Common Diseases of the Grapevine in Michigan

Michigan State University Extension
MSU Extension Fruit Bulletins - 17329401
11/30/99

Common Diseases of the Grapevine in Michigan Michigan State University Extension Extension Bulletin E-1732 October 1994

D.C. Ramsdell
Department of Botany and Plant Pathology
AG FACTS

Black Rot

This disease, caused by the fungus Guignardia bidwellii, is
common throughout Michigan, the Eastern United States and
Canada. If uncontrolled, it can be devastating in terms of
crop loss. Well-timed, effective fungicide sprays can give
almost 100% control.Symptoms and Disease CycleSmall,
circular reddish-brown spots 1/8 to 1/4 inch in diameter
appear, usually on the lower leaves, in mid- to late-June
(Fig. 1). The lesions are the result of spring ascospore
infections originating from old black rot fungus infected
berries and canes. Only a few hundredths of an inch of rain
are necessary to promote ascospore release and subsequent
leaf infection. The lesions develop a black border and
small black, pimple-like pycnidia are found in the
reddish-brown part of the lesion. The herbicide Gramoxone,
often used in vineyards for weed control, can cause
similar-appearing leaf spots, but these do not show the
black pycnidia in the lesions.

Ascospores and conidiospores (secondary spores) infect the
blossoms during each rain. However, the results of early
blossom and fruit infection are not visible until mid-July
or early August, when infected grape berries begin to
shrivel in the clusters. Leaf, cane and tendril infection
can occur only when the tissue is young, but berries can be
infected until almost fully grown if an active fungicide
residue is not present.Mid-season infection of green fruit
is evidenced by a whitish circular spot 1/8 inch in
diameter on the fruit at the point of infection. About two
weeks later, after the fungus has grown throughout the
berry, it begins to shrivel and look like a hard bluish
raisin (Fig. 2). These eventually fall to the ground and
are an inoculum source for the rest of the season. American
varieties such as Concord and Niagara are quite susceptible
to the pathogen; Delaware and certain French
-American hybrids are moderately resistant. Aurore is a
particularly susceptible variety. (See Table 1).

Control
Fungicidal control is very effective. Good spray coverage
is essential. Consult the MSU Fruit Spraying Calendar
(Extension Bulletin E-154) for recommended fungicides,
rates and timing of sprays.

Downy Mildew
This disease can be extremely serious in grapes and will
cause severe crop loss. The fungus Plasmopara viticola
causes downy mildew.

Symptoms and Disease Cycle
The fungus overwinters in diseasedleaves on the ground.
Spores are released in the spring and spread to the leaves
and berries by splashing rain or wind. During years with
warm, extended wet periods during bloom, fruit clusters may
be partially or totally killed (Fig. 3). Clusters infected
early in the season soon become covered with a white fungus
growth. Attacks later in the season are restricted to
cluster stems, or to individual berries in the cluster
which shrivel. The older infected berries harden and turn
red or yellow instead of ripening.Leaf symptoms appear in
early to mid-July. Small, pale yellow circular lesions 1/8
to 1/4 inch in diameter are seen first on the upper surface
of the leaf (Fig. 4). On the underside of the leaf are
corresponding areas of whitish-grayish fungus growth (Fig.
5). The fungus produces sporangia, which liberate zoospores
into a film of water and spread the infection during
prolonged rains or dew. Later, the leaf tissue dies in the
area of the lesion. If lesions are numerous, they will
coalesce, causing large areas of the leaf to turn dark
brown or black. Badly damaged leaves will prematurely fall
leaving berries exposed to sun scald. On certain
French-American hybrid and European (Vitis vinifera)
varieties cane tips will become blighted, exhibiting
distortion and twisting and may be covered by
whitish-greyish sporulation. See Table 1 for a rating of
varietal susceptibility.

Control
Downy mildew is comparatively easy to control with
fungicides. Apply fungicide sprays just before bloom,
during bloom (if bloom is slow), after bloom, 10-14 days
later, about August 1 and again about September 1. It is
most economical to use a fungicide or combination of
fungicides that will control both black rot and downy
mildew. Consult the MSU Fruit Spraying Calendar (Extension
Bulletin E-154) for fungicide rates.

