Fungous Diseases Of The Grape

The grape is ravaged by four or five fungous diseases in America,
unless the utmost vigilance is exercised to keep the parasites in
check. Happily for commercial viticulture, there are regions, as we
have seen in the description of grape regions in Chapter I, so
fortunate in their freedom from fungous diseases that there is little
uncertainty in grape-growing and but small expense in controlling
diseases. Also modern science has discovered the life history of all
the important diseases and devised fairly effective means of combating
them.

All of the fungous parasites of the grape in America are indigenous,
having long subsisted on wild vines. They are, therefore, all widely
distributed, and as cultivation has presented to them great numbers of
grape plants in continuous areas, the diseases have increased rapidly
in intensity, at times have swept like wildfire through grape regions
devastating and utterly ruining great areas of vines. Means, however,
are now at hand in remedial and preventive treatment, which, while
because of cost may not permit the grapes to be grown profitably in
all parts of America, do permit their culture for home use in
practically all agricultural districts in the country.

Black-rot.

This is the most widely distributed and the most destructive fungous
disease of the grape in the region east of the Rocky Mountains.
Fortunately, it is unknown on the Pacific coast. The disease is caused
by a parasitic fungus (Guignardia Bidwellii) which gains entrance to
the grape plant by means of minute spores distributed chiefly by wind
and rain. Black-rot passes the winter in mummied grapes, on dead
tendrils or on small, dead areas on the canes. In the spring, the
fungus spreads from these spots to the leaves and forms brown leaf
spots about a fourth of an inch in diameter, or oblong, black spots on
the shoots, leaves, petioles and tendrils. Later the disease spreads
to the fruits, not usually attracting attention until the berries are
at least half grown. Soon after the ravages of the fungus become
apparent on the berries, the fruits turn black, shrivel and become
covered with minute black pustules which contain the summer-spores.
Figure 44 shows the work of black-rot. In the winter and spring,
another form called the winter- or resting-spore is produced upon
these old, shriveled, mummied berries, and these carry the disease
over from one season to another.

Since the disease is carried through the winter in mummied fruits and
diseased wood, the desirability of destroying these mummied grapes and
the leaves and prunings of infected vines as soon as possible is
apparent. This treatment, however, is not sufficient, and the disease
can be effectually controlled only by thorough spraying with bordeaux
mixture (4-4-50). The first application should be made just before the
grape blossoms; the second, shortly after blossoming. The amount of
material applied matters less than evenness in distribution and
fineness of the spray as applied. In rainy seasons, perhaps a third or
a fourth application should be made in regions where the disease is
serious; the third is made when the berries are the size of a pea; the
fourth, as the berries become large enough to touch each other.

Downy-mildew.

Downy-mildew (Plasmopara viticola) rivals black-rot for first place
among fungous diseases of the grape. It is found in all grape regions
east of the Rocky Mountains but does most harm in northern localities.
Like black-rot, downy-mildew attacks all the tender growing parts of
the vine, but is chiefly found on the foliage and is usually less
destructive than black-rot. As first seen on the foliage, the work of
the fungus appears as greenish-yellow, irregular spots upon the upper
surface which later become reddish-brown. At the same time on the
under surface of the leaf, a thin, white downy growth puts forth. The
spores of the fungus are produced on this downy growth, and under
favorable conditions are distributed by wind and water to all tender
parts of the vine, where they germinate and begin their work of
destruction. The fruit is attacked when partly grown, as shown in Fig.
45, becoming covered with the gray down of the fungus, the "gray-rot"
of the grape-grower. If the berries escape the disease until half
grown, the fungus causes a brownish-purple spot that soon covers the
whole grape, giving the disease at this stage the name of "brown-rot."
Besides the summer-spores, another form of reproductive bodies is
produced in the winter to carry the fungus through the resting period.

