Grape Diseases

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  • Angular leaf scorch

    Lesions are initially yellow or reddish and confined by major veins. They later become necrotic and surrounded by yellow or red margins. Late-season infections may look like freckled spots and can cause premature defoliation. Infected flower clusters dry up. Unlike Botrytis blight, this disease infects only the berry stems, not the rachis. The pathogen overwinters in fallen infected leaves. The disease may seem absent in most years but can be severe in years with prolonged rainy weather.

  • Anthracnose

    Some table grape varieties are particularly susceptible. Symptoms occur on all aboveground parts of the vine, particularly on young tissues. Leaves develop numerous dark brown spots, 1/25 to 1/5 inch (1 to 5 mm) in diameter. As the centers fall out, lesions take on a “shot-hole” appearance. Severe infections curl and distort leaves. Lesions on shoots are sunken and dark brown with grayish centers. On green berries, “bird’s-eye” spots are purplish brown or bleached with a dark edge. Berries remain firm, or crack and shrivel. The fungus overwinters in infected parts of the vine, and spores are dispersed by wind and rain splash in the spring. Anthracnose can be severe in rainy years.

  • Armillaria root rot

    Armillaria root rot affects many woody plants, including grapes. Vineyards planted on old orchard sites or newly cleared forestland may be at risk. Aboveground symptoms are stunted shoots, yellow or red leaves, wilting and premature defoliation. Symptoms are most obvious in late summer, when vines may completely collapse and die. White, feltlike fungal mats occur below the bark near the soil line. Infected tissues have a distinct mushroomlike odor when moist. Black, shoestringlike strands (rhizomorphs) may be present on bark and in the soil. In the fall, clumps of golden-brown mushrooms may appear at the base of the vine. The fungus spreads to neighboring vines via root contact and rhizomorphs, resulting in distinctive clusters of dead vines within the vineyard. Armillaria can survive for years on dead roots and old tree and vine stumps in the soil.

  • Bitter rot

    Bitter rot gives the berries a bitter taste that is detectable in wine. After flowering, the fungus infects the berry stem and remains latent until the berry is mature. Then the fungus rapidly invades the berry and sporulates in concentric circles, darkening and roughening the surface. Within a couple of days, the berries soften and easily detach. Berries that do not fall off shrivel up, similar to black rot-infected berries. The optimum temperature for infection is 82 to 86ºF (28 to 30ºC), but infection can occur at temperatures as low as 54ºF (12ºC). Fruit injury by insects, birds or cracking can cause bitter rot to spread rapidly throughout the cluster. The fungus invades wounds and overwinters in plant debris and bark of 1-year-old canes.

  • Black rot

    Black rot can affect all new growth – leaves, petioles, shoots, tendrils and berries – but it is most destructive to fruit clusters. Fruit infections occur from bloom until the berries become naturally resistant (about 3 to 5 weeks after bloom in most varieties). The first symptom, a whitish dot within a rapidly expanding brown area, appears 10 to 14 days after infection. Within a few days, the berry starts to shrivel and becomes a hard, blue-black mummy. If berries are infected close to the onset of natural resistance, lesions remain localized. The fungus over-winters in mummies within the vine or on the ground. Ascospores are released shortly after bud break until about 2 weeks after bloom and are dispersed by wind and rain. Infected tissues can also yield conidia, which are dispersed by rain splash and cause secondary infections. The optimum temperature for disease development is 80°F (27°C). At this temperature, the wetness period required for infection is only 6 hours (see table below). Number of wetting hours required for black rot infection at various temperatures. Ave. temp. (F) / Hr. of leaf wetness 50 / 24 55 / 12 60 / 9 65 / 8 70 / 7 75 / 7 80 / 6 85 / 9 90 / 12

  • Botrytis bunch rot

    Botrytis bunch rot is a fruit rot, but it can also affect other plant parts. In spring, buds and young shoots may be infected and turn brown. In late spring, V-shaped or irregular brown patches may appear on leaves. Inflorescences may become blighted and wither away. Some flower infections remain latent until veraison. Once infections become activated, they spread rapidly from berry to berry. Compact clusters, powdery mildew infection, hail and insect damage, high nitrogen content and rain cracking can predispose grapes to infection. The disease is favored by temperatures of 59 to 68ºF (15 to 20ºC) and spreads rapidly during rainy periods, especially close to harvest. In certain cultivars, slow developing late-season infections are termed “noble rot” because they contribute to the production of exceptionally sweet wines. The fungus overwinters in mummified fruit and other infected plant parts.

