Muscadine Grape Diseases and Their Control

Fruit Disease Information Note 12
W.O. Cline, Extension Plant Pathologist




[General Information] [Symptoms] [Control]
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General Information

The muscadine grape (Vitis rotundifolia) is a native grape species found in the southeastern United States. It grows wild throughout North Carolina and will survive in areas where other grape species will not. The fruit is generally harvested as single berries rather than in bunches. As the species name implies, the leaves are round and unlobed, though serrated at the edge. Both purple-fruited "black" and bronze-fruited "white" muscadines are seen in the wild. Since the early 1950s, many improved cultivars have been developed for commercial production of fresh fruit and for winemaking.

Although far more resistant to most diseases than are bunch grapes (V. labrusca, V. vinifera), muscadines can suffer extensive losses in commercial plantings if diseases are left unchecked. Fruit rotting diseases can reduce yields and make berries unmarketable for either wine or fresh sales. Fruit grown for fresh consumption must be adequately sized and free of blemishes caused by disease. 

Symptoms of Major Diseases of Muscadine Grapes

Angular leaf spot (Mycosphaerella angulata) is an important leaf disease. The symptoms of angular leaf spot first appear as faint, light yellow spots on the leaves. As the yellow spots become more noticeable, irregular brown flecks develop in the center of the spots. As the growing season progresses, the disease often increases and causes extensive defoliation by harvest. If heavy defoliation occurs, the total yield and also the quality of grapes are reduced.

muscbit1.jpg (4323 bytes)Bitter rot (Greeneria uvicola, syn. Melanconium fuligineum) can be a very destructive fruit disease. Infection occurs shortly after bloom. Just before harvest, infected berries turn black with spore-bearing structures (acervuli) erupting through the skin of the fruit. Rotted berries may or may not fall to the ground before harvest. The fungus infects fruit stems, leaves and young shoots.

muscrus1.jpg (6046 bytes)Powdery mildew (Uncinula necator) attacks berry clusters and young berries just after flowering. Infected berries are covered with a white fungus growth. As the berries enlarge, the fungus growth disappears, but the grapes become rough-skinned (russetted) and may crack. Loss of yield results from both berry drop and reduced size of berries.

muscrr1.jpg (4427 bytes)
Ripe rot
(Glomerella cingulata; imperfect stage Colletotrichum sp.) Also causes a berry rot near harvest and in recent years has become the most damaging fruit rot disease. Rotted berries turn uniformly dark brown over part or all of the berry and have pink or orange spore masses on the surface. Ripe rot infections can occur at any stage of fruit development, but fruit infected in the green (unripe) stages does not rot until it begins to ripen.  Once infected grapes begin to rot and produce spores in the vineyard, the disease can spread rapidly to other ripe fruit.  The most devastating losses to this disease occur on susceptible cultivars during rainy harvest seasons. Generally speaking, dark skinned cultivars (Noble, Pride) are more resistant, while bronze cultivars (Carlos, Scuppernong, Magnolia) are more susceptible.

muscmac1.jpg (4513 bytes)Macrophoma rot (Botryosphaeria dothidea) causes small, sunken, black fruit spots that are round with distinct edges in the early part of the season. As harvest approaches, these spots may develop a greasy-looking soft rot around the initial lesion. A halo develops around the black spot and the entire fruit may rot just before harvest. Infections are sometimes not visible until the soft rot stage occurs.

muscbrl1.jpg (5986 bytes)Black rot (Guignardia bidwellii f. muscadinii) causes a circular brown leaf spot and a black scab on berries. Occasionally, lesions occur on the young stems and tendrils. Black rot seldom causes much damage in sprayed vineyards.

muscpie1.jpg (7692 bytes)
Pierce's disease
(Xylella fastidiosa) is the primary disease limiting production of bunch grape species (V. labrusca, V. vinifera) in the southern U.S. This bacterial pathogen is of little importance on most muscadine grapes, occasionally causing a marginal leaf burn on susceptible cultivars such as Carlos. The cultivar Pride is highly susceptible and may be killed by the bacterium. Growers should avoid propagating from symptomatic vines. Chemical control is not available.   For more information on Pierce's disease on bunch grapes, see the information note at Texas A&M.

musccg1.jpg (8823 bytes)Crown Gall (Agrobacterium sp.) is a disease caused by a bacterium which has frequently been associated with gall formation on grape vines. Galls are fleshy, irregularly shaped growths. The disease usually occurs in association with freeze injury, and galls may form all along the length of the trunk and cordons. Fall planting exposes new vines to freeze injury and should be avoided. Symptoms of crown gall are far less severe on muscadine grapes than on bunch grapes.

Control

Resistance.  Before establishing a planting, investigate the possibility of using disease-resistant cultivars. For instance, among large-fruited bronze cultivars, 'Fry' is very susceptible to ripe rot, while 'Triumph' is not. As mentioned earlier, dark-skinned cultivars tend to have less ripe rot.

Cultural Practices.  Home and commercial growers alike can reduce diseases through cultural practices. Mowing or otherwise reducing undergrowth near vines will improve air movement through the vineyard. Timely harvesting along with removal of leftover fruit at the end of the season should also help reduce fruit rots. Avoiding excess late-season fertilizing can reduce both disease and the likelihood of winter injury to cordons and trunks of vines.

Fungicides.  Results from experimental tests for control of muscadine grape diseases have shown that a regular spray program with an effective fungicide plus an insecticide is highly beneficial and profitable. An effective disease control program is essential to produce the high yields of quality grapes that are possible with the newer varieties. In new vineyards, the disease control program should begin in the second or third season after planting. Repeated early season applications of fungicides (May-June-July) are the most effective. In home plantings, fungicides are generally not necessary.
 

Relative Effectiveness of Various Fungicides at
Stated Rates for Control of Grape Diseases
(0 = ineffective or injurious; xxxxx = very effective or very safe)
Fungicide (Rate/Acre)

Angular
Leaf Spot

Bitter Rot

Powdery
Mildew

Ripe Rot

Macrophoma
Rot

Black Rot

Plant Safety

Benlate 50WP
(1.0lb/ac)

XXXXX

XXX

XXXXX

X

XXXX

XXXXX

XXXXX

Captan 50WP
(4.0lb/ac)

XXXX

XXXX

0

XXX

XXX

XXXXX

XXXXX

Wettable Sulfur
(4.0lb/ac)

0

0

XXXXX

0

0

0

XXXX

Nova 40WP
(5.0oz/ac)

0

XXXX

XXXX

0

X

XXXX

XXXXX


Other Resources


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Outside North Carolina, look for your state extension service partners.

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Recommendations of specific chemicals are based upon information on the manufacturer's label and performance in a limited number of trials. Because environmental conditions and methods of application by growers may vary widely, performance of the chemical will not always conform to the safety and pest control standards indicated by experimental data. All recommendations for pesticide use were legal at the time of publication, but the status of registration and use patterns are subject to change by actions of state and federal regulatory agencies. 

Published by the North Carolina Cooperative Extension Service
Distributed in furtherance of the Acts of Congress of May 8 and June 30, 1914. Employment and program opportunities are offered to all people regardless of race, color, national origin, sex, age, or disability. North Carolina State University at Raleigh, North Carolina A&T State University, U.S. Department of Agriculture, and local governments cooperating.

Last update to information: April, 2000
Last checked by author: April, 2000
Web page last updated on 07 April 2000 by A.V Lemay.