Man's relationship with fruiting plants began long before the origins of agriculture in 8000-10,000 BC, when all human beings practiced the hunter-gatherer lifestyle. Fruits gathered from the wild were mainstays of our diet, being excellent sources of fiber, vitamins, and other healthful or medicinal compounds unbeknownst to us then. While cereal grains such as wheat and barley were probably the first crop plants domesticated by humans, several of today's fruit crops were not far behind since they were native to the very same area - the fertile crescent of Asia Minor. Domestication of wild fruiting plants may have been inadvertent; the first groves of fruit trees probably sprang from seeds thrown in waste heaps at the edge of villages. Careful observation and selection for useful traits such as larger size, better taste, and higher yield started the transformation of those wild plants into the crops we cultivate and enjoy today. During the age of discovery, fruits, seeds or live plants were often taken on transoceanic voyages, and exchanges in both directions helped spread many crops throughout the world. Christopher Columbus and his contemporaries may not have realized the impact they would have on agriculture and society when they brought crops such as coffee and citrus to the new world, and returned to Europe with previously unknown, but now common foods like cocoa and pineapple.

Today, we have well established world trade networks and sophisticated cultural and postharvest technologies that allow fruits to be enjoyed throughout much of the year, instead of mere weeks per year like our ancestors experienced. Global trade has made formerly rare and exotic treats derived from fruit crops commonplace in countries with no hope of cultivating the plants. Fruit crops are important agricultural commodities, adding tens of billions of dollars per year to the global economy, and being major sources of income for developing countries. Worldwide, over 100 million acres of land has been devoted to their production, and the livelihood of literally millions of farming families depends on continued global trade.

This text is designed to acquaint you with the basics of the botany, production, economic value, general culture, and food uses of the world's major fruit crops. It is formatted as a reference text to allow quick retrieval of essential facts and figures. The following outline is used throughout the text to facilitate information retrieval and keep the discussion as uniform as possible from crop to crop:
 

• I. Taxonomy
• II. Origin, history of cultivation
  
• III. Folklore, medicinal properties, non-food usage
  
• IV. Production 
• V. Botanical description
  
• Plant
• Flowers
• Pollination
• Fruit
• VI. General culture:
  
• Soils and climate
• Propagation
• Rootstocks
• Planting design, training, pruning
• Pest problems
• VII. Harvest, post-harvest handling
  
• VIII. Contribution to diet
  
• IX. References and further reading

One would think that the term fruit crop would be clearly defined - not so. In fact, one of the most frequently asked questions from this web site is "...is the tomato a fruit or a vegetable?" My usual reply is "both", as it is clearly a fruit in the botanical sense, but a vegetable from a culinary perspective. There are also legal definitions on the books, since in some instances vegetables are taxed and fruits are not (or vice-versa). I think it wise to avoid culinary and legal definitions since these change over time and across regions, and the botanical definition of a fruit, being invariable, is a safer bet. I have chosen to define fruit crop as: a perennial, edible crop where the economic product is the true botanical fruit or is derived therefrom. The term perennial eliminates crops grown as annuals such as tomato, pepper, melons, and corn, even though the harvested product is the true botanical fruit. Annual cultivation practices differ markedly from those of perennial crops, and to call these fruit crops would only increase the existing confusion. Note that strawberry, an herbaceous perennial, is included in the text despite the fact that in recent decades, much of the acreage is replanted annually. The word edible eliminates perennial crops whose fruits are used for fiber like Kapok (Ceiba pentandra), or strictly industrial oils like tung nuts (Aluerites fordii). The true botanical fruit is the ripened ovary plus any associated parts, and contains the seeds of the plant. While most would not consider coffee and cacao fruit crops, they fit my definition since they're perennials, and coffee and cocoa are just roasted, ground up seeds of the fruit. African oil palm, coconut and olive might not strike you as fruit crops either, but again, they fit the definition since coconut, olive and palm oils are edible and derived from a true botanical fruit (all drupes). A nut is a dry, indehiscent fruit with a hard shell, and accordingly, several nut crops are included in the text.
 