Downy-mildew, like black-rot, spreads most rapidly and does most
injury in hot, wet weather. As with practically all diseases of the
grape, much can be accomplished in the way of control of the disease
by destroying infested leaves, shoots and berries which contain the
winter spores, but these sanitary measures are not sufficiently
effective and vineyards must be sprayed as recommended for black-rot,
except that the first application should be made before the
blossom-buds appear.

Powdery-mildew.

Less troublesome than downy-mildew in the East, powdery-mildew
(Uncinula necator), unless checked, is capable of destroying the
entire crop of European grapes on the Pacific slope. In the East it
sometimes causes great loss on the several varieties known as "Rogers
hybrids" and, curiously enough, is often a rather serious disease of
the Concord. The disease is caused by a superficial fungus which
passes the winter on fallen leaves and also on the canes. The spores
begin to germinate a few weeks after the grape blossoms, but the
disease is not often found until the grapes are nearly half grown. The
fine white filaments of the fungus, which constitute the vegetative
portion of the parasite, then attack the leaves, shoots and fruit,
sending up short irregular branches on which great numbers of spores
are borne. These give the upper surface of the leaf a gray, powdery
appearance, hence the name. Eventually the diseased leaves become
light brown and if the disease is severe, soon fall. Infected berries
take on a gray, scurfy appearance, speckled with brown, are checked in
growth and often burst on one side, exposing the seeds. The berries,
however, do not become soft and shrunken as when attacked by the
downy-mildew. The disease passes the winter in resting-spores produced
late in the growing season. Powdery-mildew differs from other fungous
diseases of the grape in being more prevalent in hot, dry seasons than
in cold, wet ones.

In eastern America powdery-mildew is controlled by the treatment
recommended for black-rot. When black-rot is not prevalent, two sprays
with bordeaux mixture are recommended; the first in early July and the
second about two weeks later. On the Pacific coast, however,
powdery-mildew or "oidium" as it is often called there, the name
coming from Europe, is more cheaply and more successfully combated by
dusting with flowers of sulfur. Dusting is often done by hand or with
perforated cans but this is wasteful and uncertain, and any one of
several sulfur-sprayers may be used which does the work better.

Anthracnose.

Another widespread disease is anthracnose (Sphaceloma ampelinum),
called "birds-eye-rot" because of the peculiar spots produced on the
affected fruits, which attacks leaves, shoots and fruits of the vine.
It first appears on the leaves in small, irregular, dark brown sunken
spots with a dark margin. Later it appears on the fruits, having much
the same appearance though the spots are usually larger and more
sunken, the disease being most characteristic on the fruit, however.
Frequently two or more spots unite and so cover the greater part of
the berry. The fruits become hard, more or less wrinkled, and the
diseased area often ruptures, exposing the seed, much as with
powdery-mildew. The spores of the fungus are produced in great numbers
on diseased areas during the growing season and are borne on
thread-like filaments which live throughout the winter in the tissues
of the vine and are ready for new growth in the spring. Winter-spores
have not yet been discovered.

Anthracnose is widely distributed in eastern America but seldom causes
great or general loss, most of the commercial grapes being relatively
immune to the disease. A few sorts rather commonly grown in home
vineyards, as Diamond, Brighton and Agawam, suffer most from
anthracnose. Spraying with bordeaux mixture, as recommended for
black-rot, is usually sufficient to keep the disease in check.

Dead-arm disease.

A troublesome disease of recent appearance is now doing considerable
damage in the Chautauqua grape-belt along the shores of Lake Erie,
being most common on the Concord. From the fact that it is usually
found on one arm of the vine it is called "dead-arm disease"
(Cryptosporella viticola.) The disease is caused by a fungus which
passes the winter in small, black fruiting bodies in the dead parts of
the vine. Early in the spring the fungus spreads by means of spores to
the young shoots and later in the season attacks mature berries,
producing small, black, oblong spots of black-rot. Sooner or later, if
the diseased shoot is not cut off, the fungus spreads to the arms or
trunk of the vine, producing a slow, dry rot which eventually kills
the affected part. Fortunately, the presence of the disease is quickly
detected by small yellowish leaves, much crimped about the margin.

