Article by Mark Rieger, Professor of Horticulture, University of Georgia
Grapes - Vitis spp.
- European or wine grape - Vitis vinifera L.
- Fox, Concord or American Bunch grape - V. labrusca L.(syn. V. labruscana Bailey)
- Muscadine grape - V. rotundifolia Michaux
Grapes belong to the Vitaceae family. The genus Vitis is broadly distributed, largely between 25° and 50° N latitude in eastern Asia, Europe, the Middle East, and North America. Additionally, a few species of Vitis are found in the tropics - Mexico, Guatemala, the Caribbean, and northern South America. These are over 100 species in the literature, 65 of which are thought to be genuine and another 44 which are questionable, probably interspecific hybrids. Vitis is split into 2 subgenera:
- Euvitis - "True grapes"; characterized by elongated clusters of fruit, berries that adhere to stems at maturity, forked tendrils, diaphragms in pith at nodes. Also called "bunch grapes" .
- Muscadinia - Muscadine grapes; Characterized by small fruit clusters, thick-skinned fruit, berries that detach one-by-one as they mature, simple tendrils, and the lack of diaphragms in pith at nodes. There are only 2-3 species in this section; the rest are in Euvitis.
Three important species, and one hybrid group:
V. rotundifolia - cultivated muscadine grapes.
Very fruity aroma, thick-skinned fruit, in small clusters or borne singly, ripen over an extended period. Make good juice, also eaten fresh. Only of local importance in the South. Since muscadines are resistant to Pierce's disease and phylloxera, they may have uses in breeding or as rootstocks for vinifera grapes.
Cultivars: 'Scuppernong'- oldest cv, bronze skin; many bronze-skinned cultivars are called 'Scuppernong'.
V. labrusca L. (syn. V. labruscana Bailey).
American bunch grape, Fox grape, Concord grape. Valuable in breeding; hybrids used in wine production or as phylloxera resistant rootstocks for vinifera grapes. Major use is for sweet grape juice (Welch's) and associated products - jelly, jam, preserves, some wine.
'Concord' - major cultivar, 80% of production; others - 'Niagara', 'Isabella', 'Delaware', 'Catawba'. Some cultivars are probably interspecific hybrids with other native American grapes and vinifera grapes.
V. vinifera L. - "Old world grape", "European grape".
This is the major species of grape, accounting for >90% of world production. It was probably domesticated more than 5000 years ago in the Middle East.
- V. rotundifolia - cultivated muscadine grapes.
French-American hybrids. The introduction of the phylloxera ("grape root louse") to Europe in 1860 created a need for resistant rootstocks. V. labrusca and other species native to the host range of the phylloxera (north-central USA) were hybridized with V. vinifera to produce a range of stocks with resistance. In addition to use as rootstocks, some hybrids were found which had both resistance and excellent wine quality attributes.
Origin, history of cultivation
- V. vinifera. This species is thought to be native to the area near the Caspian sea, in Asia Minor. Seeds have been found in excavated dwellings of the Bronze-age in south-central Europe (3500 - 1000 BC). Egyptian hieroglyphics detail the culture of grapes in 2440 BC. The Phoenicians carried wine varieties to Greece, Rome, and southern France before 600 BC, and Romans spread the grape throughout Europe. Grapes moved to the far east via traders from Persia and India.
- V. labrusca. This species is found growing wild from Maine to the South Carolina Piedmont, as far west as the Tennessee mountains. It was seen first seen by Viking explorers before Columbus' voyages, who named the maritime provinces of Canada "Vinland" meaning "grape land" due to the abundance of wild grapes growing in the forests. Early settlers to the northeast domesticated this species after European grapes failed to grow, probably due to cold injury.
V. rotundifolia. Muscadines are native from Virginia in the north through central Florida, west to eastern Texas. This species has been enjoyed by southerners since antebellum times, and has received little attention outside of the southeast.
Folklore, medicinal and non-food uses
- Cancer therapy - Historically, "grape therapy" was used. Up to 15 lbs of grapes/day for 6 weeks; claimed to cure cancer. However, this could have caused more cancer than it cured, since grape seeds and skins contain tannins, which are carcinogenic. In 1996, scientists discovered the chemical in grapes that shows anti-carcinogenic activity - "resveritrol". Originally identified in a tropical legume, this compound was found later in grapes (particularly red) and wine. It can inhibit tumor formation in three ways - stopping DNA damage, slowing/halting cell transformation from normal to cancerous, and slowing tumor growth. Resveritrol haas anti-inflammatory properties and may be very useful for colon cancer prevention. It is still experimental, but may be a useful cancer drug in the future. Resveritrol is also found in peanuts and mulberries.
