_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.
[Illustration: 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.