Citrus (PROSEA)

Plant Resources of South-East Asia Introduction

List of species

Citrus L.

Protologue: Sp. Pl.: 401 (1753).

Family: Rutaceae

Chromosome number: x= 9; usually 2n= 18; there are some triploids and tetraploids; aneuploidy has also been reported.

Major species

• Citrus aurantifolia (Christm. & Panzer) Swingle - lime - see separate article.

• Citrus maxima (Burm.) Merr. - pummelo - see separate article.

• Citrus medica L. - citron - see separate article.

• Citrus ×paradisi Macf. - grapefruit - see separate article.

• Citrus reticulata Blanco - mandarin - see separate article.

• Citrus sinensis (L.) Osbeck - sweet orange - see separate article.

Information on C. aurantium L. (sour orange), C. hystrix DC. (Mauritius papeda), C. limon (L.) Burm.f. (lemon) and C. macroptera Montr. (Melanesian papeda) is summarized in the chapter on minor edible fruits and nuts.

Vernacular names

• General:
• citrus (En)
• Agrumes (Fr)
• Indonesia: jeruk
• Malaysia: limau
• Papua New Guinea: muli (Pidgin)
• Burma: shouk
• Cambodia: krauch
• Laos: mak
• Vietnam: cam.

Origin and geographic distribution

Citrus occurs naturally from India and southern China to northern Australia and New Caledonia. There trees can still be found in the wild state, e.g. a few wild species such as Citrus indica Tanaka and Citrus halimii B.C. Stone, populations of C. maxima in remote parts of Borneo, the "Papeda" group - especially C. macroptera -locally in Malaysia and Indonesia. Most Citrus species are only known as cultivated and naturalized plants. The earliest records on cultivation come from China: 2200 BC! Cultivation in South-East Asia should be at least as old. Following the conquest of Alexander the Great, Citrus species gradually found their way to the Mediterranean, from where they reached the New World. These introductions were by no means complete: many leading Asian cultivars were not imported in the western hemisphere until the 19th and 20th Centuries. Citrus is now grown throughout the tropics and subtropics, roughly between latitudes 44°N and 35°S.

Uses

Citrus has always been mainly used as a fresh fruit, but now the production of juice is predominant. Several other edible products are made from the fruit, including the rind, e.g. marmalades, candies. Pectin is made from the peel, and citric acid from lemons and limes. The pulp is used as cattle feed.

The fragrant flowers, fruits and leaves are used to extract essential oils. The rind of the fruit is the most convenient source, but sour orange flowers yield the most expensive essence. Both leaves and fruit - or peel - may serve as spices or condiments. Medicinal uses of citrus are few, but flavonoids from the inner cortex have medicinal value and may have anti-tumour properties.

Production and international trade

Taken as a group citrus vies with grapes and bananas for first place in tonnage produced worldwide. With more than 40 million t in 1987, sweet oranges top the citrus production list, followed a long way behind by mandarins (8 million t), lemons plus limes (nearly 6 million t) and grapefruits plus pummelos (4.5 million t). Statistical data are more fragmentary for the smaller crops; for the category "citrus fruits not specified elsewhere", a production of 1.2 million t is given for 1987.

In South-East Asia sweet orange, grapefruit and lemon are of little importance. Mandarins take first place and the minor citrus fruits play a relatively large role in the region. Citrus growing in South-East Asia is not confined to home gardens; production in (small) orchards predominates. Almost the entire region is rather poorly supplied with citrus fruit.

International trade in citrus fruits and products is big business. South-East Asia, however, exports only a tiny fraction of its production and almost exclusively to other Asian countries (e.g. Thai pummelo to Hong Kong).

Properties

Mature citrus fruits contain 77-92% water; when there is drought, water is withdrawn from the fruit to the leaves. Starch, which is present in young fruits, practically disappears in maturing fruits. The sugar content in the edible part may vary from 2-15%; it is usually around 12% in mature sweet oranges. There is less than 2% protein in the edible portion. In addition to 1-2% citric acid, sweet orange fruit may contain small amounts of tartaric, malic and oxalic acids. Lemon and lime fruits contain as much as 6-7% citric acid.

