Diseases

Fruit Rots

Black Rot (Guignardia bidwellii): This is probably the most damaging grape disease in New England. Most loss is caused by damage to the berries, though leaves, tendrils and new shoots are also damaged. The fruit is susceptible from fruit set until veraison; resistance increases as fruits transition from pea-size to veraison.

This disease is caused by a fungus that overwinters in mummified berries and stem lesions. Mummies on the soil surface release spores when rain soaks them in the spring. There is a continuous production of spores throughout the spring and summer. These are carried to new plants by wind. The duration of leaf wetness required for infection to occur varies with temperature (see Table 53). Young tissue is infected in less than 12 hours between 60–90˚F. Spores germinate and produce mycelium resulting in symptoms in 8 to 25 days, depending on the weather. New leaves and half-grown berries are most susceptible. Secondary infections occur when new spores are produced on the current year’s infections. Secondary spores are produced into August, and are spread by splashing rain.

On leaves, infections appear as yellowish-tan spots in late spring. These roughly circular spots enlarge and become reddish-brown with a dark outline. Lesions are roughly circular in shape. Shoots develop sunken, elliptical black lesions, up to 2 cm in length. On the berry, symptoms do not appear until the fruit is half grown. Lesions start as a small whitish dot and quickly engulf the whole berry. The infected area develops a reddish brown color. The berry wrinkles and blackens completely within a few days. These fruit become mummies that are very hard and stony, and supply inoculum for the following year.

Management: Sanitation is very important. Destroy all mummies and canes with lesions. Remove infected tendrils from vines. Plant grapes in locations having good air circulation, taking advantage of prevailing winds and sun. Black rot is more likely to occur near woodland borders than in full sun, and it occurs much more severely in wet years than in dry ones. Protectant fungicides offer good control if applied initially when the shoots are 10-16 cm long and continued until the berries contain approximately 5% sugar. Strobilurins (Abound, Elite, Flint, Sovran, Nova and others) are excellent eradicant and protectant materials. See pest management schedule for recommended materials and timing. Varietal resistance is another control option (see Table 59). 

Bitter Rot (Greeneria uvicola): Bitter rot, while most common in southern grape regions, may infect grapes in New England. If 10% of the berries in a wine pressing are infected with bitter rot, the wine can be undrinkable. Bitter rot may be easily confused with black rot. Infected berries first develop brownish, water-soaked lesions. The bitter rot fungus infects ripe grapes, and unlike the black rot fungus, does not infect green berries. Bitter rot susceptibility increases at veraison. Lesions often have concentric rings in white-fruited varieties. Berries turn brown but retain their shape. Within 3-4 days, black pustules erupt on the berry. If overripe berries become infected, they are not easily detected, because pustules do not form. These berries are the most bitter, and the most likely to be mistakenly harvested.

Warm, humid weather at the time berries ripen favors the disease. The fungus grows rapidly, and berries can rot in 5 to 7 days. Wounding promotes fungal growth.

Management: Promote good air circulation for good drying in the vineyard. Fungicides used for the control of other diseases usually will also control bitter rot. If conditions are right for infection, late season sprays should not be omitted. Most varieties have some degree of resistance to the fungus.

Botrytis Bunch Rot (Botrytis cinerea): Botrytis rot can cause serious losses in susceptible varieties. While some rot is acceptable in wine grapes, and may even be desirable, the disease can get out of control. The fungus that causes the disease is present in grape mummies, debris on the vineyard floor and in organic matter around the planting. Spores are released in moist, cool weather in spring, and then throughout the growing season. These first spores infect blossoms at the end of bloom. A second infection occurs at berry maturity. The fungus uses senescing or dead material as a base to spread into healthy tissue. Botrytis-infected berries are at first soft and watery. The berries usually become covered with gray, fuzzy fungal mycelium within a few days. Rotted berries shrivel, then drop to the ground to eventually become mummies.

Management: Good air circulation and vineyard sanitation are helpful. Leaf removal around the clusters has shown excellent control of the disease in California. White-fruited varieties (particularly Riesling and Seyval) are highly susceptible. Protective fungicides should be used when wet weather occurs near bloom and berry ripening. See pest management schedule for recommended materials and timing. Fungicides should be used thoughtfully to avoid promoting resistance by the fungus.

