By HENRY SNYDER MAUGER, PH.G.
(From an Inaugural Essay).
It is well known that in some localities many of the best varieties do not succeed solely on account of their foliage being destroyed more or less by mildew, and the criterion of a useful grape depends solely upon its freedom from mildew on the leaves, and not on account of the flavor or other good qualities of the fruit; so we find that the most popular varieties are not those of the highest merit in flavor, but those that are least affected with mildew on the foliage and fruit. The most prevalent form of mildew on the leaves of our native grapes is known as Peronospora viticola, Berk. This is always found on the under surface of the leaf; it commences in small spots of a brownish color which adhere closely to the leaf ribs, and when the conditions are favorable it spreads rapidly and destroys the vitality of the part attacked. Its presence is made apparent by a yellowish tinge which may be seen on the upper surface of the foliage, and in clear weather these spots become brown-colored, afterwards crisp and dry, and ultimately the leaf is more or less destroyed. This appearance on the foliage is sometimes termed sun-scald, but it hardly need be stated that the leaves would not be injured by the sun were it not that their vitality is impaired by mildew; yet we frequently meet with cultivators who maintain that their vines are free from mildew, while they admit the foliage is scalded by the sun and drying off.
By the time its effects are thus visible the mildew is not so easily discerned, or it may have run its course and left but little evidence of its presence in an active state, and this may be the reason why many grape-growers show so little knowledge of the disease. Hence the origin of so many seemingly conflicting opinions relative to the exemption of varieties of grape from mildew, owing to the effects produced by this disease being attributed to other supposed causes.
It is a disputed question whether or not mildew will attack perfectly healthy vegetation. By many persons it is held that fungoid growths only appear on disorganized vegetable or animal matters; that, previous to the appearance of the mildew on leaves, some disturbing cause has been at work on the plant, and the partial decomposition which has resulted from the unhealthy state forms proper conditions for the development of the fungus. From this reasoning it follows that, previous to the appearance of mildew, there must exist a disorganization of vegetable tissue; and before a remedy can be suggested we must first endeavor to discover the cause of the incipient disease which allows the development of the fungoid growths.
The peronospora is never found on grape-leaves which are always dry. The predisposing cause of this particular species of fungus is an excess of retained moisture on the foliage, either from continued wet and damp weather, or from heavy night dews succeeded by calm days. Grape-vines trained on trellises protected by a covering at top, so as to prevent the radiation of heat from, and the consequent deposition of dew upon, the surfaces of the leaves are never troubled by this fungus.
It is also a common observation that grape-vines growing through and over trees are never seriously injured by mildew, the protection afforded by the leaves of the tree preventing it. Branches from the same root, some of which are allowed to ramble over a tree, and others trained upon an ordinary trellis, will afford good examples as to the benefits, of protection in preventing mildew. Hence it may be inferred that a good locality for vineyards is one where there is exemption from late spring frosts, from late dews during summer nights, and from early frosts in autumn; and the best results will be found where all these conditions exist, and failures will follow in proportion to their deficiency. So far as concerns entire freedom from the mildew under consideration, the conditions are found on sloping hillsides contiguous to well-defined valleys. It has long been observed that in clear, still nights during summer, dews are less frequent upon the sides of hills than they are in the neighboring valleys.
The appearance of hoar-frost in valleys during the early winter and spring seasons is produced by conditions of temperature similar to those which cause the heavier deposition of dews in these localities. During clear nights currents of cool air run downwards on the inclined lands to the bottom of the valleys. These currents are the result of the sudden depression of temperatures sustained by the surface of the earth in consequence of rapid radiation, by which the stratum of air in immediate contact with that surface becomes specifically heavier by condensation, and descends into the valley, which then rapidly cools, while the warm air of the valley is lifted up, and impinges on the sides of the hills, and so far as this warm stratum extends there is no condensation of moisture such as occurs in the low grounds in the form of heavy dews in summer, and which in cool weather freezes and becomes hoar-frost. The effects of this stratum of warm air upon vegetation on hillsides is very well defined where early autumn frosts have destroyed the foliage of the trees below a certain line, which is sometimes called the vernal line, or line of no frost; above this line, and within the limits of the extent of the warm stratum or zone, vegetation is unharmed. The altitude to which this line reaches above the bottom of the valley is dependent upon the mean temperature of the day and night, or rather upon their comparative difference at the time of its occurrence; when the temperatures of both are high, the lower places only are affected by the frost, but when low, the frost will extend higher up on the hills. If we consider the climate conditions of localities where grapes do well, we will find that they are those which are nearly exempt from dews, and, as a consequence, all varieties of grapes retain their foliage during summer. In other words, the distinguishing peculiarity of a good grape climate is that of the entire absence of mildew on the foliage of the grape, and this is entirely independent of cultural processes of manipulation or training, or of the quality of the soil in which they are planted, although the latter may sometimes exert an auxiliary influence.