Consumption of citrus fruits and juice is much enhanced by their reputation as good sources of vitamin C, about 50 mg per 100 ml juice. Vitamin P (also called citrin) and vitamin A are also present. The principal glucoside in most citrus fruits is hesperidin, but in grapefruit and pummelo it is naringin. Citrus peel contains much pectin.

Description

• Small trees armed with axillary single spines; youngest branches angular, compressed, older branches often spineless. All parts aromatic when bruised, glandular.
• Leaves alternate, 1-foliolate or simple, usually thin, chartaceous to coriaceous; petioles often winged or marginate and articulated with the blade; blades entire to crenulate, with few lateral veins.
• Inflorescences racemose-corymbose, axillary, or flowers single; flowers bisexual or functionally male through abortion of the pistil; calyx cup-shaped, 4-5-lobed; petals (4-)5(-8), thick, linear; stamens 4(-10) times as many as petals; disk annular, short; ovary subglobose, 8-18-celled; style cylindrical, stigma globose-capitate.
• Fruit a berry (hesperidium); peel (rind) leathery, outer layer (exocarp) densely glandular and usually yellow or orange, inner layer (mesocarp) dry and white; endocarp consisting of several segments filled with pulp-vesicles; endocarp spongy; pulp-vesicles firm to soft, filled with watery, sweet, sour, or bitter juice, with or without distinct oil droplets.
• Seeds angular-obovoid, pale, containing one to many white or green embryos.

Growth and development

Polyembryony is common in citrus and seedling populations are often largely of nucellar origin and hence true to type, the zygotic embryo being suppressed. C. medica , C. maxima and some cultivars of other species are mono-embryonic. Seedlings have clear juvenile characteristics, such as a spiny habit. Shoot growth is indeterminate, but ends suddenly by the abortion of the shoot tip. The roots bear few root hairs, nutrient absorption - especially P - being largely taken over by mycorrhizas.

In the humid tropics there is no synchronous flushing; at any time of the year extension growth may be resumed anywhere in the tree. If floral initiation was completed, resumption of growth will be associated with flowering. Thus, unless synchronous flushing is imposed by a distinct dry season, flowering may occur off and on over a long period of time, so that fruit is often harvested over a period of 8 months or longer.

The viability of pollen may be low or nil (as in "Washington Navel" sweet oranges) and female sterility occurs in some grapefruits and lemons, but the ability to produce parthenocarpic fruit usually leads to satisfactory crops, even in the absence of pollinator trees. In South-East Asia neither incompatibility nor parthenocarpy seem to play an important role; orchards planted with a single cultivar still produce good yields of seeded fruit.

Fruit set is generally high, but many fruitlets drop during a few waves of abscission which regulate the crop load. In mandarins, particularly those with numerous seeds per fruit, fruit drop is suppressed, resulting in a heavy crop of small fruit and biennial bearing. The fruit is non-climacteric and generally ripens 7-8 months after flowering.

Other botanical information

Citrus taxonomy is complicated because the species concept has run to extremes: Engler distinguished 9 species, Swingle 16 and Tanaka 166; about 800 binomials exist! Innumerable cultivars are known; many are interspecific or compound hybrids. Since polyembryony is common and autotetraploid forms emerge spontaneously, natural and artificial hybrids behave like normal species.

Two major systems of Citrus taxonomy have been developed: by Swingle and by Tanaka. Swingle's system, widely followed in botany, is used here.

Ecology

Citrus is grown in 3 areas with distinct climates: 1) subtropical, mostly from 30-40°latitude: a Mediterranean climate with winter rain and hot, dry summers; 2) semi-tropical, from 20-28°latitude: dry winters and hot, wet summers (Florida, Brazil); and 3) tropical, within 20°from the equator: no prominent cold period and rainfall well distributed over the year or concentrated in one or two wet seasons.