  • Crown gall

    This bacterial disease is particularly damaging to vinifera grapes and interspecific hybrids. The major symptom is fleshy galls on the lower trunk near the soil. Galls may also form up to 3 feet high on trunks and canes and on below-ground plant parts. Initially, galls are cream-colored and fleshy, but later they turn brown and woody. Affected vines appear weak, and portions of the vines above the galls may die. They may also be more prone to freeze injury. Young vines may be girdled by galls in one season. The crown gall bacterium lives in the soil and enters the plants through wounds caused by freeze injury, mechanical damage, grafting or insect damage. Crown gall may be confused with natural callus growth at graft unions.

  • Downy mildew

    Downy mildew is a widespread, serious disease of grapevines. Initial leaf symptoms are light green to yellow spots, called “oil spots” because they appear greasy. Under humid conditions, white, downy spore masses can be seen on the lower leaf surface. These spores are wind dispersed. The lesions eventually turn brown as the infected tissue dies. Severely infected leaves drop prematurely, which can reduce winter hardiness of the vine. Infected flower clusters dry up or become covered with white spores under humid conditions. Infected berries turn a mottled dull green or reddish purple and readily fall from the cluster. Although berries become resistant to infection within 3 weeks after bloom, the rachis remains susceptible for several weeks longer. The pathogen overwinters in infected leaves on the ground. In spring, spores are carried by rain splash to new leaves, where they require a film of water for infection. Lesions appear 5 to 17 days after infection. The disease can spread rapidly under warm conditions with frequent rain or dew. Use the 10-10-10 rule to decide when to start scouting for downy mildew: at least 10 cm (4 in.) of shoot growth, 10 mm (0.4 in.) rainfall and temperatures of 10 degrees Celsius (50 F) during a 24-hour period.

  • Eutypa dieback

    Eutypa dieback is a progressive disease of the woody tissues of the grapevine. It is mainly found in older vineyards. Symptoms may not show for several years after infection. Initial symptoms usually appear on one arm and are best observed in mid- to late spring when shoots are 10 to 12 inches (25 to 30 cm) long. Leaves are cupped, yellowish and smaller than normal. Shoots are stunted and have fewer and smaller fruit clusters. Severely infected arms or vines develop fewer shoots each year and eventually die. Below the bark, a canker can usually be found surrounding an old pruning wound. The fungus releases spores from the canker once the bark has weathered off. Most spores are released during late winter and early spring when temperatures are above 32ºF (0ºC) and more than 1/25 inch (1 mm) rainfall or snowmelt occurs. The fungus infects vines primarily through pruning wounds, which remain susceptible for a month or more.

  • Fanleaf degeneration

    Fanleaf degeneration affects vinifera cultivars. It is characterized by fan-shaped leaves with toothed margins, proliferation of shoots, short internodes and zigzag growth. Foliar symptoms appear early in spring and persist through the growing season. Sometimes leaves show a bright yellow mosaic or yellow vein banding with little or no malformation. Fruit clusters are small with poor fruit set, irregular ripening and shot berries. The causal virus is spread by dagger nematodes and planting material. The virus is not transmitted through seeds and has no natural weed hosts. Roots from infected vines can be a source of infection even after the mother plant has been removed.

  • Flavescence dorée

    Flavescence dorée affects only vinifera grapes and interspecific hybrids. Labrusca cultivars are resistant. Symptoms usually appear the year after infection and either get progressively worse until the vine dies or disappear in an apparent recovery. Symptoms include delayed or no bud break and progressively shortened internodes. In summer, vines take on a weeping posture, and shoots become rubbery and fail to lignify. Characteristic black pustules may be seen in longitudinal rows near the bases of shoots. The leaves have golden yellow or reddish patches and curl downward. Growing points become necrotic, and flower and fruit clusters shrivel up and fall. The pathogen overwinters in infected canes and is spread by a leafhopper. Symptoms may resemble those of certain virus diseases or potato leafhopper damage.

  • Flyspeck

    Flyspeck is characterized by small, black specks on the berries. The specks are caused by a fungus that grows superficially on the fruit surface. The fungus also infects many cultivated and wild hosts, which serve as a reservoir of inoculum. High relative humidity is conducive to development of this disease.