Powdery Mildew
Powdery mildew is most severe on French hybrids and
European (Vitis vinifera) grapes. However, in some years it
can cause moderate damage to American grapes. Leaves
heavily infected by powdery mildew are less able to
manufacture food (photosyn-thesize) resulting in decreased
plant vigor and increased chance of winter injury to the
vines.

Symptoms and Disease Cycle
Leaf symptoms of powdery mildew appear in early to mid-July
as a white powdery or dusty fungus growth on the upper
surfaces of the leaves and other green parts of the vine
(Fig. 6). Severely affected leaves turn brown and fall.

Attacks later in the season are restricted to the berries
and cluster stems. Infected cluster stems can cause
shelling of ripe fruit. Infected berries turn hard, brown,
and fail to properly mature. In late summer and early
autumn, minute black fruiting bodies called cleistothecia
form on infected parts; this is the overwintering stage of
the fungus. Some varieties are more susceptible than
others. Consult Table 1 for relative varietal
susceptibility.

Control
Fungicide sprays, beginning about two weeks after bloom
followed by another spray about three weeks later and a
third one about September first, will control powdery
mildew fairly well. Consult the MSU Fruit Spraying Calendar
(Extension Bulletin E-154) for suggested fungicides and
rates.

Eutypa Dieback
The disease Eutypa dieback was earlier called "dead-arm"
due to an error made by plant pathologists many years ago.
What was called "dead-arm" was really the symptoms of two
diseases: Eutypa dieback, caused by the fungus Eutypa
armeniacae and phomopsis leaf and cane spot disease caused
by the fungus Phomopsis viticola. This latter disease will
be described later. Eutypa dieback is present in about 10%
of the Concord grapevines in Michigan. It is a very serious
and costly disease.

Symptoms and Disease Cycle
The most striking symptoms appear in the late spring and
early summer when new shoots are 6 to 12 inches long. The
shoots are stunted and the leaves are cupped upward,
smaller than normal and yellow or yellow-streaked (Fig. 7).
These symptoms are caused by a deep-seated wood rot of the
arms or trunk; probably a toxin is given off by the fungus.
As the disease progresses over several years, one or more
arms may die (hence the old name of "deadarm") as well as
the whole vine.

Careful examination of the arms or trunk will reveal that
the initial infection occurred at a pruning wound. Around
this old wound is a dark "stroma" (Fig. 8) composed partly
of grape bark tissue and partly of fungus tissue. The
stroma contains fungal fruiting bodies called "perithecia"
whose ascospores are shot out from autumn through May
whenever the temperature is above freezing and rainfall
occurs.

Very small amounts of rain will trigger ascospore release.
The spores are wind-borne and infect pruning wounds on the
vines. When sawing through a diseased arm or trunk, a dark
pie-shaped area of diseased wood is often present. It is
unusual that the disease is infective during the winter
months instead of the summer. It takes two to three years
after infection for symptoms to show, and four or five
years for stroma to form. There is no known varietal
resistance to this disease.

Control
Removal and burning of diseased arms or the entire vine is
recommended. Sometimes a new vine can be trained from a
sucker. However, these often show disease symptoms after a
few years. Double-trunking has also been successfully done.
If one trunk becomes diseased, it can be cut off leaving
the other which may remain disease-free for some time.

Phomopsis Leaf and Cane Spot and Fruit Rot Disease
This disease is caused by the fungus Phomopsis viticola.
Symptoms were originally thought to be part of the
"dead-arm" disease. It was not until the mid-1970s that
this confusion was rectified.

Symptoms and Disease Cycle
The fungus overwinters in the canes producing fruiting
bodies (Fig. 9). Spores infect new leaves in the spring
during rainy weather. The leaf symptoms appear in early to
mid-June as small angular dead spots (Fig. 10). The lower
leaves are the first to show infection. Later in the
season, canes, tendrils, leaf petioles and even cluster
stems may show elongated, brownish or purplish lesions 1/4
inch long (Fig. 11). If the disease is severe, the fungus
enters the grape berries, probably through the pedicel
(berry attachment to the cluster stem). Mechanical
harvesting will often shake many berries off the vine ahead
of the machine, causing considerable crop loss."Niagara"
grape is very susceptible to this disease. "Concord" is
less susceptible, but where fungicidal control has been
lacking, losses can occur.