The growth rhythm of the trees differs in the 3 zones and the scope for manipulation to produce out-of-season crops increases from zones 1 to 3. The period from flowering to maturity is 1-2 months shorter in the tropics. The difference in the period from flowering to harvest may even be several months more, since in the tropics fruit quality declines if fruit is left on the tree after reaching maturity; in the other zones the winter season permits the delay of harvesting.

In regard to fruit quality there are also great differences between the zones. In the tropics fruits tend to be large, have a high juice content but a low level of citric acid; acid content of the sour citrus fruits (lemon, lime) however, is as high as in the other zones. The fruit of cultivars selected for zones 1 and 2 is often rather insipid when grown in the tropics and there is a striking difference in colour: in the absence of cold nights the fruits remain green or turn yellow in the tropics, instead of becoming bright orange.

Citrus species escaped from cultivation (e.g. in Florida) show a clear preference for moist but well-drained sites with fertile soil, near rivers or lakes. On these sites the trees form a dense undergrowth in the shade of hardwood trees, in keeping with their natural occurrence on the best sites in tropical rain forests.

Shade reduces yield and very humid conditions greatly complicate disease control. No shade is used in cultivation and drier conditions are preferred. Coupled with higher yields and concentrated crop loads in seasonal climates, this adds up to a more stressful situation. That is why good soil, irrigation and adequate nutrition assume great importance in citrus orchards.

At higher latitudes and altitudes temperature becomes limiting. Although citrus grown in zone 1 withstands slight frosts, extension growth stops below 13°C; optimum daytime temperatures are in the range of 25-30°C. Heat requirement differs for species and cultivars; it can be usefully expressed as a "heat index" in degree-hours, calculated by totalling the temperatures above 13°C but below 38°C for each hour during the growing season. Such calculations indicated, for instance, that in Colombia at 3°N "Washington Navel" orange could be grown up to 2100 m elevation, "Valencia" orange to 1600 m and grapefruit to 1200 m.

Crop water use is 120-160 mm during a hot month. Even on the best of soils irrigation is needed if the dry season lasts more than 3 months. During a period of drought, extension growth ceases and floral development can take place, also on the shoots that stopped growing most recently. When this is completed, rain or irrigation provokes flowering of the trees within four weeks. Water requirements remain high until the fruit approaches full size. Thereafter the soil may be allowed to dry, since this gradually enforces quiescence on the tree, a condition for synchronous flowering for the next crop.

Excessive rainfall during flowering or fruiting is fatal for the crop. With increasing rainfall, drainage becomes more important and heavy soils more unsuitable. It is sometimes said that citrus can grow on almost any soil. However, for the trees to grow and yield well, the physical condition of the soil should be good; control of water and nutrient supplies by the grower can do the rest. Thus, one cannot have high expectations of attempts in Java to grow the trees on mounds in paddy fields, nor of citrus growing in Thailand on heavy clays - even acid sulphate clays - in the central delta.

The optimum pH is 5-6; at higher pH a range of deficiencies (Zn, Cu, Mn, Fe) comes to the fore. High levels of boron or soluble salts are toxic. Strong winds damage the fruit and may even uproot trees, but since wind velocity in South-East Asia is usually low, wind screens are generally not recommended.

Propagation and planting

Citrus spp. can be propagated from seed, on their own roots (air layers, cuttings) or on a rootstock (by budding or grafting). The seed of most cultivars produces nucellar seedlings which are true to type. Many trees in home gardens in South-East Asia are indeed seedlings. Nucellar seedlings also serve as uniform rootstocks for nurserymen, the common commercial propagation method being budding. Air layering (mandarin and pummelo in wet areas) comes second.

The drawbacks of seedlings are the difficulty of guaranteeing the nucellar origin, and juvenility which delays the onset of flowering. Air layering requires more labour than budding, but is the fastest way to get a bearing tree. Micro-grafting is being promoted to obtain disease-free plants and speed up their multiplication.