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Plant Names

The scientific or Latin name of a plant is extremely important, as common names vary with location and language spoken. Taxonomic classification places a plant in a large group of related plants (a family), and assigns to it an official name, based largely on Latin and Greek root words. Scientific names generally consist of two italicized words, the first denoting the genus, the second a species within that genus. For example, Malus domestica is the name for the cultivated apple, where Malus is the genus and domestica the species name. In Latin, Malus is a noun meaning apple, or alternatively evil, bad or wrong. [The dual meaning probably stems from the biblical story of Eve and the forbidden fruit in the Garden of Eden]. The species name domestica is an adjective meaning "around the house", thus the entire name translates roughly to the domesticated apple. Last but not least, someone always has to take credit for things, so the authority is tacked on to the scientific name denoting the person who named the plant. In the case of the apple, it was a botanist named Borkhausen, so the precise, full name for the apple is:

Malus domestica Borkh.

Linneaus, the father of botany, named many plants, which explains the capital letter "L" found at the end of many plant names, such as Pyrus communis L. (pear). The authority is often omitted in popular literature, on nursery tags, or in plant catalogs, so outside of scientific literature it is not often seen. The authority is not italicized, but typically abbreviated, and there are literally thousands of these abbreviations in use today.
  
To make matters more complex, some plants have been named or renamed several times, so two or more scientific names may be found for the same plant (see sidebar). For example, the Asian pear, Pyrus pyrifolia (Burm. f.) Nak., also is named Pyrus serotina L., so you may see either name used. Yes, this somewhat defeats the purpose of using a unique, scientific name for a given plant, but opinions vary as to which is "right".

We are all familiar with different types of fruits within a species, such as 'Red Delicious', 'Golden Delicious', and 'Granny Smith' apples. Horticulturists refer to these subspecies as "cultivars", which means "cultivated varieties". The term "variety" often is used interchangeably with cultivar, although purists prefer the latter. Note the convention of enclosing the cultivar name in single quotes. The precise name of that green, crisp apple we see in grocery stores is therefore:
 
Malus domestica Borkh. 'Granny Smith', or alternatively written as

Malus domestica Borkh. cv. Granny Smith.
 
An even finer level of detail is found in some plants, the strain or sport. This is equivalent to a sub-subspecies, or a form, where within a cultivar we have several slight variations that have horticultural importance. An example would be 'UltraEarli Fuji' apple, which is a strain of 'Fuji' apple that ripens a bit earlier than the original 'Fuji'. As in this example, strains are often given a new cultivar name if they become commercially important. Another example is 'Red Max', a strain of 'McIntosh' apple with deeper red color. While this seems like the splitting of hairs, it is useful to note that, in this example, 'Red Max' and 'McIntosh' are genetically more similar than are 'Fuji' and 'McIntosh'. A few more points will help clarify the use of plant names:
 

• After the first reference to a genus in the text, the genus name is often abbreviated using only the first letter. For example, in the chapter on apple, I discuss species related to Malus domestica and refer to these as M. floribunda, M. sargentii, M. micromalus, etc., hoping that you understand that the "M." stands for "Malus" in each case; this saves valuable typing time for the author and a few drops of ink for publishers.
• A multiplication sign "X" placed between the genus and species denotes an interspecific hybrid within that genus. An example is the name for the cultivated strawberry, Fragaria X ananassa, which was originally developed by crossing Fragaria chiloensis with Fragaria virginiana.
• Some genera of plants are so poorly characterized, so diverse, or contain hybrids with complex parentage derived from so many species, that it is convenient to refer to them as a group using the abbreviation "spp.". Among fruit crops, blackberries are a good example of this, often referred to as Rubus spp.

 

Cultivars

For each crop, I discuss only the major cultivars or groups of cultivars. Even brief descriptions of the many cultivars grown would require a doubling of the length of the text. Cultivars change over time, and vary across different fruit growing regions, making it impossible to cover adequately and keep the text as concise as possible. Several other texts or sources are devoted to cultivar descriptions, including some listed below, and I have included one or two references for each crop that give more detail on cultivars.
 

