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FRUIT, CITRUS

within the world citrus production. For instance, in the United States, Florida produces a large proportion of the sweet oranges, the majority of which are used in pro- cessing. California produces a higher quality sweet or- ange, with emphasis on navel varieties, which is eaten fresh and largely exported. Countries such as Spain and Morocco produce large quantities of mandarins for ex- port to the United Kingdom and northern Europe. Some of the Southern Hemisphere countries export to major Northern Hemisphere producers during the off-season.

As with any industry, there have been changes over the years. In the last decades of the twentieth century, the trend has been toward increased global trade and yearlong availability in most major markets. Processed products have grown in importance. In fresh fruit, the trend has been toward easy-peeling, seedless mandarins and sweet oranges. In grapefruit, the pigmented varieties are becoming predominant.

Production

Citrus is produced in slightly different ways in different areas. Commercial production is more uniform through- out the world than is local or personal production, but there are some differences here as well. Many of the dif- ferences are in the nature of farming inputs rather than the production of trees. For instance, fertilization and ir- rigation are necessary in most areas. However, a more industrialized producer in an exporting country may uti- lize drip irrigation with inorganic fertilizers injected through the drip system, while a producer for the local market in a poor country or area may use manure and flood irrigation.

Citrus can be grown from seed; however, there are some disadvantages. In some cases, seedlings are not true- to-type with the mother tree; due to juvenility factors, seedling trees do not usually bear fruit until they are nearly a decade old; and they are vulnerable to unfavor- able soil conditions, diseases, and so forth. For these rea- sons, most citrus produced throughout the world utilizes budded (grafted) trees.

A budded tree consists of two parts: the scion, which is the fruit variety, and the rootstock, which supports the scion in the soil environment. Rootstocks are chosen based on a number of factors, including compatibility with the scion, resistance to diseases or pests, adaptation to soil conditions, effect on fruit quality. Citrus root- stocks can be grown from seed, since the commonly used rootstocks are apomictic (and hence true-to-type), and there are no confirmed seed-transmitted systemic dis- eases of citrus. Production from seed is easier than from cuttings, the common method of production for root- stocks for most other tree crops.

The rootstock is usually of an appropriate size for budding about nine months to a year after germination, when it is about the diameter of a wood pencil. The scion variety is budded onto the rootstock by making an inci-

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sion into the bark of the rootstock, inserting a bud re- moved from the scion variety, and wrapping it with tape. A callus should form between the rootstock and scion tis- sues in two to four weeks. With appropriate training, the young tree is ready for planting in the field in about an- other year.

Once planted, it is usually about two to three years before the tree begins to produce fruit. Full production is usually achieved at about ten years of age. Under ap- propriate conditions, citrus trees may live a long and pro- ductive life and achieve a fairly tall height. This was common in many older citrus-producing areas. Since about the 1970s, citrus production has become more cyclical, like that of other tree crops, and the life of an orchard may be no more than twenty to thirty years.

Citrus requires relatively little cultural manipulation compared to crops such as grapes and deciduous trees, which require pruning and extensive training. In some areas, however, such as the Mediterranean basin, man- darins and sweet orange may receive somewhat more ma- nipulation than in areas such as California. Lemons grow vigorously upright and require more frequent topping. Irrigation and fertilization are necessary. Certain pro- duction problems or challenges in citrus have been suc- cessfully managed with the application of plant growth regulators. This is more established in citrus than in most other perennial crops.

In contrast to the relatively low cultural inputs for citrus, disease and pest management in this crop is more critical and challenging than for many others. Because citrus is grown in warm areas of the world, reproduction of insect pests is rapid and insect pressure can be great. The individual insect pests vary greatly with geographic area. Compared to other crops, citrus is also subject to a larger number of systemic, graft-transmissible diseases caused by virus and viruslike pathogens that can poten- tially devastate industries. The most important world- wide is the tristeza virus, which destroyed many thousands of hectares in California and South America starting in the 1930s. This has been managed in some areas by certification programs requiring the use of virus- tested propagative materials and in a few cases with erad- ication programs. Other diseases, such as greening and citrus variegated chlorosis, are equally deadly but less widespread throughout the world.

Citrus is harvested by hand. At this point, there have not been any widely accepted methods of mechanical har- vest. The time of harvest is dictated by the market or in some cases by legal maturity standards. Citrus is more forgiving than some other crops in that harvesting can be delayed somewhat and fruit quality is not decreased too much by the extra time on the tree. This varies with variety. However, if fruit are left on the tree too long, quality deteriorates as acid levels decrease and the taste becomes insipid. Other fruit quality problems can also occur. After harvest, citrus can be stored at low (refrig-

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