- Dentifice - Ashes of burned branches used as an abrasive paste in England.
- Healing aid - V. compressa juice is used for healing in tropical Asia.
- Grappa - A powerful alcoholic drink (100 proof) is distilled from fermented skins, seeds, and stems which are left over from pressing the juice. Often used as an after-dinner drink in Italy. Many types of flavoring added (like orange or lemon peel) to improve flavor.
- Grapes have been used to make wine since prehistoric times. The Greco-Roman wine God was Dionysus-Bacchus; the "Oschophoria" was his festival (carrying of the grapes). The drunken state was believed to result from possession by the gods, allowing one to see the future. Wine has been symbolic of the blood of life in several religeons.
World - 58,120:
1998 production down from more typical 65 million MT/yr in the early 1980's. Trend is to reduce production, especially within the European Economic Community (EEC). Acreage - 18,195,000; down from 23,295,000 acres in 1980; >60% of area is in Europe. There are 861,000 acres in the USA.
Average Yield - 7759 kg/ha or 6966 lbs/acre; stable. About 11 lbs/vine at typical spacings, but highly variable.
- Italy - 9,208 6. China - 2,439
- France - 6,800 7. Iran - 2,315
- USA - 5,948 8. Argentina - 2,021
- Spain - 4,818 9. Chile - 1,575
- Turkey - 3,650 10. Germany - 1,408
Italy typically had 11-12 million MT, France 9-10 million, and Spain 5.5-6.5 million MT in the 1980's. Area in Spain has been reduced by 449,000 ha since 1980. Chinese production has increased almost 10-fold since 1980, and Chile surpassed the perennial Germany in the top 10 only recently with its rapidly expanding wine and table grape industries. With the emergence of the EEC, pressure has been placed on Italy to down-size its industry, by reducing area planted and setting lower yield maxima. The Italians obviously don't like this, since they believe their quality can be maintained without imposing severe yield restrictions as in France. Both France and Italy have reported steady decreases in acreage and production since the 1980's.
In contrast to Europe, US production has been steadily increasing during the 80's, but leveled off in the early 1990's. The US enjoys one of the highest production efficiencies in the world; yield/area 19,446 kg/ha or 7.2 tons/acre. In France and Italy, average yields are 7,746 and 9,964 kg/ha, respectively, due to emphasis on wine.
5,087,000 MT (1997 USDA); value = $2.5 billion.
Utilization: Wine - 50% Table - 10-15% Raisins - 25-30% Canned - < 1% Juice, jelly, etc. - 6-9%
|Foreign Trade (1996, % of production)||Exports||Imports|
- California - 90% of US production. 661,000 acres.
Wine - 43-50%
Raisin - 24-44%
Table - 10-12%
- Washington 4-5% - 69% juice/processed, 31% wine
- New York 2% - 70% juice/processed, 30% wine
Plant: a liana or woody vine. Leaves are often large (8-10" in width) sometimes deeply lobed as in many V. vinifera cultivars, or rounded with entire or serrate margins. Tendrils occur opposite leaves at nodes.
Flower: Flowers are small (1/8 inch), indiscrete, 5-merous, borne in racemose panicles opposite leaves on current season's growth. The calyptra, or cap is the corolla, which abscises at the base of the flower and pops off at anthesis. Species in Euvitis may have 100+ flowers per inflorescence, whereas muscadine grapes have only 10-30 flowers per cluster. Also, vinifera and concord grapes are perfect-flowered and self-fruitful, whereas some muscadines have only pistillate flowers.
Pollination: Most grapes are self-fruitful and do not require pollinizers; however, pistillate muscadines (e.g., 'Fry', 'Higgins', 'Jumbo') must be inter-planted with perfect-flowered cultivars for fruit set. Since parthenocarpy doesn't exist, all grapes require pollination for fruit set. Even seedless cultivars like 'Thompson Seedless' are not parthenocarpic; rather, the embryos abort shortly after fruit set. This condition is called "Stenospermocarpy".
Fruit: Fruit are berries, with 2 to 4 seeds; ovaries contain 2 locules each with 2 ovules.
Thinning is not practiced for most types; crop load is controlled through meticulous pruning. However, French-American hybrids may set 4-5 clusters of fruit per shoot, and require cluster thinning for development of quality and proper vine vigor.