Some helpful references on taxonomy in relation to fruit crops:
 

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The center of diversity of a plant species denotes the area of the world where the species evolved and is found growing in the wild. It is interesting to note that many of the fruit crops (and crop plants in general) grown in the United States are not native to North America. Perhaps more interesting is that several cultivated fruits are not found in the wild, indicating that man has hybridized or selected the species over time to make it very different from its wild progenitors. It is not surprising, therefore, that many of the world's most common fruits are native to the area where agriculture had its roots - Asia Minor and China.

Knowledge of soil and climatic conditions occurring in the native range give us clues about site selection and cultural methods that should be employed when growing these crops in foreign areas. Also, plant breeders often return to these centers of diversity to collect germplasm useful for disease and pest resistance or other traits. A brief history of cultivation is provided for each crop in the text. Lessons from history have been valuable in shaping the way we grow crops today.
 

Evans, L.T. 1998. Feeding the ten billion: plants and population growth. Cambridge University Press, Cambridge, UK.

Upshall, W.H. (ed). 1976. History of fruit growing and handling in the United States and Canada 1860-1972. Regatta City Press, Kelowna, BC, Canada.

Fruits have more than just nutritional value to us; some may contain anti-cancer compounds while others may cause health problems or even death. In this section, I have compiled information from various sources on the folklore, myths, healing properties, and symbolism that has surrounded fruits for centuries. Some of the information on medicinal properties is well-documented, but some of it is speculative. Consult the sources below for more information on medicinal properties of fruits and other plants.
 

The production in terms of metric tons (MT) per year (1 metric ton=2200 lb or 1000 kg) or percentage of total is given for the world and the United States. Generally, publication of this data runs about 1-2 years behind the times. In the text, I have used the year 2002 since these data were fairly solid by 2004 when the book was written. Primary sources for this are the Food & Agricultural Organization of the United Nations (FAO), and the USDA Agricultural Statistics Service. In some cases, state agencies or commodity groups have been contacted for these data. Most states in the USA have agricultural statistics services that produce annual reports of agricultural production. Much of this data can now be accessed from the Internet, which is generally more up to date:

FAO - http://apps.fao.org/default.htm

USDA - http://www.usda.gov/nass/pubs/agstats.htm
 
Table 1.1 The world's top 20 fruit crops ranked in terms of weight of production per year (FAO 2002).

Vines or lianas are woody plants that are trained to have a single trunk at the base, but use twining stems or tendrils to support the canopy. Vines rarely have large trunks like trees since they support themselves by climbing on taller plants in nature, or on trellises in cultivation. As a result, vines spend little of their energy on supportive wood, while growing very tall and maximizing leaf exposure to sunlight.

The floral morphology of a fruit crop is important in determining the mode of pollination (i.e., wind or insect) and type of fruit that will arise when the ovary matures. A complete flower is one possessing all four fundamental appendages: sepals, petals, stamens and pistils. An incomplete flower lacks one or more of these features. The position of the base of the pistil, or ovary, with respect to the other three appendages is important in identification, and also partially determines what the fruit type will be. The two most common positions are inferior (epigynous) and superior (hypogynous). A third possibility is "half-inferior" (perigynous), as found in the stone fruits (Prunus spp.). Figure 1.4 shows that a superior ovary sits above the point of attachment of the sepals, petals, and stamens, whereas the inferior ovary is embedded within the receptacle, below the point of attachment of the other floral organs. A perigynous ovary sits within a hypanthium or floral cup. Different fruit types arise from the flowers shown, partly as a result of the difference in ovary position.

Figure 1.4  Superior and inferior ovary position in idealized flowers (above). Below, examples of ovary position in fruit crops: left, a superior ovary in thornless key lime flowers; middle, an inferior ovary of an Asian pear at petal fall; right, a perigynous ovary surrounded by the hypanthium in sweet cherry.

A perfect flower possesses both male (stamens) and female (pistil) parts, whereas an imperfect flower may be either staminate (functionally male, having only stamens) or pistillate (functionally female, having only a pistil or pistils). The style is very short or lacking in some pistillate flowers, like in  pecan, but the stigma and ovary are always present if the pistil is functional. If staminate and pistillate flowers are borne on the same plant but in different locations, the species is termed monoecious. If staminate and pistillate flowers occur only on different plants, the species is termed dioecious. One can see the ramifications for pollination and orchard design: a dioecious species such as pistachio or kiwifruit must be planted in an orchard with male plants near females for pollination. Aside from pollination, the male plants are useless since they do not possess ovaries that will ripen into fruit.