In illustration of the conditions which constitute a good grape climate mention may be made of the high lands bordering Keuka Lake, in Steuben County) New York. These steep hillsides are covered with vineyards which extend for several hundred feet above the level of the lake; the soil is shaly, and in many places the surface is very thickly covered with loose stones. On these hillsides mildew is comparatively unknown, the Catawba, Iona, Delaware, and indeed all varieties of native grapes, except those which require a longer season than the climate affords, mature to a degree of perfection which they fail to attain in more southern but less favorably situated localities. The influence of the lake is also well illustrated in the freedom from mildew on the vines which are planted quite close to the water. Higher up the valley beyond the lake, while the vines are equally as healthy on the hills there as they are on those in the near neighborhood of the lake, the plants suffer from mildew on the lower grounds, showing that the radiation of heat from the water during night has the effect of preventing dews even on low grounds near the lake. Here we have two factors, both of which are favorable to a healthy condition of vines, or rather they prevent mildew, which is the prime result, if not the cause of unhealthiness, so far as atmospheric influences are concerned. The first of these is owing to the elevation above the valley; during the day heat accumulates in the valley, and forms a stratum of warm air, which is lifted up as the colder air rushes down the slope after sundown, and wherever this warm air strikes the hillsides dews are not found.
The second important factor is the influence of the water in effecting a healthy condition of local climate. The ameliorating influence of an extensive body of water is well understood, and a noted illustration of its value is found on the shores and on the islands of Lake Erie, which have long been popular for the extent and excellence of the vineyards and the superior qualities of the fruit which they produce. This success is fairly attributable to the modifying effect of the body of water upon the atmosphere, which secures a comparative immunity from heavy night dews during the season when vegetation is most active. The heat which the water accumulates during summer has the further effect of warding off the frosts of autumn and early winter, thus virtually lengthening the season to an equality with the climates of latitudes several degrees southward, so that grapes which ripen perfectly in the vicinity of the lake fail to mature in localities immediately beyond its influence.
For all cultural purposes it is sufficiently accurate to assume that the hardiness of a grape simply depends upon its immunity from mildew. On the other hand, when a variety of our northern native grapes is said to be too tender for our winters, it simply means that it is so subject to mildew that the growths fail to ripen, as all of our native grapes of the Northern States, and indeed foreign grapes also will stand the winters, provided their young yearly growths become thoroughly matured; the summer climate rather than the winter climate decides the question of hardiness, so that when a seedling grape is announced as being perfectly hardy and exempt from rot in the berry, it may be true as far as hardiness is concerned, in the climate where it originated, if it happened to be a specially good climate, but it does not follow that it would be hardy in other parts of the country, as hundreds who have purchased such plants can abundantly testify.
Another form of mildew that may sometimes be seen on grape leaves is a species of Erysiphe. This form appears on the upper surface of the leaves, also on the surface of the fruit, its appearance being somewhat similar to a dusting of fine flour, and may be brushed off without leaving any apparent marks of injury, but its effects are to retard growth. Young, green shoots once covered with this fungus cease to grow, and will remain green until the frosts of winter destroy them. When the fruit becomes severely attacked it cracks open, and the seeds will protrude. Green shoots will also crack if the mildew attacks them severely. Unlike the peronospora it abounds mostly in the early part of the growing season. Sudden changes of the weather from heat to cold will produce it, but our native grapes do not suffer materially at any time from this kind of mildew.
The American Journal of Pharmacy, Vol. 59, 1887, was edited by John M. Maisch.