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Hort 403 - Lecture 32
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Lecture 32
Citrus

Citrus: Citrus spp., Rutaceae

  • Citrus are subtropical, evergreen plants originating in southeast Asia and the Malay archipelego but the precise origins are obscure.
  • There are about 1600 species in the subfamily Aurantioideae.
  • The tribe Citreae has 13 genera, most of which are graft and cross compatible with the genus Citrus.
  • There are some tropical species (pommelo).
  • All Citrus combined are the most important fruit crop next to grape.
  • The common features are a superior ovary on a raised disc, transparent (pellucid) dots on leaves, and the presence of aromatic oils in leaves and fruits.
  • Citrus has increased in importance in the United States with the development of frozen concentrate which is much superior to canned citrus juice.
  • Per-capita consumption in the US is extremely high.
  • Citrus mitis (calamondin), a miniature orange, is widely grown as an ornamental house pot plant.

    		•  fig. 01
    fig. 02 fig. 03

    History

  • Citrus is first mentioned in Chinese literature in 2200 BCE.
  • First citrus in Europe seems to have been the citron, a fruit which has religious significance in Jewish festivals.
  • It was mentioned in 310 BCE by Theophrastus.
  • Lemons and limes and sour orange may have been mutations of the citron.
  • The Romans grew sour orange and lemons in 50-100 CE; the first mention of sweet orange in Europe was made in 1400.
  • Columbus brought citrus on his second voyage in 1493 and the first plantation started in Haiti.
  • In 1565 the first citrus was brought to the US in St. Augustine.

    • Taxonomy

  • Citrus classification based on morphology of mature fruit (e.g. juice sacs and vesicles) and is considered confused at present.
  • The citrus species originated from species with compound leaves, and although leaves are simple, all contain wings as appendages.
  • Leaves of pomelo, grapefruit, oranges, kumquats can develop considerable resistance to cold.
  • Many do better where cold temperature or at least frost is common.
  • They are truly subtropical plants, but there are differences.
  • Pomelos never found of good quality in subtropical climates.
  • Taiwan, for example, has excellent pomelos in the south but quality lowers in the north.
  • Diploid chromosome number is 2n=18; triploid and tetraploids are now found, and there are many sports (mutations) found in citrus.
  • Tyosabura Tanaka, a Japanese taxonomist and an authority on the Old World Citrus included 145 species and is thus considered a "splitter."
  • The classification of Walter T. Swingle, a USDA scientist who did much of his work in Florida, includes only 16 species and he is considered a "lumper."
  • Swingle classification is relatively simple and contains 16 species.
  • Many hybrids exist among the species which complicates the horticultural classification.

    		•  fig. 04

    Leaf Morphology

    Cultivated Familiar Citrus Species Include:

  • C. aurantifolia (lime, sour orange)
  • C. grandis (pomelo, shaddock; pamplemousse is the French term for pomelo)
  • C. limon (lemon)
  • C. medica (citron)
  • C. paradisi (grapefruit)
  • C. reticulata (mandarin)
  • C. sinensis (sweet orange)
  • Fortunella margarita, F. japonica (cold hardy kumquat, fruits that are sweet on the outside, tart on the inside).

    		•  fig. 05

    Cultivars

  • Sweet orange (Citrus sinensis).
    There are four classes:
  • Common round orange such as a 'Valencia'
  • Navels ('Washington Navel, syn ='Bahia', 'Jaffa')
  • Blood oranges (red pigmentation in the flesh, found in Mediterranean areas, and require cool weather
  • Acidless
    •  In Florida citrus, sweet orange production has three seasons:
  • Early (before December) - 'Navel', 'Hamlin, 'Parson Brown'
  • Midseason (December to February) - 'Queen', 'Pineapple'
  • Late (February to Summer) - 'Valencia', one of the best quality oranges for juice and fresh fruit.
    •  Note: Citrus can be stored on the tree from February to Summer.
    It does not ripen like an apple, but when fruit gets too old it loses juice and becomes dry.