Figure 1.5. Stick figures of different inflorescence types commonly seen in fruit crops. See glossary for complete definitions.

Pollination
Pollination is the transfer of pollen from the anther to the stigma. This is usually mediated by the wind or an insect. In some cases, hummingbirds (wild pineapple), bats (wild bananas), or other animals may play the role of pollinator, but most fruits are pollinated by the familiar honey bee (Apis mellifera).

Once the pollen is transferred, the pollen grain will germinate on the stigma, and the pollen tube will grow downward until it reaches an ovule. This may take a few days. The pollen tube, under control of the tube nucleus, allows for the movement of the two generative nuclei through the style and into the ovule. The processes of pollination and fertilization are depicted in Figure 1.6.

Figure 1.6. Simplified diagram of pollination and fertilization within a typical flower pistil. Some of the details within the embryo sac (exploded view at right) have been omitted for clarity. See text for explanation.


Upon release of the generative nuclei within the embryo sac, the process of double fertilization occurs, involving two fusion events (hence the name, "double fertilization"): one generative nucleus unites with the egg nucleus, and the other generative nucleus unites with two polar nuclei, yielding the zygote and endosperm, respectively. You may want to review the details of double fertilization in one of the references listed below (which will show all of the other details left out for the sake of clarity). Suffice it to say that the end-product of double fertilization is the seed. When a fruit containing seeds is deposited in a suitable place, the seeds germinate, and the life cycle of the plant is completed.

At this point, one might ask: why is pollination necessary for fruit culture, given that seeds are undesirable in fruits from a marketing standpoint? The short answer is that seed development is the prerequisite for fruit set. From the plant's perspective, a fruit functions in seed dissemination. Therefore, investing a great quantity of photosynthetic energy into a fruit that doesn't contain viable seed is wasteful. Unpollinated and unfertilized ovaries are therefore dropped from the plant shortly after bloom, as there is no need to invest resources into fruits that cannot aid in the reproductive success of the species. Physiologically, the developing embryos within seeds produce growth regulators that prevent abscission of fruitlets, and cause the fruit tissue in the vicinity of the seed to grow. Thus, dropping of unfertilized ovaries after bloom is "pre-programmed", and prevented only if there is one or more developing seeds present. In fruits with multiple seeds, poor pollination and low seed set results in misshapen or asymmetrical fruit that are often unmarketable (Figure 1.7).

Having said that, it must have dawned on you by now that there are several types of seedless fruit, and the above argument does not always hold. Fruits that set and mature without seed are termed parthenocarpic, and these species, although rare in nature, have been exploited by horticulturists to a great extent. Examples include Persian lime, some seedless oranges and tangerines, banana, and pineapple. Seedless grapes are perhaps the best known seedless fruits, but are not truly parthenocarpic. They undergo pollination and fertilization, but seed development is aborted shortly afterwards; this situation is termed stenospermocarpy.

Cross-pollination and self-pollination are important terms. Some fruit crops require genetically distinct pollen for fertilization and fruit set to take place, and set fruit poorly if their own pollen is used. These species are planted in orchards with two or more cross-compatible cultivars to favor cross-pollination. On the other hand, some species set more than enough fruit when pollinated by themselves. Such self-pollinating species can be grown in large orchards composed of a single cultivar, which are easier to manage. The cultivar used as a source of compatible pollen is referred to as a pollinizer for cross-pollinating species. Note that transfer of pollen between one flower and another on the same tree is still considered self-pollination, as is the case when pollen is transferred between flowers on separate trees of the same genotype. The key to cross-pollination is having a genetic distinction between the pollen source and recipient.

Many cross-pollinating species exhibit self-incompatibility, such that fertilization by their own pollen is disfavored or prevented through physical or biochemical factors. There are different degrees of self-incompatibility, and many self-incompatible species will produce a few fruit even when self-pollinated. Thus, a single apple tree in your backyard may have a bushel or so of fruit, since apples are not completely self-incompatible, but the same tree may produce several bushels if cross-pollinated. Horticulturists have coined the terms "self-fruitful" and "self-unfruitful" to describe cultivars that can set commercial crops, or cannot set commercial crops (respectively) when self-pollinated. Thus, self-fruitful and self-unfruitful are economic or horticultural terms, whereas "self-incompatible" or "cross-incompatible" are botanical terms.

