The Raisin Industry / A practical treatise on the raisin grapes, their history, culture and curing

by Gustavus A. Eisen

POWDERY MILDEW OR UNCINULA.

General Notes.

—This disease of the grapevine is caused by the growth of parasitic fungus known in Europe as Oidium Tuckeri, and in this country as Uncinula spiralis or powdery mildew. I am satisfied the two names signify the same fungus, only the European form has never been found as highly developed as the American one, which has on that account received a name of its own. If the two are identical, then the European Oidium, which for many years caused the destruction of the transatlantic vineyards, was imported to that country from this. The Uncinula spiralis is undoubtedly native on our indigenous vines.

Characteristics.

—The mildew appears in two different stages, one in the spring when the vines are in blossom, the other again later in the summer when the fruit is more advanced. The first stage of the mildew resembles a fine cobweb spun between the flowers of the bunch. If allowed unrestricted sway, the flowers will drop off, the fruit will never set, or set only imperfectly, and the crop will be a great loss or even a total failure. Generally the inexperienced vineyardist does not perceive the mildew until too late. A slight touch to the vine will then bring down all the young fruit or blossoms like a shower, and the stem of the bunch will be seen to be entirely bare, or with only a few scattered berries. This form of the Uncinula mildew has not been as scientifically investigated as would be desirable, and nothing is known as regards its development. It is possibly a primary generation and early stage of the later Uncinula. I believe this form of the mildew is identical with the disease which is called Colure by the French, and which is characterized by the dropping of the young, undeveloped grapes. The first appearance of this mildew is always accompanied by white, salty excrescences on the edges of the grape leaves. Whether they are directly or indirectly connected with the fungus is not known.

The later form, the powdery mildew, and the form which has given this mildew its name, appears later in the season, when the grapes are half grown or more. It then takes the shape of fine powder-like patches or blotches on the upper side of the leaves, stems or berries. These spots are of a dull gray or whitish gray color, and smell strongly of mold or mushrooms. If these mildew spots when young are rubbed smooth, especially on the green stems or berries, we see below them, in the epidermis of the vine, the mycelium or stem of the fungus spreading in all directions from a central point, like the roots of a tree or plant. This part of the mildew corresponds with the stem and root of a plant, while the upper, powdery part is the one which produces the spores or the seed, conidia and peritheca, all of which are reproductive organs. The grapes thus attacked gradually dry up or crack open. The leaves are eaten through and dry up, and the whole plant becomes badly diseased, and may even die.

Powdery Mildew (Oidium Form), Greatly Magnified.

History and Distribution.

—The powdery mildew or Oidium was observed for the first time in the year 1845 in hothouses in England. It immediately began to spread, and in a few years infested all the vine districts of the Old World. Before any remedy had been discovered, many vine districts were so injured that they have not since been able to recover. Thus in 1850 and 1851 France suffered greatly from this mildew, and the Island of Madeira, which for three hundred years had produced the finest wines, had its grapevines so injured that they up to this time have not again produced as good a quality of grapes as before the advent of the disease. The Grecian Islands as well as Morea were also visited by the powdery mildew, and though the latter is now kept in control, the general opinion is that the quality of the currants is not as high as it was before the mildew appeared. Now there is probably no place in the Old World where grapes are not attacked by this mildew, although some places are injured much more than others. Adjoining vineyards are often differently attacked, some being even entirely free, while others are visited yearly. Young vines are less attacked than old ones, and in favorable places the mildew seldom infests vines before they are two or three years old. Elevated places and localities exposed to winds and cold are generally attacked by the first stages of this oidium, while its second or last stage prefers low, damp places exposed to dew or fog.

The American form of the powdery mildew or Uncinula spiralis differs in some respects from the European Oidium, not as to its effects, but as to its microscopical characteristics. The Oidium occurs in Europe only with certain generative organs called gonidia, while the American Uncinula also develops so-called peritheca. It is more than probable that both fungi belong to the same species, but until these perithecal organs have been found on the European Oidium, the proper name for our mildew must be Uncinula, and not Oidium. It is also probable that the Uncinula fungus is a native of this continent, and that it from here has spread to Europe, where the natural conditions are such that only the gonidial form of the fungus has been able to develop. In general appearance and in their effects the Uncinula and Oidium are identical.

The Oidium appears sooner on poor soil and on exhausted vines, and vines in which the flow of the sap for some reason or other has been checked are more subject to the mildew than those which are yet in full growing vigor. Elevated vines on trellises which are much exposed, and vines which are so covered up that the air has little access, are the first ones to be attacked, and those which will suffer the most. The powdery mildew affects all varieties of grapes, but some kinds more than others. The Muscats are among those which suffer considerably, and if not sulphured would in severe cases neither set nor bear suitable grapes. The Malaga is less affected, and so is the Sultana. In new districts the Uncinula does not appear until the vines are older. Thus in the Fresno district the earliest vines did not suffer from mildew until they became five years old, but now the mildew would destroy the grapes every year, in case they were not treated with sulphur.

Remedies.

—The most common and perhaps the best remedy is powdered sulphur. The latter is applied either with the dust can or “dredger,” or with bellows. The dust can is used when the vines or vine shoots are yet small, and the bellows when the vines are larger. The first sulphuring should be done when the young shoots are six inches long, immediately before the bloom, and the second time when the berries are well set. Sulphuring as a regular vineyard operation will be more fully discussed further on.