    Orange: 2001 World Production
    Continent 1000 tonnes Chief countries
    World 61,094  
    Africa   4,812 Egypt (1,714), S. Africa (1,041), Morocco (708)
    North America 16,736 USA (11,240), Mexico (3,886), Costa Rica (405)
    South America 19,884 Brazil (16,924), Argentina (861), Venezeula (500)
    Asia 13,329 India (3,200), China (2,860), Iran (1,800)
    Europe   5,855 Spain (2,734), Italy (1,935), Greece (950)
    Oceania       477 Australia (470), New Zealand (4), Tonga (1)

    Sweet orange, Citrus sinensis

    fig. 06

    (Round orange) 'Valencia'

    		 fig. 07

    Navel orange 'Washington Navel'

    fig. 08

    Valencia chimera green and yellow

    		 fig. 09

    Valencia orange, Israel

    Round oranges (for juice)

    fig. 10

    Fortune

    		 fig. 11

    Fremont

  • Tangerines Citrus reticulata
    Highly prized for easy peeling (zipper skin).
    Some consider four horticultural groups.
  • Satsuma - Common in Japan, are seedless and sections are exported.
    These are often called mandarins.
  • Hybrids such as 'King' and 'Temple' orange Large and good quality that are easy peel.
    Many believe that these type of fruit are the future of the fresh fruit citrus industry.
  • Mediterranean.
  • Common mandarins (called tangerines).
    •  Tangerines are small erect trees that fruit at the end of branches.
    The branches tend to be erect.
    The skin is loose and is hard to pick without damage but is the highest priced fruit.
    In Florida 10% of tangerines can be legally added to frozen orange concentrated for color.
    None are cold hardy.

    Tangerine: 2001 World Production
    Continent 1000 tonnes Chief countries
    World 16,613  
    Africa       998 Egypt (483), Morocco (263), Algeria (110)
    North America       756 USA (472), Mexico (250), Jamaica (16)
    South America   1,825 Brazil (905), Argentina (474), Peru (128)
    Asia 10,471 China (5,640), Japan (1,532), Iran (650)
    Europe   2,470 Spain (1,653), Italy (636), Greece (90)
    Oceania         93 Australia (87), New Zealand (6)

    fig. 12 fig. 13

    Tangerines along roadside in Japan

    Mandarin, Citrus reticulata

    fig. 14

    Satsuma

    		 fig. 15

    Fairchild

    fig. 16

    Clementine

    		 fig. 17

    Wilking

    fig. 18

    Pixie

    		 fig. 19

    Kinnow

    fig. 20

    Frua

    		 fig. 21

    Kara

  • Grapefruit (Citrus paradisi)
    Probably arose as a hybrid between sweet orange and pomelo (Shaddock) in Jamaica and long known as the forbidden fruit!
    A Captain Shaddock introduced pomelo to Barbados in 1696.
    There are four horticultural types, all due to mutations from a single clone:
  • white flesh and seedy, the original grapefruit ('Duncan')
  • white flesh and seedless ('Marsh')
  • pink and seedless ('Thompson')
  • pink-red and seedless ('Redblush').
    •  The reddish seedless grapefruit now completely dominates the market.
    More high quality grapefruit is grown in Texas than Florida because of the higher temperatures.
    In Florida, early grapefruit were induced to bear early by spraying with arsenic to induce peel injury and thus ethylene production, one of the best kept secrets of the grapefruit industry.

    		•  fig. 22

    Grapefruit: 2001 World Production

    Continent 1000 tonnes Chief countries
    World   4,831  
    Africa       445 S. Africa (207), Sudan (67), Tunisia (50)
    North America   2,813 USA (2,240), Mexico (212), Cuba (170)
    South America       390 Argentina (191), Brazil (66), Paraguay (48)
    Asia   1,115 Israel (318), China (275), Turkey (140)
    Europe         54 Spain (29), Greece (8), Portugal (8)
    Oceania         14 Australia (12), New Zealand (2)