Highly self-incompatible cultivars or species are often referred to as "self-sterile", which is technically incorrect. The term "sterile" implies that there is either no viable pollen or no viable eggs to be fertilized. If a cultivar is truly male sterile, for example, its pollen could not fertilize its own or any other egg; the pollen is simply non-functional. Likewise, a female sterile cultivar will not produce viable seed regardless of the pollinizer used. The rabbiteye blueberry (Vaccinium ashei) is a good example of misuse of these terms. When two cultivars of rabbiteye are inter-planted, fruit set is often 50% or more on both cultivars, but if either is planted alone, only about 2% of flowers will set fruit. The latter causes people to think that rabbiteyes are self-sterile. But, since fruit sets on both cultivars when cross-pollination occurs, this indicates that the pollen and eggs of both cultivars are viable (not sterile); rabbiteyes are simply highly self-incompatible.

Fruit
Fruits are matured ovaries plus any associated flower parts, and contain the seeds of the plant. The ovary may be subdivided into two or more carpels (then termed compound ovary) each bearing one to many ovules. Individual carpels develop into sections of a whole fruit, as with citrus where each familiar segment of the fruit represents one matured carpel. If the ovary is not subdivided, then it is termed simple. The ovules will mature into seeds if fertilized.

The ovary has three layers of tissue: the exocarp (outermost), mesocarp (middle), and endocarp (innermost) (Figure 1.8). These layers may develop into distinct parts of the fruit. Generally, the exocarp becomes the fruit peel or skin, the mesocarp becomes the fruit flesh, and the endocarp becomes the innermost part of the flesh or a specialized tissue surrounding the seed(s), like a pit. In many cases, however, the three layers are indistinguishable, and the term pericarp is applied to denote all ovarian  tissues surrounding the seed(s).

Figure 1.8. Fruits are matured ovaries, and contain the seeds of the plant. Ovaries sometimes have distinct layers of tissues, as labeled above, which develop into distinct structures of the fruit as shown for a drupe. Not all fruits have such clearly demarcated tissues.

Fruit types are good identification criteria for plants, and are often determined by floral morphology, particularly ovary position (i.e., superior or inferior). The major fruit types in commercial fruit crops are: pome, drupe, berry, hesperidium, aggregate, accessory, multiple or syncarp, and nut. Use the glossary for descriptions of fruit types, and/or the fruit key below.

