swarmspores, which really initiate the disease, begins in February and continues until June. They are formed and discharged within 24 hours. The swarmspores or their parent spores, the conidia, are spattered in drops of water or mud by heavy rains to the leaves of the vine. After reaching the leaf, the swarmspore, which is still a naked protoplasmic mass, soon comes to rest and surrounds itself with a thin wall. Then a germtube is developed which penetrates the leaf by way of a stoma, always on the lower side of the leaf. If a swarmspore is on the upper surface of the leaf, it swims over the edge of the same to the lower side. The germtube within the tissues develops into mycelium which brings about the various changes in the affected parts as evidenced by the symptoms already described. Within a short time the fungus masses itself beneath a stoma, through which opening several (four to ten) conidiophores pass. These grow from most of the stomata in the infected region, each conidio-phore reaching a height of about one-fiftieth of an inch. In mass they present a whitish, downy appearance - the Downy Mildew. These structures form branches, on the ultimate tips of which are borne conidia. The conidia are blown to other susceptible parts of neighboring vines, where new spots are formed. In the autumn, affected leaves fall to the ground, carrying the fungus within them. Before winter arrives oospores, as many as 200 to the square centimeter, are formed. In the spring the oospores are set free by a rotting of the leaves, germination occurring in the soil.
Fig. 63. - Downy Mildew on grape - berries.
A temperature of 77° to 82.4° Fahr. is best for the growth of the fungus. Shaded, moist situations are favorable for its development. Long, dry periods do not kill the pathogene, although such conditions seriously check it. Heavy rains of short duration followed by sunshine and winds are not favorable to the spread of the fungus. Most injury is done during hot, wet weather.
Since the fungus hibernates in old fallen leaves it is commonly recommended that these be plowed under early in the spring. The real value of such an operation, however, is not definitely known. The vines should be sprayed to protect them against attacks of the Downy Mildew pathogene in any case. Use bordeaux mixture 5-5 - 50, making five or six applications during the season. The first should be made just before the blossom buds open; the others at intervals of two weeks.
Istvanffi, G. Etudes sur le rot livide de la vigne. Hongrois Royal Inst. Cent. Ampel. Ann. 4: 1-260. 1913. Viala, P. Les maladies de la vigne, pp. 57-185. 1893. Farlow, W. G. On the American grape - vine mildew. Bussey Inst.
Bul. 1: 415-425. 1876. Gregory, C. T. Spore germination and infection with Plasmopara viticola. Phytopath. 2: 235-249. 1912. Shear, C. L. Insect and fungous enemies of the grape east of the Rocky Mountains. U. S. Agr. Dept. Farmers' Bul. 284: 30 - 32.
1907. Gregory, C. T. Studies on Plasmopara viticola (Downy mildew of grapes). Int. Cong. Vit. Rept. 1915: 126 - 150. 1915.