Fruit size and cluster length are increased through GA application on 'Thompson Seedless' and other table cultivars. GA is applied @ 10-15 ppm when 50% of flowers on 50% of clusters are in bloom, followed by 40 ppm sizing spray 1 ( 2) weeks later. This opens the cluster, prevents crushing of berries, and reduces disease.
A) Soils and Climate
Soils: Grapes are adapted to a wide variety of soil conditions, from high pH and salt, to acidic and clayey. Rootstocks allow adaptation to various soil situations. However, deep, well-drained, light textured soils are best for wine grapes. Highly fertile soils are unsuited to high quality wine production, since vigor and yield must be controlled. Irrigation is detrimental to grape internal quality, and sometimes illegal for wine grapes, but is beneficial for table and raisin grapes where high yields are desired.
Climate: Vinifera grapes can be generally characterized as requiring a long growing season, relatively high summer temperatures, low humidity, a ripening season free of rainfall, and mild winter temperatures. All of these attributes are found in mediterranean climates. Concord and muscadine grapes are obviously adapted to humid, temperate climates, with muscadines requiring longer growing seasons and milder winters than concords.
Cold hardiness is a major limiting factor for vinifera grapes. Damage to primary buds occurs at 0 to -10°F (-18 to -23 C), and trunks may be injured or killed below -10°F. Labrusca grapes are more cold hardy than vinifera or French-American hybrids, but will experience some damage at -10 to -20°F. Muscadine grapes are the least cold hardy, being killed below 0°F, and injured in the single digits °F.
The number of days from bloom to maturity, or heat unit requirement increases as follows:
Labrusca (least) < French-American hybrids < European < Muscadine.This generally corresponds to 150-200+ frost-free days, with 165 to 180 best for vinifera, or 2000 to 3500 heat units.
Internal quality and hence wine quality is affected by summer temperature. Cool climates favor lower sugar and higher acid development, whereas higher temperature favor high sugar and low acids. Generally, longer growing seasons and warmer temperatures are required for red wine cultivars, and cooler temperatures for white cultivars. This is why red wines are produced mostly in Italy, southern France, Spain and Portugal, while white wines are produced in northern France and Germany.
Humidity is another limiting factor for vinifera grape culture, due to disease susceptibility. Grapes cannot tolerate high RH or rain during harvest. Muscadines, however, grow much better in humid climates.
Chilling requirement is highly variable among grape species; some grapes can be grown in the tropics and have little or no CR. Labrusca grapes generally have high CR's, 1000 - 1400 hr. European grapes have low CR's, 100-500 hr. Muscadines have intermediate CR's.
The most common method of grape stion production is bench grafting, although rooted cuttings (where phylloxera is not a problem), T-budding, layering (difficult-to-root types like muscadine), and to a limited extent, tissue culture are used in various situations.
Vitis vinifera was propagated on its own roots from the beginning of recorded history until about the 1870s. The grape phylloxera (Dactylosphaera vitifolii, Homoptera), also called the "grape root louse" (but is actually an aphid), was introduced into Europe from eastern North America in the 1860s, where it caused the most significant pest-related disaster in all of fruit culture. The search for resistant rootstocks led horticulturists to the native range of the phylloxera, eastern N. America, where various species of American grapes had coexisted with the pest for millennia, and thus were resistant to it. Most grape rootstocks used today are numbered clonal selections of hybrids of V. riparia, V. rupestris, and V. berlandieri (e.g., "420 A", "3309 C").
D) Orchard design, pruning, training:
- Design. Most grapes are trellised and grown in long narrow "hedgerows", spaced about 9-15 ft between rows depending on training system. Typically, there is 3-8 ft between vines in a row, and 9 feet between rows. However, in arid regions like Sicily and southern Spain, grapes are grown as free-standing "bushes", with head training, at very low densities due to low rainfall.
Pruning and Training. There are probably more training systems for grapes than for all other fruit crops combined. The most important ideas are spur vs. cane pruning, head vs. cordon training, and balanced pruning.
Spur vs. Cane Pruning.
"Spur" or "short pruning": leave only a stub with 2-6 buds on each renewal cane.
Short spur pruning - 2-3 buds. Long spur pruning - 5-6 buds.
For use on: Cultivars with fruitful basal buds. Cultivars that are excessively vigorous. Wine cultivars where high quality juice is more important than high quantity.
"Cane" or "long pruning": 9-16 buds retained at base of last year's canes.
For use on: Cultivars with unfruitful basal buds (e.g., 'Thompson seedless'). Cultivars which lack vigor or are low yielding.
Head vs. Cordon training.
Head - Permanent part of vine consists of trunk and 4-5 small stubs at top called arms.
