THE PRODUCTION OF TABLE GRAPES IN CALIFORNIA       Larry E. Williams

ABSTRACT
The production and consumption of fresh grapes in California began shortly after the "Gold Rush" began in 1848. Shipments of fresh grapes from California back to the eastern parts of the United States began as early as the 1870's. Table grapes are produced in California from the deserts of southern California (the Coachella Valley) to the southern San Joaquin Valley (from south of Bakersfield to just north of Fresno). Today, approximately 85% of the table grapes produced are seedless. The Thompson and Flame Seedless cultivars comprise approximately 57% of the table grapes produced. The acreage of the seeded cultivar, Red Globe, has increased 250% in the last five years, going from 2470 ha in 1993 to 6223 ha in 1997. The typical trellis system consists of a stake and a crossarm (T trellis) for most cultivars and locations. Various forms of overhead arbor systems are also being used for new vineyards. Most of the vineyards are planted on their own roots excepting in areas where nematodes are a problem. Many of the seedless cultivars are sprayed with gibberellic acid (GA₃) at various phenological stages to increase berry size. Canopy management practices include the removal of lateral shoots, shoots without clusters and basal leaf removal. Almost 80% of the grapes are packed in the field. The packed fruit is pre-cooled and fumigated with sulfur dioxide prior to shipping. Packed fruit is kept at 0^oC and fumigated periodically for long-term storage.

PRODUCTION AREAS
The Coachella Valley is located approximately 160 km east of Los Angeles. For the period from 1993 to 1997 grapes produced in the Coachella Valley comprised 13% of the total table grape production in California. This area is important, as the fruit from this valley is the first to reach the market each year. The weather can be extremely hot with temperatures during July and August approaching 45 to 50^oC. However, budbreak in that area generally occurs the middle of January and harvest takes place in May, prior to the extremely high temperatures. Most of the acreage in that area is sprayed with hydrogen cyanamide (H₂CN₂) to overcome the lack of chilling during the dormant portion of the growing season. The use of evaporative cooling also is used there to increase the accumulation of chilling units by the grapevines. It has been demonstrated that grapevines need approximately 400 hours of temperatures less than 3^oC to ensure a favorable budbreak. The soils can be very shallow and the texture ranging from sandy to sandy loam. Due to the high evaporative demand and shallow soils the vineyards must be irrigated. The majority of vineyards in the Coachella Valley use low volume irrigation systems (drip irrigation). The water supply comes from the Colorado River. The major cultivars grown there are Perlette, Flame Seedless and Thompson Seedless.

The table grape production area in the San Joaquin Valley extends from south of Bakersfield to just north of Fresno, a distance of 160 km. Approximately 60% of the table grapes produced in California come from Kern County (Bakersfield is the principle city) and the southern portion of Tulare County (just north of the city of Delano). Around 14% of the table grapes are produced in Fresno County. Most of the acreage devoted to vineyards in the San Joaquin Valley is located on the eastern portion of the valley. The soil textures range from sandy to sandy loam with adequate rooting depth. Water for irrigation purposes may come from the mountains or the ground, both of generally good quality. New vineyards being planted in the southern San Joaquin are using a variety of trellis systems and new cultivars being developed by different agencies (the United States Department of Agriculture and University of California).

CULTIVARS USED
In California grape cultivars are classified according to the type of grape being produced, i.e. raisin (dried grapes), table (fresh grapes) or wine. The Thompson Seedless cultivar is classified as a raisin grape, however, it is also the number one cultivar used to produce table grapes. For the period from 1993 to 1997 the yearly production of Thompson Seedless used for table grapes averaged 211 thousand metric tonnes. The next highest producing cultivar was Flame Seedless (164 thousand metric tonnes). The ranking of other cultivars for production (highest to lowest) during that time period was Red Globe (a late maturing, red seeded cultivar), Ruby Seedless (a late maturing red seedless grape), Perlette (the earliest seedless white grape) and Sugraone (also known as Superior Seedless, an early white seedless grape). The success of Red Globe is due to the fact that it is primarily exported to Asian markets. New cultivars being developed include those with white, red and black berries. They are being developed to fill particular niches in the market place, having different colored fruit and time of maturity and to reduce production costs by having naturally large berries. One example is Fantasy Seedless, a cultivar with black berries that ripens during mid-season.