Every citrus cultivar can be budded on virtually any member of Citrus and related genera. However, only a few rootstocks are commonly used, in South-East Asia mainly rough lemon (Citrus jambhiri Lush.) and Rangpur lime (a misnomer, as it is presumably a hybrid of lemon and mandarin). The first requirements for a rootstock cultivar are seediness and a high level of polyembryony to ensure uniform seedling populations. Add rapid, upright growth of the seedlings (e.g. rough lemon) and the rootstock will be popular with nurserymen. The grower is more interested in stock-scion interactions, which are prominent in citrus, e.g.: the stock adapts the scion to heavy soils; the stock is resistant to (soil-borne) diseases and/or imparts disease tolerance to the scion; the stock restricts tree vigour and size; all stocks affect yield and/or fruit quality. Current choice of rootstocks in South-East Asia is largely based on old findings, accumulated before anything was known about diseases transmitted through propagation. Now that healthy budwood is becoming available, the suitability of various rootstocks has to be assessed anew.

To produce budded plants, freshly extracted rootstock seeds are sown, preferably after disinfecting them in well-agitated water at a temperature of 51°C for 10 minutes. The nursery must be located on soil that has not previously been used for citrus. The seeds are set out in beds at 30 cm × 5 cm. When the plants are some 15 cm tall they are transferred to nursery rows at a spacing of 1 m × 30 cm, all the weaker and deviating plants being discarded. Weeds should be controlled by hand or by a trifluralin weedkiller. Healthy, uniform plants are budded when the stems measure 8 mm or more in diameter. Usually the bud is inserted in an inverted T-cut, made at a height of 20-30 cm.

Budwood should be round in cross-section and mature. Petioled buds are cut, including a sliver of wood. After 2 weeks bud take is assessed and the rootstocks are lopped by cutting halfway through the stem just above the inserted bud and pushing the top over. This breaks apical dominance, but still allows assimilates to be supplied to the roots. Lopping forces the inserted buds to sprout. As soon as the sprouts are capable of absorbing the sap stream, the broken rootstock tops are cut off. Other buds which leaf out on the rootstock are rubbed off. At the end of the first flush the scions are tipped to induce the emergence of side branches, the tree frame.

Rapid growth in the nursery is important to limit the propagation phase to two years or less and to maintain uniformity of the plants, which greatly facilitates culling. With proper selection and 90% bud take, half the initial number of rootstocks should produce good plants. The timing of propagation is determined by the availability of fresh seed and the preference for field planting early in the rainy season.

For delivery the plants are carefully lifted with a digging fork and bagged if necessary. Bare-rooted plants must be covered by wet bags and planted the same or the next day. Bare-rooted plants can be disinfected by immersion in water at a temperature of 43°C for 10 minutes to control foot rot.

Husbandry

Usually little attention is given to citrus grown in home gardens, whereas citrus orchards are managed rather intensively. Home gardeners are content with an asynchronous growth rhythm, as it ensures the production of some fruit for home use during a large part of the year. However, yields are not high, presumably because twigs often resume extension growth before floral development has been completed.

Orchards, on the other hand, are predominantly sited in drier areas, where a period of drought is followed by simultaneous flowering. Irrigation after a dry period of 6 weeks or more brings about a flush with bloom. This starts a cropping cycle which is reinforced by the fruit, because the fruit load checks erratic flushing. A synchronous growth rhythm greatly facilitates orchard management as each activity - pruning, manuring, crop protection, fruit thinning, harvesting - can be properly timed and given undivided attention.

Unfortunately, rapidly mounting disease problems have led to the decline of traditional centres of citrus production in South-East Asia and the crops have shifted to other - generally less favourable - areas. Starting with diseased plants on soils with often defective physical conditions, the growers cannot withhold water long enough to impose a clear-cut crop cycle.

The usefulness of pruning citrus is doubtful, partly because the response to pruning is hard to predict. Although the trees can bear prolifically, no system of replacement pruning has evolved in which ageing bearing wood is cut out to make room for new twigs. However, pruning is generally recommended to shape the framework of the young tree and to eliminate crossing and inward-growing branches.