Cordon - Permanent part of vine consists of trunk and 1-4 long, straight shoots trained along a wire (cordons). Spurs or canes are developed at regular intervals along cordons.
The amount of buds left to produce fruiting shoots is based on pruning weight in winter. Thus, the number of fruiting buds left on the vine are proportional to bearing capacity. To start, the pruner removes most of the wood he believes will be necessary, bundles it, and weighs with a small hand scale. 70 to 90% of previous season's growth is removed. For a given cultivar, a formula is applied to calculate the number of buds to be retained based on pruning weights; generally, 20 buds are left for the first lb of prunings, and 10-20 for each additional lb of prunings.
- Spur vs. Cane Pruning.
'Sangiovese' - cordon training, spur pruning (older vine).
'Sangiovese' - A small head trained grape vine has been cane pruned, leaving one shoot with several buds. The cane will be bent downward at the tip into the “Tuscan Bow” system. Note the renewal spur on the left side of the head for next season’s cane.
A close up of a arm that has been spur pruned, leaving stubs with only 2-3 buds
E) Backyard Considerations
Backyard considerations. Muscadine and some labrusca grapes grow well in the southeast, but vinifera grapes do poorly. Diseases and cold damage prevent good results with familiar vinifera cultivars like 'Thompson seedless', or 'Cabernet Sauvignon'. Muscadines are adapted to the Piedmont and coastal plain regions, whereas certain cultivars of labrusca should be grown in only the Piedmont and mountain regions.
- Construct a sturdy, single-wire trellis in a sunny location; the wire should be 5-6' above the ground. Double-wire trellises can be more productive, but have worse disease problems.
- The labrusca cultivars 'Fredonia', 'Delaware', and 'Niagara' have done well in the southeast, as have some of the eastern seedless cultivars like 'Mars', 'Venus', and 'Reliance'.
- Perfect-flowered muscadines are easiest to grow since they do not require pollinizers; 'Southland', 'Triumph', 'Dixieland', 'Cowart', 'Carlos' (for wine) and 'Noble' (for wine) are some examples.
- Train young vines along a stake or string tied to the wire; try to develop a straight trunk the first year, and cordons (or a head) the second year. Remove fruit for the first 2 years if it occurs to avoid stunting the vine.
- Prune annually in late winter, or you will end up with a tangled, unproductive mass of foliage!
- Use a cordon system with short (3") spur pruning for all muscadines; space spurs about 6" apart on cordons.
- Use either a head/cane or cordon/spur system with labrusca cultivars, leaving a maximum of 40-50 buds/vine.
- Muscadines are relatively disease and insect free; don't spray unless something unusual occurs. Insects to watch are grape berry moth, grape curculio, grape root borer, and Japanese beetles.
- Labrusca grapes, unlike the resistant muscadines, require spraying for best results. Black rot causes problems in most years; sprays begin at bud break and continue until harvest as needed. Grape berry moth, grape curculio, Japanese beetles, and other fruit pests can cause problems from bloom until harvest; scout at least weekly and spray when insects are present or feeding injury is seen.
Harvest, post-harvest handling
- Table grapes. Time of harvest is determined mostly by appearance, including color and size of berries. The stems of the cluster also turn a wood or straw color when berries are mature.
- Raisins. Since drying ratio should be minimized for maximum yields and quality, it is best to allow grapes to ripen fully on the vine before picking. Water content decreases and Brix increases during maturation, so waiting as long as possible maximizes yield. 'Thompson Seedless', the major raisin cultivar, reaches 22-23° Brix at full maturity, and this is easily determined with a refractometer. In the central San Joaquin valley of California, damaging rains can be expected after Sept. 20, so time of harvest is a compromise between maximizing Brix and yield, and harvesting early to avoid potential crop damage/loss due to rain. A general rule is to harvest when Brix reaches 22° or by Sept. 1, whichever comes first.
Wine grapes. Harvest criteria, and hence date, depend on type of wine to be made. For any given type, sugar ( Brix) is perhaps the most important parameter, although pH is also very important - pH is more difficult to adjust than sugar in the winery, and wines with pH above 3.6 are potentially unstable. Brix/acid ratio and total acidity are also important, and Brix/acid ratio together with Brix has been used to accurately predict wine quality.
Generally (California standards):
- White wine - Brix = 19.5 to 23 % acidity 0.70 pH 3.3
- Red wines - Brix = 20.5 - 23.5 % acidity 0.65 pH 3.4
- Concord juice grapes. The most important parameter is sugar content; Welch's won't accept grapes with <15 Brix.