IRRIGATION AND MINERAL NUTRITION
Quality aspects of table grapes include berry size and lack of blemishes (to include sunburn, a browning of the fruit). Therefore, irrigation management is important to insure berries of adequate size and the development of a canopy that will shade the fruit to prevent sunburn and provide enough leaf area to produce carbohydrates for vine growth. Studies conducted by the author indicate that berry size and vine productivity is maximized at approximately 80% of full vineyard evapotranspiration (ET_c). At this amount of water an adequate canopy develops to provide shade and for carbohydrate production via photosynthesis. Vineyard water use at full ET is determined by multiplying seasonal crop coefficients that have been developed by the author and potential or reference ET (ET_o). Potential ET can be obtained from the California Irrigation Management Information System operated by the California Department of Water Resources. Table grape irrigation requirements range from 450 to 550 mm between budbreak and harvest in the San Joaquin Valley. Potential ET during that time frame is about 1000 mm. Vineyard water use in the Coachella Valley from budbreak to harvest is only slightly greater than vineyard water use in the San Joaquin Valley. This is due to the fact that the months with the highest evaporative demand occur in the Coachella Valley after vines are harvested, which generally takes place in May. The application of water may be via drip or other low volume methods or on the soil surface via flood or furrow irrigation. The majority of table grape vineyards are now being irrigated via drip irrigation. One interesting aspect of the irrigation research conducted on table grapes is that water use will decrease during the period a girdle remains open. Girdling, the removal of the phloem portion of the vascular tissue around the trunk with a specialized knife, is used on many of the cultivars to increase berry size or to enhance maturity.
The most common mineral nutrient deficiencies experienced in table grape vineyards in nitrogen and zinc. Potassium and boron deficiencies are less common. Assessing mineral nutrient status of the vine is usually accomplished by tissue analysis. Petioles opposite a cluster are collected at bloom and sent to a laboratory. Deficient and adequate values of the macro- and micro-nutrients in dried petiole tissue have been established for Thompson Seedless and are currently being established for other cultivars and scion/rootstock combinations. If deemed necessary nitrogen applications may be applied after budbreak or shortly after berry set, two weeks after bloom (flowering). A foliar application of zinc may be applied shortly before bloom.

OTHER MANAGEMENT PRACTICES
The most common row and vine spacings of established vineyards are 3.66 and 2.44 m (1121 vines per hectare), respectively. Some of the newer plantings are going to narrower vine and row spacings, with equipment size and trellis system used determining actual distance. Many of the table grape cultivars can be spur pruned (leaving a 3 bud spur) and trained to either bilateral or quadrilateral cordons. Thompson Seedless is head trained and cane pruned. The canes are generally 10 to 12 buds in length. The most common trellis system is the T system, consisting of a 2.41 m stake at each vine (driven 0.5 m into the ground) and a crossarm 0.9 m in length. The height of the crossarm from the cordon wire can be 0.46 m above the cordon on spur pruned cultivars. For Thompson Seedless, the canes are tied to a wire with a 0.9 crossarm above it. Some T systems may have more than one crossarm. The overhead trellis system is being employed in newer vineyards. The most popular is the double slant or gable system, however, other types are also being used.

CULTURAL PRACTICES USED TO ENHANCE
There are numerous cultural practices that growers will use to enhance fruit quality. The use of these practices will differ from cultivar to cultivar. Pruning is one means to adjust potential crop. Studies by the author on Thompson Seedless, Flame Seedless, Ruby Seedless and Perlette in the Coachella and San Joaquin Valleys indicate that bud dissections are useful in determining fruitfulness for the coming year. It was found that the number of cluster primordia found by dissecting the buds during vine dormancy was highly correlated with the actual number of clusters counted one month after budbreak the following growing season. Therefore, one could prune their grapevines to leave a desired number of clusters per vine or leave more buds per vine if potential fruitfulness was lower than expected.