Control
Fungicide sprays applied at the 1-inch shoot length stage,
again at the 4- to 6-inch shoot length stage (first black
rot spray) once during bloom and again at pea size will
give good control. For specific fungicides and rates,
consult the MSU Fruit Pesticide Handbook (ExtensionBulletin
E-154).

Crown Gall
This is a bacterial disease caused by Agrobacterium
tumefaciens. The main symptom consists of galls formed on
the roots, crowns and/or trunks (Fig. 12). Crown gall can
arrive or spread in a vineyard by several means. Once
established, the bacteria can live in the soil for many
years.

Infection of grapevines can occur directly through root
wounds or as the result of bacteria being rain-splashed
onto upper portions of the plant. The bacteria can also be
spread by shears during the pruning process. Sometimes
nursery stock will arrive already infected with crown gall,
thus, inoculating the soil into which it is planted. The
vines can become weakened and stunted if severely infected.

Control
Reject planting stock with galls or suspicious swellings.
Dipping roots and crowns before planting into a suspension
of a hypovirulent strain of Agrobacterium sp. such as
Galltrol@ will protect against infection by resident crown
gall bacteria at the planting site. Disinfect shears
between cuts when pruning vines are known to be infected
with crown gall.

Peach Rosette Mosaic Virus Disease
Concord and Catawba and Niagara grapevines are infected
with peach rosette mosaic virus (PRMV) in a large
percentage of vineyards in Michigan. This virus is
soil-borne and is spread from plant to plant by the
root-feeding dagger nematode (Xiphinema american
um). Dandelion, curly dock and carolina horsenettle are
weed hosts for the virus. SymptomsWhere the disease exists,
there are "holes" in the vineyard (Fig. 13). Infected
vines are usually umbrella-shaped (Fig. 14) because the
virus causes the canes to grow somewhat crookedly.
Internodes are shorter than normal (Fig. 15), and leaves
are distorted (Fig. 16). Berry cluster shelling (Fig. 17)
will occur on vines that have been infected for several
years. The virus is seed-borne in grapes. The practice of
spreading grape pomace in the vineyard should be halted,
because this can reintroduce the virus into the vineyard.
The dagger nematode will spread the virus from infected
grape seedings to healthy vines. Delaware grapevines are
less susceptible to PRMV than are Concord, Catawba and
Niagara varieties.

Control
Control of this disease includes pre-plant soil fumigation
of vineyard sites to kill the dagger nematode vector. If
diseased vines are present in an established vineyard, all
infected vines will need to be identified and removed. The
soil should then be tilled for one growing season. Fumigate
late-summer or autumn soil prior to replanting with
certified virus-tested clean stock. See MSU Extension
Bulletin E-806 "Vineyard Preparation for Nematode and Virus
Disease Control" and the MSU Fruit Spraying Calendar
(Extension Bulletin E-154) for further recommendations
concerning control by soil fumigation.

Tomato Ringspot and Tobacco Ringspot Disease
French hybrid grapevines (especially blue-fruited
varieties) are susceptible to infection by these two
viruses. The symptoms are the same regardless of which
virus causes the disease. Tomato ringspot (TmRSV) is more
prevalent than tobacco ringspot virus.

Symptoms
Leaves on infected canes are yellow and smaller than
normal, vines appear stunted (Fig. 18) and internodes are
abnormally shortened (Fig. 19). Vines lose vigor and will
often die of winter injury if infected with either virus.
The dagger nematode, Xiphinema americanum spreads this
disease by feeding on the roots of infected weeds
(chickweed, dandelion, plantain and others) or grapevines
and then feeding on the roots of healthy vines.

Control
See control section for peach rosette mosaic virus.

Fungus disease susceptibility1 and sulfur sensitivity 2 of
American, French Hybrid, and Vinifera (European) grape
varieties.