B. Harvest method
- Table grapes. Individual clusters are judged for maturity by pickers, and clipped from vines with minimal handling, and placed in plastic lugs. Vines are harvested 2-3 times over a period of several weeks, since grapes ripen rather slowly and hold their condition on the vine for weeks following ripeness. Clusters are often picked into retail containers to minimize handling.
- Raisin, wine and juice grapes. Mechanical harvest is the norm for all processed grapes, although most wine varieties are picked by hand. Special wines such as those with noble rot are made from hand-picked berries from special areas within a cluster. Mechanical harvesters generally have horizontally pivoting arms which smack the curtain, or vibrating fingers which dislodge clusters and berries from clusters. Machines harvest 5 MT/hr (5.5 tons/hr), equivalent to about 100 hand pickers. Raisin grapes are often harvested 4-8 days after cutting canes off, which causes berries to abscise more easily from clusters. Canes are pruned but left in place, and harvested mechanically when berries are loose. This allows a neat, single layer of de-stemmed berries for harvest onto continuous paper trays.
C. Post-harvest handling, packing
- Table grapes. Shipping occurs immediately after harvest when possible, but some storage may be necessary when volume exceeds demand. Grapes are pre-cooled in forced-air rooms, where fumigation with SO2 occurs.
- Raisins. Prior to picking or mechanical harvest, soil between E-W rows is smoothed to accommodate trays or papers used for field drying. Grapes are picked and placed onto drying papers in single layers, or in small clusters. The top layer browns and shrivels in 7-10 days, and the berries are turned. When they are dry enough (13-15% moisture) that juice cannot be squeezed out when pressed, they are ready for boxing. Machines collect raisins from continuous paper trays, and fruit are boxed. Curing involves the first few weeks of storage, where water is transferred from wetter to drier berries, and moisture content equalizes within the lot - this is referred to as "sweating".
Wine. The science of wine-making is called enology. Entire courses and curricula are available at some Universities for specializing in this area. The basic processes are these:
- Harvested grapes are dumped into bulk bins ranging from a few pounds up to 10 MT and taken to the de-stemmer/crusher, where the juice ("must" = juice+skins+seeds+stems for red; juice only for white) is extracted. Initial quality evaluations are made and the vinification process is decided upon. Adjustments of sugar level and acid can be done at this point.
- Sulfur dioxide is added to prevent "oxidative casse" and other forms of enzymatic oxidation, and color deterioration, and selectively activate certain yeasts for fermentation and kill bacteria and other undesirable microbes.
- Must is transferred to large fermentation vats (25 to 500 hl) for sugar alcohol conversion. Red wine is produced by partial fermentation with skins where anthocyanin pigments reside; Ros- is produced by limited contact of must with skins; White wine is produced with no contact with skins. Fermentation temperature is higher for reds (24-28 C) than whites (14-18 C). Fermentation is stopped by racking or dispensing wine into containers; sometimes SO2 is used to stop the process and act as a preservative.
- Wines are filtered once, then maybe aged in oak barrels for some time prior to bottling; time limits are imposed by governments or individual enologists. Generally, red wine is stored in large (100-200 hl) barrels, and white in small ( 2 hl) barrels. White wine is aged for only short periods of time (< 1 yr), whereas some reds may be aged for up to 10 yr.
- Bottling takes place at appropriate age, after ultra-filtration for clarity and antisepsis.
- Raisins - may be stored for long periods of time at room temp due to low moisture content.
Fresh grapes - vinifera grapes may be stored for relatively long periods of time at 31 F and 95% RH - 90 to 180 days, depending on cultivar. Labrusca grapes are more perishable, lasting only 14-56 days under similar conditions. Muscadines can be stored for up to 3 weeks at 32 F and 90% RH.
Contribution to diet, food uses
Dietary value, per 100 gram edible portion:
Water (%) .......................................... 81
Calories ............................................. 67
Protein (%) .......................................... 0.6
Fat (%) ............................................... 0.3
Carbohydrates (%) ............................. 17
Crude Fiber (%) ................................... 0.5
% of US RDA*
Vitamin A ........................................... 2
Thiamin, B1 ........................................ 3.6
Riboflavin, B2 ...................................... 1.9
Niacin ................................................ 1.7
Vitamic C ........................................... 8.9
Calcium ............................................. 1.5
Phosphorus ........................................ 2.5
Iron ................................................... 4.0
Sodium .............................................. ---
Potassium ......................................... 3.7
* Percent of recommended daily allowance set by FDA, assuming a 154 lb male adult, 2700 calories per day.