  • Grapevine decline or Esca

    Grapevine decline affects both young and old vines. Young vines often show stunted growth, small trunk size and reduced foliage. On older vines, yellowish or reddish patches may appear between leaf veins in mid- to late season, eventually leading to marginal and interveinal burning. Berries may show poor maturation and purplish gray flecks “measles.” The entire vine or part of it may die suddenly, usually during hot periods. Sometimes shelflike mushrooms can be found on the trunk. Causal fungi can infect vines through roots and pruning wounds and become systemic in the plant. Infected vines are often symptomless, so the disease can easily spread via planting material.

  • Leaf blotch

    Leaf blotch can affect many types of grapes but is most often found on leaves of American rootstock cultivars. Leaf lesions appear after mid-season. Lesion size ranges from 1/25 inch to 2 inches (1 to 50 mm). Small lesions have dark margins, and large lesions have light-colored, zonate rings. Stalked fruiting structures are produced within 3 to 4 days of the appearance of the lesion, usually on the lower leaf surface. The fungus may also sporulate on overripe berries. The fungus overwinters in infected plant debris.

  • Leafroll

    Symptoms are most obvious in fall. Infected vines are slightly smaller than healthy vines. Leaves become yellow or reddish purple as the season progresses; the main veins remain green. By late summer, the leaves start rolling downward, beginning at the base of the shoot. At harvest, fruit clusters are small, poorly colored and low in sugar. The disease does not kill the vine but will remain chronic. Not all infected vines show symptoms. The leafroll virus spreads primarily via infected nursery stock and the grape mealybug. Within-field spread by mealybug is very slow.

  • Nematodes

    Plant-parasitic nematodes are microscopic roundworms that live in soil and feed on plant roots. Aboveground symptoms are poor growth, low yields and an “off” color. The symptoms may resemble those of nutrient deficiencies or virus diseases. Belowground symptoms include poor root development, root browning, root swelling and stunting or death of feeder roots. In new vineyards, nematodes may cause poor establishment and weak growth of young vines. Nematodes seldom kill vines but contribute to a steady decline in vigor. Dagger nematodes can also transmit certain viruses. Nematodes spread with soil and plant roots. Once established in a vineyard, nematode infestations tend to be permanent.

  • Peach rosette mosaic decline

    This disease occurs only in Concord and Catawba grapes in Michigan. Symptoms appear 3 to 4 years after infection. The plant canopy is umbrellalike with shortened and crooked internodes. Leaves are misshapen with a flattened base. Clusters are scraggly and may shell berries. Infected vines lack vigor, are prone to winter injury and may die after several years. The virus is spread by nematodes, infected planting stock and grape pomace. The virus also infects peaches and perennial weeds such as dandelion, horse nettle and curly dock. Boron deficiency and fanleaf degeneration may mimic this disease.

  • Phomopsis cane and leaf spot

    Infected leaves have small, yellowish spots with dark brown centers and may be puckered. On petioles, shoots and rachises, elongated black spots or streaks develop that make the tissue brittle. Most shoot lesions occur on the basal three to six internodes. Young tissues are most susceptible. Symptoms appear 21 to 30 days after infection. Rachis and berry infections become apparent later in the season. Infected rachises wither, causing berries or entire clusters to drop prematurely. Berries turn brown and shrivel. Prolonged rainy, cold weather in spring and early summer promotes the disease. The optimum temperature for infection is between 59 and 68ºF (15 to 20ºC). The fungus overwinters in bark of infected canes.

  • Pierce’s disease

    Initially, only a few shoots start to show symptoms in mid- to late summer. Leaves show scorching from the margin inwards and drop off, leaving the petiole attached to the shoot. Flower clusters may set berries, but these tend to dry up. In fall, infected shoots mature in a patchy manner, leaving “islands” of green tissue surrounded by dark brown mature wood. In spring, bud break on infected vines may be delayed as much as 2 weeks, and new shoots are stunted. An infected vine may die the first year after infection or may live for 5 or more years, depending on the cultivar, the vine’s age and climatic conditions. Pierce’s disease is caused by a bacterium that lives in the xylem and is vectored by sharpshooter leafhoppers and spittlebugs. The bacterium is present in native plants such as grasses, sedges, bushes and trees. The range of insect vectors determines the range of the disease.