    Grapefruit (Citrus paradisi)

    fig. 23

    Marsh

    		 fig. 24

    Redblush

    Lemon (Citrus limon)

    fig. 25

    Lime (Citrus aurantifolia)

    fig. 26

    Lemon & Lime: 2001 World Production

    Continent 1000 tonnes Chief countries
    World 10,885  
    Africa       612 Egypt (301), S. Africa (106), Sudan (61)
    North America   2,704 Mexico (1,547), USA (907), Guademala (131)
    South America   2,269 Argentina (1,180), Brazil (578), Peru (224)
    Asia   3,605 India (1,400), Iran (850), Turkey (500)
    Europe   1,654 Spain (961), Italy (537), Greece (145)
    Oceania        40 Australia (34), New Zealand (4), Tonga (2)

    Lemon (Citrus limon)

    fig. 27

    Eureka

    Lime (Citrus aurantifolia)

    fig. 28

    Bearss Seedless

    Sweet Lime (Citrus limettiodes)

    fig. 29

    Sweet Lime

    Lime rootstock

    fig. 30

    Rangpur Lime

    Other Citrus: 2001 World Production

    Continent 1000 tonnes Chief countries
    World   5,309  
    Africa   3,741 Nigeria (3,240), Guinea (210), Sierra Leone (80)
    North America         25 Mexico (12), Guatemala (4), Trinidad (4)
    South America         17 Ecuador (13), Suriname (4)
    Asia   1,489 China (575), Syrian Arab Rep. (275), Japan (260)
    Europe         26 Italy (15), Spain (7), Greece (3)
    Oceania         11 New Zealand (8), Australia (1), Samoa (1)

    Citron (Citrus medica)

    fig. 31

    Etrog

    		 fig. 32

    Corsican

    Kumquat (Fortunella margarita)

    fig. 33

    Citrus Hybrids

    Prefixes and suffixes used in naming of citrus hybrids

  • C. paradisi (grapefruit) - suffix ummelo or ela
  • C. sinensis (sweet orange) - prefix or, suffix ange
  • C. reticulata (tangerine, mandarin) - prefix tan, suffix andarin
  • C. limon (lemon) - prefix lem
  • P. trifoliata (trifoliate orange) - prefix citr

    • fig. 34

    Hybrid: tangerine × orange = tangor

    fig. 35

    Murcott

    Hybrid: tangerine × pomelo = tangelo

    fig. 36

    Page

    Lime rootstock

    fig. 37

    Minneola

    Ecology

  • Citrus grows best in a band 30-40º latitude on either side of the equator.
  • It is a true subtropical crop.
  • Good growing temperatures are 75-80ºF, but 40ºF good for rest, development of acidity, and color.
  • Quality is best where there is a certain amount of low temperature.
  • Peel color is especially related to temperature.
  • Low temperature brings out orange color.
  • Some citrus such as 'Valencia' will even regreen if warm temperature interrupt the maturation period.
  • High temperatures lower acidity, and produce larger insipid fruit.
  • Pomelo is the exception to most citrus.
  • It developes good quality and color in high temperatures.
  • Also true for grapefuit, hybrid between pomelo and sweet orange.

    • Nucellar Embryony

  • Apomixis - development of an embryo without the fusion of male and female gametes.
  • Polyembryony - occurrence of more than one embryo in a seed.
  • Nucellar embryony - embryos form from nucellar tissue.

    • Nucellar embryony in citrus means that most embryos are vegetative not zygotic.
    They are produced from nucellar tissue and are therefore clones of the mother tree.
    Pollination is needed to trigger nucellar development.
    Nucellar embryos begin development as soon as pollination occurs while zygotic embryos take four weeks to develop; thus, nucellar embryos crowd out the zygotic embryos.
    There are various degrees of nucellar embryony.
    Some cultivars produce only nucellar embryos, and some vary in the percentage of nucellar embryos.