Nutrition is very important to maintain tree vigour and prevent deficiencies as the trees come into bearing. Micro-nutrients (especially Zn, but also Fe, Mn, Cu; sometimes B and Mo) strongly affect growth and yield. Recommended foliar levels for the major nutrients in California are: 2.50 N, 0.15 P, 1 K, 4 Ca, 0.50 Mg and 0.25 S (pure elements, not their oxides, in per cent dry matter). Nutrients should be balanced, in particular K against Ca and Mg. Nitrogen is needed at the rate of 3.5 kg N per tonne fruit; high N levels are detrimental for fruit growth and quality.

Irrigation frequency depends very much on soil type and depth: on deep volcanic soils in Java the interval may be extended to 3 weeks; trees on cambered beds on heavy clay soils in the Central Province of Thailand need water every few days in spite of the regulated water table! Heavy mulching, particularly on such defective soils, helps to maintain soil moisture in the topsoil which is more accessible to the roots, and to release nutrients gradually.

Diseases and pests

Citrus growing in South-East Asia - in particular the major commercial species - is in dire straits, owing to several debilitating diseases. The indiscriminate use of infected budwood and marcots has contributed to the rapid spread of these diseases.

About 60 citrus virus diseases have been described, most of them in recent years. Identification is difficult and only for a few is something known about the occurrence in South-East Asia. It is not easy to assess the damage because: both mild and virulent strains may occur; the susceptibility of species and cultivars differs and so do the symptoms; symptoms may only become manifest after years, or not at all (symptomless carriers); the cumulative effects of 2 or more viruses are rather unpredictable.

Tristeza is the main citrus virus in South-East Asia. Around 1920 it killed trees on the very susceptible sour orange rootstock, so that other rootstocks had to be used. Even trees on these stocks suffer from stem-pitting, leading to unthrifty growth and premature yellowing of fruit in sweet orange and dieback in lime. Tristeza is transmitted by several aphids; the black citrus aphid (Toxoptera citricidus) is the main vector, also in South-East Asia. Inoculation of clean nursery stock with a mild tristeza strain gives protection against the more virulent strains; this may be an acceptable compromise where the virus cannot be eradicated.

The name psorosis refers to a group of viruses which cause vein flecking of young leaves. Psorosis A is probably widespread in Indonesia, but symptoms commonly develop on older trees; tree decline before that age due to other causes may obscure the incidence of psorosis A. Bark lesions, leading to bark scaling and secretion of gum are other symptoms. Although no vectors have been identified, there appears to be some natural transmission. Some forms of psorosis are transmitted through seed of citranges and Poncirus trifoliata (L.) Raf.

Xyloporosis has not been identified in Indonesia, but it is a virus that could be serious in mandarins, including Rangpur lime. The symptoms, wood-pitting and gum exudate, are not specific and even inoculation on indicator plants takes years to produce symptoms.

Exocortis is a viroid, a naked RNA particle, without the DNA mantle of a virus. It causes characteristic symptoms - bark scaling and stunted growth - on Rangpur lime and on Poncirus and its hybrids, the citranges. Symptoms develop after 1.5 or, more likely, 4 years. The disease is transmitted by budding and pruning tools and can remain infective on a knife for many months! Where susceptible plants are used as rootstocks, budding knives and shears have to be dipped in a 10% solution of household bleach to prevent the virus being transmitted to all new trees. Trees on exocortis-infected stocks remain dwarfed; for this reason stocks infected with a mild strain are used in high-density orchards in New South Wales, Australia.

Greening and canker are bacterial diseases which - like virus diseases -are transmitted by budding and cannot be effectively controlled by chemical treatments.

Greening is the most debilitating of all; moreover, it has an effective vector in South-East Asia, the citrus psyllid (Diaphorina citri). The bacteria cause necrosis of the phloem followed by tree decline and death. No resistant cultivars or rootstocks are known. Infection at budding gives rise to a sparse, upright tree with narrow yellow leaves, which will never bear good fruit and should be pulled out and burned. Infections on established trees cause similar, but less obvious symptoms, starting from the infected branch. The fruit is often lopsided and shows green blotches when ripe (hence the name "greening"). Antibiotics suppress the symptoms, but do not cure the plant. The psyllid spreads the disease within a tree and from tree to tree; transmission over long distances takes place through diseased planting material or budwood.