  • Powdery mildew

    Powdery mildew can infect all green tissues and give them a white to gray, powdery appearance. Colonies occur mostly on the upper leaf surface. Early berry infections can result in split berries, secondary rots and undesirable flavors in wine. Late berry infections are less obvious but can still predispose the berries to rots. In late summer, the fungus produces small, brown to black fruiting bodies (cleistothecia) on infected plant parts. Cleistothecia overwinter in bark crevices and release ascospores when exposed to moisture between bud break and fruit set. In regions with mild winters, the fungus can also survive in dormant buds, which develop into “flag shoots.” Powdery mildew is favored by high humidity and temperatures of 68 to 81 degrees Fahrenheit (20 to 27 C). Wetness is not required for infection. Temperatures above 95 F (35 C) inhibit new infections. Begin monitoring early in the season, focusing on shaded leaves and clusters inside the canopy.

  • Ripe rot

    Initially, berries show circular, reddish brown spots, which enlarge to cover the whole fruit. Salmon-pink fungal spore masses develop in a circular pattern on the fruit surface. The berries shrivel and darken as they decay and then fall to the ground. Berries are susceptible to infection at all stages of development but do not show symptoms until the berries are ripe. Disease development is favored by wet weather and temperatures of 77 to 86ºF (25 to 30ºC). The fungus overwinters in mummified fruit and infected pedicels, from which spores are dispersed in spring and early summer. Spores produced on rotting berries can infect neighboring berries.

  • Rupestris stem pitting

    Rupestris stem pitting virus causes a slow decline of vinifera cultivars and interspecific hybrids. The main symptoms are delayed bud break, poor spring growth, stunting of infected plants and a decline in yield. No leaf discoloration is observed. When the bark is peeled off the trunk, the wood may reveal the presence of small pits. This virus mainly spreads via planting material.

  • Septoria leaf spot

    Septoria leaf spot, also called méelanose, mainly affects American Vitis and muscadine grapes. Angular, reddish brown to black spots 1 to 2 mm in diameter appear after midseason. Nearing veraison, lesions become larger with diffuse margins. Fruiting bodies may be seen with a hand lens. The area surrounding the spots may be yellow. The fungus overwinters in infected leaf debris.

  • Sour bunch rot

    Sour rot is a wet rot that spreads rapidly throughout clusters and smells like vinegar. It is caused by acetic acid bacteria and various undesirable yeasts and fungi. Unlike Botrytis bunch rot, it usually lacks fungal sporulation. Low-grade powdery mildew infections and grape berry moth infestations can predispose clusters to infection. Fruit flies are common and help spread the disease. Tight-clustered cultivars are more susceptible than others. Prolonged periods of wetness or high relative humidity are conducive to sour rot development.

  • Tar spot

    Tar spot is a minor disease that occurs mostly on wild grapevines. This fungal disease is characterized by black, slightly raised spots about 1/12 to 1/6 inch (2 to 4 mm) in diameter. A spot may be surrounded by a circular brown halo up to 2/5 inch (1 cm) in diameter. The fungus overwinters in these spots. In the spring, they release airborne spores, which infect the new leaves.

  • Tomato/Tobacco ringspot decline

    These diseases occur sporadically in vinifera grapes and interspecific hybrids. Labrusca grapes are resistant. In the first year of infection, a few leaves may show mottling. The second year, new growth is generally sparse because infected buds are prone to winterkill. Infected vines show shortened internodes with small, distorted leaves and sparse fruit clusters with uneven ripening. The third year, growth is very stunted and limited to basal suckers, and the vine eventually dies. Dead and dying vines are usually present in a roughly circular pattern in the vineyard. The viruses are introduced into vineyards with infected planting stock or by dispersal of seed from infected weeds. The virus is then spread by dagger nematodes feeding on roots of infected plants. The nematodes can retain the virus for long periods. Both viruses infect common weeds such as dandelion, sheep sorrel, common chickweed and red clover. Tomato ringspot virus also infects many fruit crops. These viruses may already be present in land used to establish new vineyards. The viruses are introduced into vineyards with infected planting stock or by dispersal of seed from infected weeds. The virus is then spread by dagger nematodes feeding on roots of infected plants. The nematodes can retain the virus for long periods. Both viruses infect common weeds such as dandelion, sheep sorrel, common chickweed and red clover. Tomato ringspot virus also infects many fruit crops. These viruses may already be present in land used to establish new vineyards.

The MSU IPM Program maintains this site as an access point to pest management information at MSU. The IPM Program is administered within the Department of Entomology, fueled by research from the AgBioResearch, delivered to citizens through MSU Extension, and proud to be a part of Project GREEEN.