    Three citrus types produce monoembryonic and therefore zygotic seed
  • 'Temple tangelo'
  • 'Clementine' tangerine
  • Shaddock, Citrus grandis (pomelo)

    • There are some important implications of nucellar embryony.
  • Rootstocks such as 'Rough Lemon', 'Sour Orange', and P. trifoliata can be produced true-to-type from seed.
    This is important for nurseries because virus is not transmitted through either nucellar or zygotic seed.
    Thus, nurseries can produce virus-free, clonal rootstocks from seed.
  • Virus can be eliminated from infected clones.
    However, because nucellar seedlings are juvenile it takes a long time to obtain productive clones.
    These can then be propagated by budding from the indexed nucellar mother tree.
  • Breeding of citrus is made difficult because of the difficulty obtaining genetically variable populations.

    • Nucellar polyembryony in Citrus

    fig. 38

    Nucellar Embryony

    fig. 39

    Flowering & Fruiting

    Seedling trees are:
  • Tall and vigorous
  • Thorny
  • Slow into coming into bearing taking 10-15 years

    • These are juvenile characteristics and is the reason why all citrus produced commercially is propagated by graftage (usually budding) despite the fact that many citrus comes up true-to-seed due to nucellar embryony.
    Flower bloom is profuse, about 40,000 flowers are produced on a 10-year old tree but only 1-5% of the flowers set fruit.
    Flower parts are in sets of five (5 petals, 20 stemmas, 5 sepals, 10 sections in a fruit, usually).
    Most citrus species are self fertile and have perfect flowers but usually produce fruit from cross pollination.
    The fruit is a berry, specifically a hesperidium.

    Seedlessness & Parthenocarpy

    Seedlessness is as very desirable character in citrus.
    Parthenocarpy is the ability to produce fruits without sexual fertilization.

    Citrus cultivars may be classified as:
  • Weakly parthenocarpic - Only a few fruit is produced without pollination (Navel oranges).
  • Moderately parthenocarpic - Fair crop is produced without pollination, while pollinated sets a good crop ('Orlando' tangelo).
  • Strongly parthenocarpic - Sets good crop without pollen ('Tahiti' lime).

    • Note: in the US, oranges are considered seedless if they have 0 to 6 seed!!
    Seedless cultivars include 'Hamlin', 'Valencia' (best quality), and 'Navels'.
    Seedy oranges such as 'Parson Brown', 'Queen', and 'Pineapple' are used only for juice production.
    Navel oranges such as 'Washington Navel' cannot be used for concentrate because it is bitter when processed.

    Rootstocks

  • As all citrus are propagated by budding, rootstocks are an important part of citrus culture.
  • In citrus, unlike apple, many scion cultivars are also used as rootstocks.
  • Common rootstocks in Florida include 'Rough Lemon' (C. limon) which is adapted to light sandy soils and 'Sour Orange' (C. aurantium) adapted to heavier soils and is more cold hardy.
  • 'Cleopatra' mandarin is an important rootstock because of resistance to tristeza virus.
  • Grafted trees bear in 2-3 years.

    • Poncirus trifoliatas (trifoliate orange)

    This is a deciduous species used as rootstocks because:
  • Produces high quality fruit on scion cultivars
  • Resistant to Phytophthora (foot rot)
  • Grows well on heavy soils
  • Produces a small tree, i.e. some what dwarfing
  • Cold hardiness is transferred to the scion

    • Poncirus trifoliate has been hybridized with Citrus to impart cold hardiness but it contains an inedible glycoside which is transferred to all hybrids.
    Protoplast hybrids between citrus species are being made at the University of Florida and Citrus Experiment Station to produce new rootstocks.

    Diseases & Pests

  • Foot (trunk) rot is caused by fungal species of Phytophthora.
    Rot develops on the base of the trunk.
    Disease is influenced by rootstock, high humidity, high temperature, and trunk injury.
    Sweet orange is very susceptible and is therefore not used as rootstocks.
    'Cleopatra' mandarin and 'Sour Orange' are very resistant.
  • Virus diseases (Tristeza, Xyloporosis, Exocortis, Psorosis) are all graft transmitted.
    Tristeza is the only virus transmitted by insect vectors (aphids and leafhoppers).
    Symptoms of virus diseases show up after 10 to 15 years but disease can be ruinous.
    A budwood registration program is underway in most important citrus producing areas to distribute virus-free certified budwood.
    These are usually produced originally from nucellar embryony or from heat treatment.
    Virus is identified by ELISA (enzyme-linked immuno sorbant assay).
  • Nematodes - Several species cause problem.
    Some rootstocks are partially resistant.
  • Arthropods - There are many citrus pests including insects (scale, aphids, others) and mites.