1. Fruit developed from two or more separate flowers, or derived from an entire inflorescence
        2. Fruit consists mostly of receptacle tissue, with tiny, ripened ovaries borne along the inner wall of the hollow
        receptacle...................................................................................................................... Syconium (Fig)
        2. Fruit consists of tightly clustered ripened ovaries plus
        the inflorescence axis............................................................ Multiple or Syncarp (Pineapple, Mulberry)
1. Fruit developed from a single flower
        3. Fruit developed from two or more separate ovaries
                4. Fruit primarily ovarian tissue; an aggregation fruitlets on a receptacle............. Aggregate (Brambles)
               4. Fruit primarily non-ovarian tissue ............................................................... Accessory (Strawberry)
        3. Fruit developed from one ovary                                       
                5. Fruit (mostly) fleshy at maturity
                        6. Fruit with a thin skin, homogenous texture throughout (except seeds) ................. Berry (Grape)
                        6. Fruit with heterogenous texture
                                    7. Outer part of fruit tough and hard, or leathery
                                            8. Septa (partitions) present, several to many, rind leathery ...... Hesperidium (Citrus)
                                            8. No septa, outer rind tough, thick .......................................... Pepo (Watermelon)
                                    7. Outer part of fruit soft; thin-skinned
                                           9. Fruit with a hard, bony endocarp surrounding the seed......... Drupe (Peach, Mango)
                                           9. Fruit with two or more seeds, center with papery or cartilaginous
                                            structure surrounding seeds..................................................... Pome (Apple, Pear)
                5. Fruit dry at maturity
                        10. Fruit indehiscent (not splitting open) at maturity
                                11. Fruit winged ................................................................................ Samara (Maple, Ash)
                                11. Fruit not winged
                                        12. Seed fused to fruit wall ......................................... Caryopsis, Grain (Corn, Wheat)
                                        12. Seed not fused to fruit wall
                                                13. Fruit wall bladder-like, loose and free from seed..................... Utricle (Spinach)
                                                13. Fruit wall not bladder-like, close-fitting to seed
                                                        14. Fruit large, with hard, bony wall .............................  Nut (Walnut, Pecan)
                                                       14. Fruit small, wall thin  .............................................. Achene (Sunflower)
                        10. Fruit dehiscent (splitting open) at maturity
                                15. Ovary compound; fruit developed from more than one carpel
                                        16. Fruit splitting into one-seeded segments at maturity, but carpels not
                                             dehiscing to release seeds ...................................................... Schizocarp (Carrot)
                                        16. Fruit splitting open to release seeds at maturity
                                                17. Two carpels separated by a thin, translucent septum
                                                       18. Fruit less than twice as long as it is wide ........... Silicle (Shepard's purse)
                                                       18. Fruit more than twice as long as it is wide ...................... Silique (Mustard)
                                                17. More than two carpels, not separated by a thin, translucent septum ..... [Capsule]
                                                       19. Capsule opening along a transverse circular line; top separating
                                                           like a lid ............................... Circumscissile capsule or Pyxis (Brazil Nut)
                                                       19. Capsule opening lengthwise or by pores, top not separating like a lid
                                                                20. Capsule opening by pores or flaps ........... Poricidal capsule (Poppy)
                                                                20. Capsule opening longitudinally, often lengthwise
                                                                        21. Capsule dehiscing through locules ..... Loculicidal capsule (Iris)
                                                                        21. Capsule dehiscing through septa .....Septicidal capsule (Yucca)
                                15. Ovary simple; fruit developed from one carpel
                                        22. Fruit opening along a single suture .............................................. Follicle (Milkweed)
                                        22. Fruit opening along two sutures
                                                23. Fruit not constricted between seeds ................................. Legume (Pea, Bean)
                                                23. Fruit constricted between seeds, sometimes breaking into
                                                    one-seed segments ................................................................. Loment (Desmodium)

The fruit bearing habit of a plant refers to the position and type of wood on which flower buds, and subsequently fruits, occur. This is important in pruning and training, because we want to encourage the type of wood that bears the fruit and minimize unnecessary vegetative growth. Some species are spur bearing, where fruit are borne on very short, slow-growing, lateral branches (Figure 1.9). Spurs develop on 2-year-old and older wood, and may grow only ¼" per year; thus, the fruit are borne at nearly the same points in the canopy from year to year. For spur-bearing species, it is important to keep good light exposure throughout the canopy because shaded spurs fail to form flower buds for next year's crop. Lateral-bearing species produce fruit from lateral buds on 1-year-old wood. For these species, it is important to stimulate ample growth each year (by dormant pruning, fertilizing, etc) so that enough fruiting shoots are available for the next year. A number of crops bear fruit on current season's growth, either laterally as in grape, or terminally as in walnut or mango. Some of the tropical crops exhibit cauliflory, where fruit are borne on large branches or trunks of trees (e.g., cacao).

Figure 1.9. The two most common bearing habits of fruit crops: spur and lateral. Spurs are simply short, lateral branches that occur on 2-year-old and older wood. Apple, pear, and sweet cherry are examples of spur bearing species. Lateral bearing species produce fruit from lateral buds on 1-year-old or current season's growth, and include peach and grape.

Thinning refers to the partial removal of flowers or fruitlets in order to improve the size of the remaining fruit. Thinning is often practiced for large-fruited species that normally set too many fruit. By thinning, one directs the available photosynthate produced by the leaves into fewer, but ultimately larger fruit, rather than many small, unmarketable fruit.   