Black Downy Powdery Phomopsis Sulfur Sensitive?
Rot Mildew Mildew Botrytis (i.e. does sulfur
cause injury?)
Variety
Aurora
(S5279) + +++ ++ +++ +++ No
Baco Noir
(Baco #1) + + + ++ + No
Cascade
(S13053) ++ + + ++ ? No
Catawba +++ +++ +++ ++ + No
Cayuga
White + ++ + + No
Chancellor
(S7053) +++ + +++ +++ +++ No
Chardonnay +++ ++ +++ +++ +++ No
Chelois
(S10878) +++ + + +++ + No
Concord +++ +++ + ++ + Yes
DeChaunac
(S9549) +++ + ++ +++ ++ No
Delaware +++ ++ +++3 ++ ++ No
Dutchess ++ +++ ++ ++ + ?
Elvira + + ++ ++ +++ No
Foch (Marechal
Foch) + + + ++ + No
Himrod ? ? + ++ ? No
Niagara +++ +++ +++ ++ + No
Riesling ++ +++ +++ +++ No
Rosette
(S1000) ++ ? ++ +++ + No
Rougeon
(S5898) +++ ++ +++ +++ ++ Yes
Seyval
(SV5-276) ++ ++ ++ +++ ++ No
Steuben ? ++ + + ? No
Verdelet
(S9110) ? + ? ? + No
Vidal
(V256) + ? ? +++ + No
Vignoles
(Ravat 51) ++ + ++ +++ +++ No1

Question mark (?) means relative susceptibility not
established;
+ = slightly susceptible, ++ = moderately susceptible, +++
= very susceptible.

2 Sulfur injury can occur on tolerant varieties when
temperatures of 85F or higher occur during or immediately
after spray application.

3 Berries are not susceptible.

Fig. 1. Black rot lesions on leaf. Note dark borders of
spots.
Fig. 2. Black rotted grapes shrivel in the cluster and
resemble blue raisins.
Fig. 3. The downy mildew fungus causes shriveling of large
portions of infected clusters. Note white mycelium on
shriveled portions.
Fig. 4 (top) & 5 (bottom). Yellow areas on upper leaf
surface correspond with white mycelial patches of the downy
mildew fungus on the lower leaf surface.
Fig. 6. The powdery mildew fungus causes white, webby
patches on leaves and other green parts of the vine.
Fig. 7. Symptoms of Eutypa dieback include stunted shoots
and yellowed leaves cupped upward.
Fig. 8. "Stroma" of the Eutypa fungus. Note the
pimple-like perithecia (arrow) containing ascospores which
are ejected from autumn to May.
Fig. 9. Fruiting bodies of the Phomopsis fungus on an
infected cane.
Fig. 10. Leaf symptoms of Phomopsis leaf and cane spot
disease appear as small angular dead spots.
Fig. 11. Elongated dark lesions of Phomopsis on cluster
stem and along leaf veins.
Fig. 12. Galls on trunk as a result of crown gall
bacterial infection.
Fig. 13. Missing vines indicate an infection center for
peach rosette mosaic virus. Dead vines have been removed.
Fig. 14. Peach rosette mosaic virus infected vines are
often umbrella-shaped.
Fig. 15. Shortened cane internodes caused by peach rosette
mo
saic virus.
Fig. 16. Distorted leaf of peach rosette mosaic
virus-infected vine.
Fig. 17. Berry cluster shelling occurs on vines that have
been infected with peach rosette mosaic virus for several
years. Note small fruit cluster.
Fig. 18. Tomato ringspot or tobacco ringspot virus
infection causes yellow stunted leaves and shoots. (Healthy
vine on left and diseased on right.)
Fig. 19. Abnormally shortened inter-nodes of tomato
ringspot or tobacco ringspot virus infected grapevines.

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Issued in furtherance of Extension work in agriculture and
home economics, acts of May 8 and June 30, 1914, in
cooperation with the U.S. Department of Agriculture. Gail
L. Imig, extension director, Michigan State University, E.
Lansing, MI 48824.

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