Ripe Rot (Colletotrichum acutatum, C. gloeosporioides, Glomerella cingulata): Ripe rot is a disease affecting grapes at or near harvest time which has largely been confined to the southeastern U.S. but has caused problems in recent years in southern New England. Rotted berries turn uniformly dark-brown over part or all of the berry and sometimes have pink or orange spore masses on the surface. As infected fruit mature, lesions appear as slightly sunken or flattened rotted areas. As lesions expand, the entire grape eventually rots, and may drop or become shriveled or mummified as it decays. Ripe rot infections can occur at any stage of fruit development, but fruit that is infected in when unripe does not rot until it begins to ripen. In these berries the fungus remains in a latent state until conditions allow it to further develop in the tissue. Once infected grapes begin to rot and produce spores in the vineyard, the disease can spread rapidly to other uninfected fruit, within the same bunch or neighboring bunches. The most devastating losses occur on susceptible cultivars during warm rainy harvest seasons. Generally, darker-skinned cultivars are more resistant while white cultivars are more susceptible.

Management: Before spring arrives, remove or disk into the soil all overwintered mummies left on the trellis and ground from the previous season. Good canopy management practices are essential for control of ripe rot. Shoot thinning, leaf removal, pruning, cluster thinning and shoot positioning are all cultural practices that open the vine canopy to increase airflow and light, reduce the amount of moisture trapped within the canopy, and allow better penetration and coverage by fungicides. Timely harvesting of all ripe grapes is recommended, to prevent overripe fruit with fungal sporulation from hanging on the vines too long. Where the disease is a problem, fungicide applications are critical during the period between bloom and pre-harvest. Captan and Pristine are the best fungicide choices for control of the disease.

Foliage and Cane Diseases

Downy Mildew (Plasmopara viticola): This disease causes damage primarily by attacking the vine, though all parts of the plant are susceptible to injury. The optimum conditions for the disease are cool to moderate temperatures, and wet weather. The disease is caused by a fungus that requires living tissue as a host. In spring, spores of the fungus come from dead tissue on the ground. Free water is required for infection, and infections may occur during high humidity throughout the season. Splashing water or handling wet plants may readily spread the spores. The spores grow into cottony masses, producing many new spores which can spread the infection. As tissue dies, it falls to the ground where the fungus overwinters. Severe epidemics can defoliate the vine.

On leaves, new infections are difficult to see. They appear first as generally angular, pale-yellow spots delimited by veins which later become brown. On the underside of the leaf the cotton-like ‘downy’ growth appears. Fruit infection occurs at two times. First, when the berries are the size of small peas, infections will cause berries to turn light brown and soft. Berries will shatter easily. Sometimes the downy growth covers the berries. During the heat of the summer, little fruit infection occurs. The second infections occur in the late summer or early autumn. These berries do not turn soft or develop downy growth, but turn dull green, then purplish-brown. Shoots and tendrils develop water-soaked lesions, become stunted and distorted, and may die.

Management: Remove debris from the vineyard floor. Maximize air circulation to improve drying. European grapes are generally more susceptible than American grapes. The most serious epidemics occur when a wet winter is followed by a wet spring and a warm summer with frequent precipitation. Fungicides should be applied when disease pressure is high. Apply just before bloom; 7-10 days later; 10-14 days later; 3 weeks later. See pest management schedule for recommended materials and timing.

Powdery Mildew (Uncinula necator): Powdery mildew causes losses by infecting leaves and berries. It is primarily a problem on European grape varieties, although American varieties may be damaged. It may be confused with downy mildew (see above). Losses are not generally heavy from the disease, but can build up over several years.

The fungal pathogen overwinters in specialized structures on or in living tissue. In spring, spores are released that attack new tissue. Unlike other grape diseases, rain and free moisture are not important to the spread of powdery mildew. Warm conditions with high relative humidity favor this disease. Wind carries newly produced spores from infected areas into new locations. Infected leaves have the appearance of being coated with a white powder. Severely infected leaves curl and defoliation may occur. Leaves of American varieties like Niagara and Concord are very susceptible. Young fruit and blossoms may be misshapen by infections; mature fruit is immune.

Management: Cultural practices can help reduce disease incidence. Planting in sites with good air circulation and sun exposure and the use of appropriate training systems which allow for good air movement are highly advisable.Use fungicides where infections are known to occur. Copper and lime sulfur dormant applications provide good early season control. However, there are label restrictions. Check with your state Extension Specialist for recommendations. Because some varieties are sensitive to sulfur, this material should always be applied at cooler temperatures (<85˚F). Strobilurin (group 11) fungicides are also effective, but care should be taken to avoid selecting for resistance by the fungus. See pest management schedule for recommended materials and timing.

Eutypa Dieback (Eutypa armeniacae): This disease also has been known as “dead arm.” It causes limbs to die back and forms cankers. Recently, it was shown to occur in conjunction with Phomopsis, causing the dead arm symptoms. Cankers are frequently found around old pruning cuts. They are usually under the bark, and show only as a flattened area on the surface. The cankers run lengthwise along the limb. Infections occur on pruning cuts in early spring. Over several years, the infection increases, causing new leaves to emerge small and yellowed. New shoot growth has shortened internodes, leaves are small and cupped and all growth is chlorotic. After about 5 years, the bark sloughs off, and eventually, the cane dies. This is seldom seen in vineyards younger than 8 years old.