Thinning is accomplished by hand usually, and is obviously very labor intensive and expensive. Flowers or fruits are removed such that a certain number of fruit per tree, or a certain spacing between fruit on a limb is achieved. For example, apples are thinned to 1 fruit per spur, with spurs spaced about 4-6" apart, resulting in the removal of about 80% of the original number of fruitlets (Figure 1.10). Thinning should be uniform throughout the canopy, as fruit in clusters will remain small even if the correct total number of fruit are left. In some species, chemicals can be sprayed on trees to kill flowers or induce drop of fruitlets. Chemical thinning is less expensive, but riskier since the degree of thinning depends not only on chemical and concentration, but on weather, cultivar, stage of fruit development, and skill of the orchardist.

Figure 1.10  Fruit thinning is commonly practiced for large-fruited species, such as apple. In this case, three fruit have been left, one per spur, spaced about 4" apart. Spacing is important; leaving three fruit on the same spur while removing all others would not yield the same increase in fruit size.

In terms of timing, the earlier the tree is thinned, the better the result. When possible, thinning at bloom provides the greatest improvement in fruit size. However, many growers wait until the threat of frost is passed to thin to make sure there will be enough fruit for a full crop. Much of the benefit of thinning is lost if delayed more than about 45 days post-bloom.

References on plant morphology, flowering and fruiting relating to fruit crops:

Harris and Harris. 1994. Plant identification terminology: An illustrated glossary. Spring lake Publ., Spring Lake, Utah.

Faust, M. 1989. Physiology of temperate zone fruit trees. Jihn Wiley and Sons, New York.

Monselise, S.P. (Ed). 1986. CRC handbook of fruit set and development. CRC Press, Boca Raton, FL.

Nyeki, J. And M. Soltesz (eds). 1996. Floral biology of temperate zone fruit trees and small fruits. Akademiai Kiado, Budapest, Hungary.

Sedgley, M. And A.P. Griffin. 1989. Sexual reproduction of tree crops. Academic press, London.

Soule, J. 1985. Glossary for horticultural crops. John Wiley and Sons, New York.

Westwood, M.N. 1993. Temperate zone pomology, 3rd edition. Timber Press, Portland, OR.

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For each crop, the main aspects of cultivation can be found in this section. The subsections include: soils and climate, propagation, rootstocks, planting design, training, and pruning, and pest problems.

Soils and Climate
Most fruit crops grow best on deep, well-drained, loamy soils, with pH of 6-7. The rare exceptions to this are noted. Climate is probably the strongest determinant of the success of fruit cultivation. Several aspects of climate are critical.

Cold Hardiness. This is the minimum temperature tolerance for the plant, often quoted in degrees F or C causing 50% or greater mortality. The flower buds, vegetative buds, and wood often have different killing temperatures, with flower buds being the least hardy. Thus, a fruit tree may survive in a northern winter, but produce little or no fruit. Maximum cold hardiness values are given for each species in the text. It is important to note that cold hardiness is not constant; in the summer, most fruit crops would be killed by relatively high temperatures (i.e., 10s or 20s °F or -12 to -2°C). As they acclimate in fall and early winter, they obtain the ability to withstand temperatures well below 0°F (-18°C) in most cases. Conversely, as the buds begin to swell in late winter or spring, several degrees of hardiness are lost per week (Table 2). In almost all fruit crops, open flowers or small fruitlets can withstand only 28 to 30°F (-1 to -2°C) without injury, making most crops vulnerable to relatively mild spring frosts. Fruit growers choose sites less prone to frost, and/or use heaters, sprinkling, or wind machines to prevent crop losses when frost occurs (Figure 1.11). Tropical crops are exceptions - most have no capacity for acclimation and are killed by brief exposure to subfreezing temperatures. Subtropical crops like citrus and date display a modest ability to acclimate and withstand temperatures 5-10°F below the freezing mark.

Table 1.2 Change in killing temperature of apple flowers as they develop during late winter and early spring (modified from Proebsting and Mills, 1978. J. Amer Soc. Hort. Sci. 103:192). Pictures below depict a few of the stages of bud development.

Figure 1.11. Peach flowers encased in ice during a frost event. Sprinkler irrigation is commonly used to protect fruit crops from frost damage in the temperate zone. The water releases heat as it freezes, keeping flower bud temperatures above the killing point of about 28°F.