Gibberellic acid (GA₃) has been used since the 1960's to increase the size of seedless table grapes. The first application of GA may be applied prior to flowering. This application seems to increase the length of the rachis (main stem of the cluster) and of the laterals coming off the rachis. A second application of GA will occur during bloom. An application at this time will reduce the number of flowers that are set. The exact mode of action for an application of GA at this time is not clear. The last application of GA will occur at berry set (approximately two weeks after flowering) or when berry size is anywhere from 4 to 6 mm in diameter (the diameter can be cultivar dependent). This application will increase berry size due to an enhancement in cell division (more cells per berry) and cell elongation (larger cell size). In California, GA is applied with spray equipment, similar to that used for the application of pesticides. In some countries, such as Chile or South Africa, this hormone may be applied manually in which individual clusters are dipped in a solution of GA.
As mentioned previously in this paper, girdling is used to increase berry size of several seedless table grape cultivars. This procedure began in California in the 1930s. Girdling is performed at berry set, similar in timing to the application of GA₃ at berry set. For cultivars that are girdled, an application of GA at pre-flowering or at flowering is required. This is due to the fact that the clusters will become too compact due to larger berries as a result of girdling. The exact mode of action of girdling is unclear. Girdling has been shown to increase the carbohydrate nutrition of the fruit as the phloem tissue is removed inhibiting the translocation of carbohydrates from the leaves to the root system. Other possible mode of actions are 1.) changing the hormone balance within the vine resulting in larger berries and 2.) a more favorable vine water status, allowing the berries to growth better. Girdling may also be used at berry softening (veraison) to enhance the color of red cultivars such as Flame Seedless. The application of ethylene producing products, such as ethrel, at veraison will also enhance the development of color in colored fruit. These ethylene releasing products have also been shown to advance the maturity, soluble solids accumulation, in Thompson Seedless and Perlette.
Other practices performed on many table grape cultivars are aimed at reducing berry number so that there are more carbohydrates available for the remaining fruit. This includes removing clusters that are poorly shaped or developed or removing berries from individual clusters. The later procedure entails the removal of groups of berries from the rachis tips or from the ends of laterals. Removing this fruit may also assist in facilitating the harvest of the remaining fruit. It should be pointed out that the approximate number of clusters growers leave per vine is dependent upon cultivar. Lastly, leaf removal in the vicinity of the clusters may be performed on many colored cultivars. As light is a requirement for the formation of many of the pigments in the berry's skin, such a procedure will assist in advancing the maturity of the fruit. The removal of shoots without clusters earlier in the growing season will also assist in allowing more light into the fruiting zone and reduce congestion around the fruit.

HARVEST AND POST-HARVEST
The majority of the table grapes in California is packed in the field. The fruit is first picked and most of the rot and undesirable fruit is removed. At the end of the row a portable stand is used for packing and it is here any further unwanted berries are removed and the fruit is graded. Each stand is equipped with a scale to insure the proper weight is packed into the boxes. Government inspectors may be present there or at the cold storage facility. Fruit quality (and its grade) is dependent upon berry size, color, and appearance. The majority of the fruit in California is packed into wooden (TKV) boxes with packed weight being 9 to 10 kg. Other types of packing also are being used dependent upon the buyer and final destination. This includes styrofoam and cardboard boxes. Many buyers demand that individual clusters be packed in plastic bags to reduce losses of individual berries. Trucks continually traverse the vineyards and transport the boxes as they are packed to the cold storage facility.
Once the fruit arrives at the cold storage facility the grapes are pre-cooled and fumigated with sulfur dioxide. The fruit is stored at 0^oC until loaded for final destination. Fumigation is used to control Botrytis cinerea and/or other pathogens and pests. A new method of fumigation with sulfur dioxide has been developed in recent years in response to regulations developed by the U.S. Food and Drug Administration (FDA) and the U.S. Environmental Protection Agency (EPA). The FDA has established a tolerance of 10 parts per million (ppm) for sulfur dioxide residues in table grapes and the EPA now classifies sulfur dioxide as a pesticide. The new method is called "total utilization fumigation". This method differs from the previous traditional method in that sulfur dioxide applied is balanced with the amount of sulfur dioxide absorbed by fruit, boxes and the cold storage room itself. At the end of a fumigation period the sulfur dioxide is usually less than 2 ppm. This method is well suited for both the initial fumigation and for grapes held in long-term storage. This level of sulfur dioxide may also mitigate bleaching of the fruit that can occur with excessive amounts of sulfur dioxide.

MARKETING
The primary markets for California table grapes are the United States and Canada. Exports have risen in recent years with approximately 153,000 metric tonnes shipped in 1997. This represented 23% of the total production of table grapes last year. The leading export markets are Latin America and Asia. The value of the export market was greater than $200 million U.S. dollars. The California Table Grape Commission coordinates the generic marketing of table grapes.