Management: Infected material should be removed; in some cases it may be necessary to remove the whole plant. Make cuts well below cankers. Destroy all prunings. Prune directly after a rain to minimize risk for infection, as the atmospheric spore load has been washed out temporarily. Prune late in the dormant season to promote rapid healing of wounds. Multiple trunk systems are recommended with renewal on an 8- to 10-year cycle. This helps minimize risk of losses due to both Eutypa dieback and crown gall. All commercial varieties are susceptible.

Phomopsis cane and leaf spot (Phomopsis viticola): This is a fungal disease that causes reddish-brown lesions on canes, leaf spots and fruit rot. Small black spots at the base of developing shoots are the first sign of infection. These areas may crack, and late in the season may appear bleached. Leaf infections appear as small, dark lesions with yellow margins. Usually the lower leaves are affected first. While berry infections are rare, and symptoms are similar to those of black rot. The fungus overwinters in lesions in wood. In spring, spores are released and spread by rain. Cool, wet weather promotes the spread of the disease.

Management: Prune and destroy infected canes. Late dormant fungicide applications help to kill the overwintering fungal fruiting bodies on the surface of the vine. Two applications of Captan (at 1” and at 6” shoots) provide good management under normal conditions. Protectant fungicides (especially Abound and Mancozeb) are helpful at preventing infection, especially in cultivars less susceptible to the fungus. See pest management schedule for recommended materials and timing. Concord, Catawba, Chelois, Delaware, Niagara, and Rougeon are the most susceptible varieties.

Anthracnose (Elsinoe ampelina): This disease, like several of the others discussed, is worst during warm, humid, and rainy growing seasons. It reduces the quantity and quality of the berries. Circular “birds-eye” lesions are produced on the leaves with brown to black angular-shaped margins. If infection is severe, numerous lesions may coalesce, making large areas of the leaf necrotic. Often lesions will be concentrated on the veins. Necrotic tissue may drop out, leaving a “shot hole.” Youngest leaves are the most susceptible.

Lesions on the stems and shoots may also be numerous; coalescing lesions will split open the tissue into the pith. Margins will be raised and purplish to brown in color. Lesions on the rachis and pedicels of the fruit cluster are like the stem lesions. If infections are numerous, berries may drop off entirely, or they may develop cracking. Spores are released from overwintering lesions on stems or berries and are dispersed by rainfall. Spores are infectious over a wide temperature range, but need water in order to penetrate susceptible tissue. Hail injury may especially favor infection by this fungus.

Management: Do not plant highly susceptible varieties in heavy soils with poor drainage. Dormant fungicide applications help to reduce the inoculum of the pathogen. Protectant sprays beginning when shoots are 5-10 cm long and continuing at 2-week intervals are recommended. A fungicide should be applied 24 hours after hail injury.

Root and Trunk or Crown Diseases

Crown Gall (Agrobacterium vitis): Crown gall is a bacterial disease that infects more than 2,000 species of plants (including grapes). Crown gall of grape is a major problem in cold climate regions. The disease affects all grape cultivars. Vines with galls at their crowns or on their major roots grow poorly and have reduced yields. Severe economic losses result in vineyards where a high percentage of vines become galled within a few years of planting.

Wounds are necessary for infection to occur. Observations suggest that freeze injury and mechanically-caused wounds are highly conducive to infection. The disease is particularly severe following winters that result in freeze injury on cold-sensitive cultivars, such as those of Vitis vinifera. Crown gall is characterized by galls or overgrowths that usually form at the base of the trunk. Aerial galls may form as high as 3 feet or more up the trunk. Galls generally do not form on roots. 

Early in the disease development, galls are small, more or less spherical, white or flesh-colored, and soft. Because they originate in a wound, the galls at first cannot be distinguished from callus. However, they usually develop more rapidly than callus tissue. As galls age, they become dark brown, knotty, and rough. The bacterium can survive in the soil for many years even in the absence of grapevines.

Management: Control procedures include: (1) planting only nursery stock that is free of any obvious galls on crowns or roots: (2) not planting into a field where crown gall has occurred previously, unless a non-host crop, such as strawberries or most vegetables, is grown for two or more years before replanting; and (3) minimizing winter injury to root and crown systems.

In addition to be above procedures, a nonpathogenic bacterium, Agrobacterium radiobacter strain K-84, is commercially available for biological control of grown gall. The biocontrol agent may be applied to roots of healthy plants when they are first set out. After planting, the control becomes established in the soil around the root zone and prevents crown gall bacterium from entering this region. However, the biocontrol agent will not cure plants that were already infected before its application.