Chilling Requirement  - this is the number of hours of exposure to 45°F (7°C) or below required each winter to satisfy dormancy and allow normal growth the following spring. The basic components of dormancy are depicted in Figure 1.12. Most temperate woody plants require between 500-1500 chill hours each winter, measured from leaf drop in autumn until February or March. If the winter is warm, and the chilling requirement is not met, then bud break is sporadic and light, and cropping is poor. If the plant receives much more chilling than needed, bud break is accelerated and often premature, resulting in frost damage. This is why it is important to match the chilling requirement of the plant to the location, and also why temperate species like apples cannot be grown in the tropics where temperatures rarely drop below 60°F. For most species, there are a few low chill cultivars that can be grown in areas with mild winters that would not support growth of traditional cultivars. ‘Flordaprince' peach and ‘Flordahome' pear are examples of fruit trees bred specifically for low chilling requirement, and can be grown successfully in Florida, southern Texas, and other warm winter locations. Tropical crops have no chilling requirements, and although they may often flower or break bud after winter in subtropical regions, they do not need the cool exposure to flower or grow normally.

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Most fruit crops are propagated by vegetative means to retain the exact fruit characteristics of the parent plant. Seed propagation generally results in highly variable fruit size, shape, color, and flavor, and creates a management nightmare, since each seedling is genetically different from the others. People recognized this a few thousand years ago and began grafting, budding, or rooting cuttings of desirable fruit crops, rather than planting them by seed. Today, almost all tree fruits are grafted or budded, and most small fruits and some grapes are grown from cuttings. Specialized nurseries produce millions of new plants each year, generally by growing rootstocks for a year, then budding or grafting them with the desired scion cultivars the following year. Many tropical fruit crops are still propagated by seed, including cashew, coffee, oil palm and coconut. They are either fairly true breeding from seed, or cannot be propagated easily by vegetative techniques.

The general process of budding or grafting is depicted in Figure 1.14. In this book, I give a brief description of the methods used to propagate a given species, but if you have not studied plant propagation recently or at all, it would behoove you to refer to one of the texts below for general information. Several terms related to propagation are described in the glossary.

Figure 1.14. Basic process of grafting or budding a fruit tree. The rootstock is grown for about one year prior to grafting or budding. Shown here is a seedling rootstock, but some rootstocks themselves are vegetatively propagated. Once the rootstock is of sufficient size, one or more buds from the desired scion cultivar are joined with it via a number of techniques.

References on propagation:

Garner, R.J. 1988. The grafter's handbook, 5th edition. Cassell Publ., Ltd, London.

• The exact growth, flowering, and fruiting characteristics of the cultivar are preserved through grafting.
  

• Juvenility is greatly reduced. Grafted trees begin fruiting at a very early age, often when only 2 or 3 years old. Seedlings of some species may not fruit until they are 5+ years old. Pomologists say that grafted trees are more precocious than seedling trees. Dwarfing rootstocks sometimes induce more precocity than non-dwarfing or seedling rootstocks.

• Rootstocks allow adaptation of scion cultivars to climates and soils normally unfavorable for growth. Some examples include:

• Tolerance of poor drainage (e.g., plum rootstocks for peach in wet soils)

• Tolerance to drought (e.g., Rough Lemon rootstock for sweet orange cultivation on the droughty sands of central Florida)

• Tolerance of high pH or salinity (e.g., peach x almond hybrid rootstocks for high pH tolerance of peach)

• Improved cold hardiness and/or bloom delay of scions (e.g., Trifoliate orange rootstock for sweet orange)

• Tolerance to soil diseases and nematodes (e.g., ‘Nemaguard' rootstock for root knot nematode resistance in peach)

• Rootstocks provide tree size control in some species. This is most widely exploited in apple, where there is a range of tree size obtainable by using different dwarfing rootstocks. For example, a ‘Red Delicious' tree on M.27 rootstock will be only 6 feet tall at maturity, but would be 20+ feet tall if grafted onto an apple seedling rootstock. The availability of dwarfing rootstocks has allowed many innovations in tree fruit culture, such as more efficient planting designs and training systems, reduced pesticide use, and an earlier return on investment for orchardists.