The Powdery Mildew, or oidium, of the vine is native to the Old World, originating on native plants in Japan. It was at one time erroneously held that this disease, like Black Rot and Downy Mildew, originated on wild vines of the eastern and central United States. The disease first appeared in Europe in 1845, when it was found in England. Two years later it reached France, and in 1848 was first observed in Belgium. By 1850 great devastation was wrought in the vineyards of France, and that same year it appeared for the first time in Spain and Italy. The following year Powdery Mildew became general over France and was then (1851) found in Hungary, Greece, Switzerland, Syria, Asia Minor and Algeria. Great damage was caused by the disease in 1854 in France. After that date growers began to learn how to control the trouble as a result of the discovery and perfection of the use of sulfur. By 1859 the use of sulfur had become so general that the crop of grapes had returned to its normal size. In 1866 the disease made its appearance in Australia. In the United States it has a wide geographical range, occurring from Massachusetts to Florida and westward across the continent of North America to the Pacific Coast. The first record of the oidium in America comes from California; it was known in that state as early as 1859, but doubtless occurred there long before that time. The disease is also found to some extent in Canada, particularly in Ontario.
The losses from Powdery Mildew of the vine are greatest in Europe. Soon after the pathogene was introduced into Europe losses of ninety to ninety-five per cent occurred in French vineyards, and the damage was so great that in certain regions vineyardists emigrated. In all countries where the disease was then known government commissions were appointed to investigate the matter, and consular reports were issued. Enormous losses were incurred in 1854 and 1855, in which latter year the climax was reached. The disease still occasions considerable annual loss. In certain regions of America, for example in the Chautauqua grape belt of western New York, in California and Oregon, Powdery Mildew ranks ahead of Downy Mildew, Black Rot and anthracnose. This is perhaps due to the ability of the causal fungus to withstand the dry atmosphere better than the fungi concerned in the other diseases mentioned. If not controlled, the oidium does considerable damage in western vineyards. In the Chautauqua region of New York growers may in cases of severe infestation lose fifty per cent of their grapes on account of shelling due to the Powdery Mildew fungus. The Lindley is very susceptible to the disease.
All green parts of the vine may be affected: leaves, canes, flowers and fruit. The young leaves at first show whitish or greenish white patches on the upper or lower surfaces. A mottled appearance is thus produced. These patches later run together until a large portion of the leaf is covered with a grayish white mildew. Sometimes this dense growth of the pathogene is accompanied by malformation and discoloration of the affected leaf. A vine with diseased foliage has a wilted and dwarfed aspect; this is particularly true in the earlier parts of the season and in warm, dry weather. An affected vine emits a moldy odor. Eventually black, pimple-like fruit - bodies of the fungus are seen scattered over the mildewed area.
Often the mildew is abundant on the shoots. It appears near the base of the canes and at first is not noticeable; later the affected portion acquires a grayish tint and it is then more easily seen. In severe cases whole canes are covered; in mild attacks the mildew is confined to patches. Later affected canes turn dark, owing to the injury to the outer bark - cells. If young canes are affected, they fail to mature properly and often they become blackened over their whole surface; this appearance is sometimes mistaken for anthracnose.
Sometimes the mildew appears on the flowers, if conditions are favorable in the vineyard during blossoming. This occurs regularly in France, but does not occur in eastern United States. Affected blossoms fail to set or develop in an aborted fashion.
Diseased young berries are dwarfed and are caused to drop. If not affected until older, the fruit continues to develop, but in an irregular fashion. As described for the canes the outer cells are injured, which accounts for a halting in the growth of affected portions. Sometimes this irregularity of development results in a cracking of the fruit, in which case the berries either become entirely dry and never ripen, or are greatly reduced in size and quality. The fruits are not attacked by the pathogene after they enter the ripening - period. On older, but still immature, berries brown spots appear; subsequently mildew becomes perceptible. The peduncles and pedicels also show the mildew. Very commonly on these parts the pathogene makes profuse growth. By harvest time the peduncles are dwarfed and withered. The development of the grayish mildew is followed by the appearance of black perithecial bodies as described for the other susceptible organs.
Sexual fruit-bodies, perithecia, of the causal fungus, Uncinula necator, probably remain on the canes and leaves or in the soil until the return of summer. Within each perithecium twenty - five to fifty ascospores are found. These are liberated from their asci, and are carried to the growing shoots and leaves. The ascospores are not all ejected at the same time; some may not be discharged until a year later. It is thought that in many cases the fungus hibernates by means of the asexual (conidial) stage; this opinion is based largely on the scarcity of the perithecia in certain regions.
Whether the fungus begins its spring activities by means of ascospores or conidia, the result is a mildewed spot. The whitish mildew is composed of mycelium and conidiophores. When a spore falls on a leaf it germinates, and a copious development of mycelium results. Haustoria are sent into the epidermal cells for nourishment. As a result of this action the cells turn brown, as described under Symptoms. From the mycelium elongated, erect conidiophores extend into the air; each bears a chain of conidia. The conidia are light and are easily disseminated. They have been found as late as the month of October.