Anthracnose (fungus
-
Elsinoe ampelina): This disease is often called "birds-eye-rot" because
of the circular, sunken, ashy gray, dark-bordered spots on the berries. It
attacks fruit stems, leaf veins, petioles, tendrils, and young shoots in addition
to the berries. The berry often cracks to the extent of exposing the seed.
The disease overwinters in old lesions. Periods of high humidity favor disease
development. Fungicides used for black rot will help prevent this problem.
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Black Rot
(fungus - Guignardia bidwellii): Black rot shows up as small, reddish-brown
spots on the upper leaf surface. In older lesions the margin is a black
line while the inner area of the spot is brown. Small, black dots are also
visible in the center of the lesion. Infected fruit shrink until they are
dried mummies. The first stage of development on the fruit is small, light-colored
lesions with black borders. In advanced stages the fruit is marked with
the small, black dots just like the foliage. Young grape foliage is most
susceptible to this disease. Disease development is favored by high temperatures
and humidity. A rainy period followed by two to three days of foggy weather
favors development of this disease. Preventative fungicides along with resistant
varieties should be used to control black rot.
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Cotton Root Rot: (See
section on Cotton Root Rot.)
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Chlorosis: Chlorosis in grapes is usually caused by iron deficiency.
American varieties are particularly prone to have this problem. The leaves
turn yellow, but the veins remain green. If not remedied, chlorosis will
decrease the yield, reduce sugar content of the fruit, and eventually
kill the vine. Two applications of iron sulfate or iron chelate during
the growing season should control chlorosis. In high pH soils, iron sulfate
and some iron chelate may become tied up and unavailable to the plant.
Foliar applications can be made, or a chelate especially for alkaline
and calcareous soils, such as Sequestrene 138Fe, can be used.
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Crown Gall (bacterium - Agrobacterium tumefaciens): Crown
gall is caused by soil-inhibiting bacteria. The bacterium often enters
wounds caused by freeze injury or hail. It stimulates the cells in the
plant tissues to grow rapidly, resulting in the formation of tumor-like,
spongy overgrowths which develop around the crown of the plant. Rain can
splash bacteria onto the aerial parts of the vine where the tumors are
often called black knots. The pathogen can move in the vessels and spread
throughout the vine. Sanitation is the best prevention. Avoid cutting
into affected tissue when pruning. Chemical and biological controls are
now available.
See
section on Crown Gall
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Dead Arm
(fungus -
Eutypa armeniacae): This fungus invades grape tissue slowly.
Spring symptoms on developing shoots and leaves adjacent to an infected pruning
stub may not appear for four to five years. Cankers form on vines, and nearby
leaves become chlorotic and dwarfed. Cordons die later, producing the "dead
arm" symptom.
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Downy Mildew (fungus
-
Plasmopara viticola): Downy mildew is common in cool, humid environments
but seldom occurs in hot, dry areas. Downy mildew is first observed as a pale
yellow area on the upper surface of the leaf. The underside of the leaf is
marked by a downy appearance. As the disease advances the infected tissue
dies and turns brown. Young stems become thickened and are often covered with
the white fruiting structures. Fruit that is infected is covered with the
white growth or it turns the berry a dull green and then brown. Downy mildew
is a particular problem in areas of high humidity. The disease develops in
temperatures of 50 to 60 degrees F. Some American grape varieties show resistance,
but additional chemical protection is usually needed. Most European varieties
are very susceptible. Fungicide application should begin before bloom and
continue at seven day intervals.
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Grapevine
Fanleaf (virus): Fanleaf is caused by a virus that also causes yellow
mosaic and veinbanding. All three are transmitted by the nematode
Xiphinema
index. Leaf symptoms that resemble a fan are very conspicuous on Mission
and French Colombard. Infected vines have shortened and more irregular internodes.
Lateral sprout development, double nodes, and stem faciations cause a bushy
appearance. Many berries shatter, and others do not develop beyond shot size.
Diseased vines should be removed.
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Leaf Roll (virus): Leaves
roll downward and turn red between the veins progressing toward the cane
tips. In California, symptoms appear in early June in non-irrigated vineyards
and in August in irrigated vineyards. Vines have fewer, smaller clusters
per vine with berries that are low in sugar. Red fruit varieties, such as
cardinal and Mission, develop fruit lacking color, and berries of white
grapes, such as Thompson seedless and Reisling, develop a yellowish-white
color instead of the normal greenish-white. In general, leaf roll decreases
fruit color, raises the acidity, and delays ripening. The disease is spread
by propagation from infected mother vines.
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Mushroom Root Rot: (See Mushroom
Root Rot.)
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Pierce's
Disease (bacterium - rickettsia-like): Early summer symptoms include
delayed shoot growth, leaf mottling, and dwarfing of new shoots. Late summer
and fall symptoms are burning, scalding, or drying of leaves; wilting or
premature coloring of fruit; and uneven cane maturity. Ribier is very susceptible,
usually dying within two years. Thompson seedless and most other French
(Vinifera) varieties die within two to five years. American (Lambrusca)
varieties often live longer than five years. No effective control is known.
Pierce's disease is spread by several types of leafhoppers, by the spittlebug,
and by grafting. Seventy-three plant species serve as disease reservoirs
and hosts for these vectors.
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Powdery
Mildew (fungus -
Uncinula necator): This fungus grows on all above
ground parts of the vine. Powdery mildew causes curling and withering of young
leaves and dark staining on the surface of mature leaves. It may appear as
a gray powdery growth on canes, and when rubbed off, it leaves web-like, dark-brown
discolorations. Other symptoms include dropping, discoloration or splitting
of berries, browning, and poor maturation of canes. American grape varieties
are rarely damaged by this disease. Since the fungus is favored by low humidity
and can grow at 90 degrees F., this disease is more common in West Texas.
Prevention is the best means to control powdery mildew. It should be controlled
by fungicides applied when foliage first develops and repeated at two to three
week intervals until berries are full size. If newer fungicides are not available,
sulfur dust applied at the rate of 5 to 10 pounds of dusting sulfur per acre
will prevent powdery mildew from developing. Apply dust when shoots average
6, 12, and 18 inches in length. Make additional applications every two weeks
until fruit matures. Reapply sulfur after rains. If early sprays are omitted,
heavy infections can be controlled with wettable sulfur at 1.5 pounds/100
gallons with a wetting agent. Do not spray table grapes with this mixture
if berries are more than one-third full size. Sulfur dust or spray can burn
leaves, shoots, and fruit when the temperature is over 90 degrees F.; no applications
should be made at such times.
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Vein Clearing
(virus): Yellowing along the leaf veins.
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