    • Citrus Production.(California)

    fig. 40

    A map of the state of California showing the major counties producing citrus.
    The counties in white all produce some citrus in addition to other crops.
    Primarily they are in the southern half of the state.

    		fig. 41

    A young orange tree recently planted showing the basin for watering and the wrapping for protection of the trunk.

    fig. 42

    Tracks between rows are left by a tank truck that fills the basin periodically.
    Later an irrigation system will be installed.

    		fig. 43

    Young orange planting 6 months after planting.
    Note the new growth.

    fig. 44

    Close-up view

    		 fig. 45

    A young orange planting about two years old.

    fig. 46

    A young orange planting with a poplar tree wind break.

    		 fig. 47

    Citrus growing areas.

    fig. 48

    A hillside planting of oranges with avocados at the top of the hill.

    		 fig. 49

    A general view showing orchard heaters and irrigation standpipes.

    fig. 50

    Valencia oranges showing blossoms, green fruit, and fruit ready for picking.

    		 fig. 51

    Non-cultivated orchard where the spacing of the ridges between the furrows is such that it matches the wheels of equipment.

    fig. 52

    An orange grove showing hedging and topping.

    		 fig. 53

    Furrow irrigation as fed by a concrete standpipe.

    fig. 54

    Orange orchard where the ground is cultivated and the irrigation ditches must be replaced after each cultivation.

    		 fig. 55

    A form of irrigation known as the "Brown arm" system.

    fig. 56

    The brown arm pipes supplying water to the furrows.

    		 fig. 57

    A close view of the low volume sprinkler unit.

    fig. 58

    Drag-line sprinkler system with low volume sprinklers that are usually left for about 12 hours before they are moved.

    		 fig. 59

    A type of over head irrigation known as "Rainbird."
    One sprinkler to 4 trees is the common spacing.

    fig. 60

    Valencia orange grove where the skirts of the tree on the right have been pruned about 2 feet up above the ground in a comparison test of fruit yield and quality where a catching frame method of picking will be used on this tree.

    		 fig. 61

    A dying tree showing the result of the Tristeza virus.

    fig. 62

    A citrus tree with Bordeaux powder sprinkled around the base to protect against brown rot gummosis.

    		 fig. 63

    Orange leaves which have been badly infested with aphid.

    fig. 64

    An enlarged view of an aphid on an orange leaf.

    		 fig. 65

    Orange leaves showing zinc deficiency.

    fig. 66

    A citrus tree which is dead, possibly the result of infection by quick decline, oak root fungus, or gummosis.

    		 fig. 67

    Orange Dog or Shallow-tail Butterfly.

    fig. 68

    Black scale.

    		 fig. 69

    Citrus Cottony cushion with a red spider on its back.

    fig. 70

    Mealy bug on a background ö 1/50 of an inch grid.

    		 fig. 71

    Mealy bug on a midrib of a citrus leaf.

    fig. 72

    Cottony cushion scale on an orange leaf with an egg mass.

    		 fig. 73

    Magnified fungus on a leaf growing on the honeydew residue left by aphis.

    fig. 74

    Young orange fruit showing thrip damage.

    		 fig. 75

    Scaleybark (psorosis) on an orange tree.

    fig. 76

    Seedling troyer orange inarch grated to seedling sweet oranges believed to be over 135 years of age.

    		 fig. 77

    Two 135 year old trees that have declined to a point where it is too late to inarch.

    fig. 78

    Closer view of a troyer seedling inarched on the 135 year old tree.
    Note the asphalt sealer at the union and the appearance of the scalybark.