The fungus is easily controlled by marking the diseased arms when the
first symptoms appear and cutting these off at pruning time. If the
vine is much mutilated by such pruning, usually suckers can be brought
up from beneath the surface of the ground to renew the vine. The
applications of bordeaux mixture recommended for black-rot are
valuable in preventing the dead-arm disease. The disease is largely
prevented by renewing the old wood of the vine as soon as the trunk
begins to show a gnarled appearance.

Shelling.

In eastern America, especially in the Chautauqua grape-belt,
grape-growers not infrequently lose a large part of the crop by the
premature falling of the grapes from the stems. The trouble is an
ancient one and is designated as "shelling" or "rattling." This
premature dropping usually begins at the end of a cluster, and
clusters farthest from the trunk are earliest affected. When vineyards
suffer badly from this shelling, the vines often take on a sickly
appearance, the foliage falling off in color and the outer margins of
the leaves drying up more or less. The fallen fruit has an insipid
taste and is, of course, worthless even if it could be harvested.

The cause of the trouble is not known. Grapes may "rattle" on high
land or low land, on poor soil or rich soil, on heavy or light soil. A
vineyard may be affected one year and not the next. Grape-growers
usually attribute the trouble to faulty nutrition, but applications of
fertilizers have not proved a preventive. Old and well-established
vineyards seem freer from the trouble than new and poorly established
plantings. The most reasonable theory as to the cause of shelling is
that it comes from faulty nutrition of the vine, but the conditions so
affecting the nutrition are not yet satisfactorily determined.

Diseases of minor importance.

Ripe-rot or bitter-rot (Glomerella rufomaculans) is a disease due to
the same fungus causing the bitter-rot of the apple. As the name
indicates, the disease usually appears on the fruit at ripening time
and under favorable conditions continues after the grapes are picked.
It may also attack the leaves and stems. The first indication of the
fungus is the appearance of reddish-brown spots which spread and
eventually cover the whole fruit. The berries do not shrivel, but the
rotted surface becomes dotted with pustules in which the spores are
borne. It is hard to tell how much damage this disease does, but it is
not usually great and the late applications of bordeaux mixture for
black-rot or powdery-mildew are very effective in controlling it.

Crown-gall, now known to be a bacterial disease which causes knots or
galls on the roots of various wild and cultivated plants, sometimes
attacks grape roots or even the vines above ground. Occasionally, the
disease is rather serious, but it is not often to be reckoned with in
the vineyard regions of America. Fungicides are useless in combating
the disease and all that can be done is to exercise great care in
planting infected stock. It is doubtful whether crown-gall ever
seriously injures vines in northern regions, although it may
occasionally do so in the South.

In California there is a somewhat mysterious disease known as "Anaheim
disease," because of its having first made its appearance in the
vicinity of Anaheim. As near as can be learned, the disease first
appeared in 1884 and then spread rapidly from forty to fifty miles
from the point where it began its ravages, causing direct and indirect
loss of many millions of dollars, and leading to the abandonment of
grape-growing in some parts of southern California. Fortunately, in
recent years the Anaheim disease is less aggressive but still does
more or less damage. The nature and the treatment of this disease are
not as yet fully determined, although several experimenters are
studying the trouble. Californians whose vineyards suffer from this
disease should apply to the experiment station at Berkeley for the
latest information in regard to it.

Coulure is another trouble of the vine in California of which little
is yet known, either as to cause or treatment. The term signifies the
failure of the fruit to set or to remain on the clusters. The trouble
occurs in varying degrees from the loss of a few berries to the
complete shelling of the fruit from the stem. It is worse in some
localities than others and in some varieties than others. Various
causes have been assigned to the disease, chief of which, and most
probable, are unfavorable climatic conditions.

Next: Control Of Insects And Diseases
Previous: Insect Pests

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