    		 fig. 79

    Morning glory climbing in a citrus tree.

    fig. 80

    Nutsedge or nutgrass of long standing in a Valencia grove.

    		 fig. 81

    Nutsedge, which is one of the most persistent weeds in some citrus groves.

    fig. 82

    Picking of oranges and citrus in general is for the most part done by hand from ladders.
    Efforts at mechanization are being engineered, but none are in general use.
    Most equipment is experimental

    		 fig. 83

    The new experimental picking bag which is supported on a pack frame.

    fig. 84

    An orange-picker using a new style pack frame picking-bag where the picker is high on a 15 foot ladder.

    		 fig. 85

    Picking bag being dumped.

    fig. 86

    A close set grove with a large truck passing between rows of trees. These large trees have been hedged to allow for passage of trucks and here show the need for another hedging. Usually these drive rows are spaced every 4th or 6th row. The pickers carry the fruit in bags to the drive roads where the fruit is placed in boxes.

    		 fig. 87

    Loading of field boxes on to the truck for hauling to the packing plant.

    fig. 88

    Experimental citrus picker called an autopicker.
    Many attempts are being made to make the harvesting of fruit easier and less costly.

    		 fig. 89
    fig. 90

    A contrast between the old trucks and field boxes as compared to the new tote bins now used at this citrus packing plant.

    		 fig. 91

    A field truck depositing metal bins in the orchard for the pickers.

    fig. 92

    Piru Citrus Association which is considered on of the most modern packing houses in the Sunkist Association.

    		 fig. 93

    A truck unloading full bins.
    Rollers permit the driver to easily push the loaded bins to the platform.

    fig. 94

    The forklift is removing the loaded bins.

    		 fig. 95

    A forklift is placing bins on a track where they are automatically dumped at the beginning of the processing line.

    fig. 96

    The bins are dumped onto a conveyor belt where the leaves, dirt, and branches fall through prior to the fruit going to the first size grader.

    		 fig. 97

    Grading as to size prior to washing.

    fig. 98

    A general view of the equipment used in washing, drying and waxing.

    		 fig. 99

    Fruit on the conveyer as it comes out of the washer and waxer on its way to be stamped and graded to size.

    fig. 100

    A general view in the packing house showing the conveyer belts and inspection stations where poor fruit is sorted out of the lot before processing.

    		 fig. 101

    The printing machine which stamps "Sunkist" on each fruit as it passes under the rollers.

    fig. 102

    Fruit on the conveyer after it has been through the printer.

    		 fig. 103

    Sizing equipment with its series of rollers spaced at increasing intervals so that the fruit falls through in progressively increasing sizes.

    fig. 104

    Sorting area where the fruit that does not meet marketable standards is rejected.

    		 fig. 105

    All rejected fruit is sent to another plant for processing into citrus by-products.

    fig. 106

    Rejected fruit being hauled to another plant where juice, concentrate, pectin, and oil will be manufactured.

    		 fig. 107

    Quantities of flat cardboard as they are received from the box manufacturer.
    Here they are placed into equipment that forms them into cartons ready to receive the fruit.

    fig. 108

    Looking down a line of workers hand packing fruit in cartons.
    Pay for this work is on a per box basis.

    		 fig. 109

    Looking down on a new automatic place-pack Sunkist packing machine.
    A series of suction cups pick up a complete layer of fruit at one time and places that layer in a carton.

    fig. 110

    Another view of the automatic packing machine showing the suction cups on the left just after they have picked up a layer of fruit.

    		 fig. 111

    A close view of the automatic packing machine with the suction cups holding a layer of oranges over the metal collar that holds the carton being packed with fruit.

    fig. 112

    A packed carton on the conveyer on its way to be closed and sealed.

    		 fig. 113

    A machine that automatically stacks a predetermined number of cartons of fruit on a pallet.

    fig. 114

    Cartons of oranges being held in a refrigeration room awaiting shipment to eastern markets.

    		 fig. 115

    Loading the packed fruit into refrigerated railroad cars.

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