Bacterial canker is caused by Xanthomonas campestris p.v. citri. The disease is extremely infectious, spreading by contact. It appears as small, brown, crater-like eruptions on leaves, stems and fruits. It occurs mainly in wetter areas on pummelo, grapefruit and lime. Lemon and mandarin are fairly resistant, orange less susceptible. In severe attacks the leaves drop and the fruit becomes unmarketable. The disease has been successfully eradicated in several countries.

Control of the above-mentioned diseases should be based on the production of clean planting material and prevention of reinfection. Programmes to produce certified clean planting material have been started in the region. The cleaning operation consists of shoot-tip grafting onto rootstocks raised in vitro. The plants are multiplied, mainly on "Troyer" rootstocks and at the same time indexed to make sure they are free of the diseases.

Multiplication proceeds via foundation blocks - situated and managed so that reinfection can be prevented - to multiplication blocks which supply the nurseries with certified budwood. Trees in multiplication blocks are destroyed before appreciable reinfection can take place, probably after 3 years. Since little is known about tree vigour, productivity, compatibility, etc. of the clean material, evaluation blocks need to be established for cultivar and rootstock trials.

However ambitious the programmes may be, there appears to be no alternative if the citrus industry in South-East Asia is to be revived. It should not be too difficult to supply the growers with certified trees, a stage that is now being reached. Reinfection in the orchards cannot be prevented, but hopefully it can be sufficiently slowed down by: 1) absence of sources of infection at the start; 2) early removal of developing infections; 3) integrated control of insect vectors; 4) possibly also by pre-immunizing trees with a mild strain of the virus concerned. It is clear that 2) and 3) presuppose specific skills and great alertness among growers, pointing to the need for intensive education.

Of the fungus diseases, Phytophthora species cause the greatest problems, which are known as foot rot and root rot. All are soilborne fungi, attacking a wide range of plants. The main species involved in the tropics is P. palmivora. In citrus the main damage is done by infections of the trunk and crown roots (by spores in water splashing up). Symptoms on the trunk include gummosis; from there the fungus can also reach other aerial parts in wet conditions. Infection of the fruit causes brown rot; in the nursery seedling blight may occur which spreads downwards from the shoot tip.

Root rot is a major killer of mandarin trees in Thailand's central delta, perhaps partly because of the shift of citrus cultivation to less suitable areas (heavy, wet soils) and the infestation with viruses and greening which weakens the trees. "Troyer" citrange is rated as one of the more tolerant stocks. Copper compounds are painted on thoroughly cleaned wounds to protect against infection. However, proper soil management remains the basis for prevention of damage by Phytophthora : excellent soil drainage (the fungus is only distributed in free water) and heavy mulching, not only to strengthen the biological equilibrium of the soil and to promote root growth, but also to prevent splashing up of spores.

Other fungus diseases, characterized by spots on leaves, branches and fruit include: Elsinoe fawcetti (scab), Mycosphaerella citri (greasy spot) and Diaporthe citri (melanose). This last fungus may also cause peduncle rot known as Phomopsis rot; the symptoms are very similar to stem-end rot caused by Diplodia natalensis. Phomopsis rot and stem-end rot are typical post-harvest rots. The fungus lingers under the calyx; picking the fruit with a twisting movement to detach the calyx reduces infection.

Orchard hygiene (removal of dead wood on which Diaporthe citri sporulates; removal of rotten fruit) reduces infections by the above fungi. Copper fungicides are commonly used in South-East Asia, although their efficacy is doubtful. Too often copper compounds are routinely added to insecticide treatments; over the years such quantities have been applied that toxic levels may be reached in the orchard soil!

The insects and mites on citrus are too numerous to list. Citrus species figure among the major hosts of many polyphagous pests. Scales do perhaps most damage, but other sucking insects are also important, especially as vectors of virus diseases and greening. Several are distributed and protected against predators by ants. Control of ants is therefore a useful preventive measure. Caterpillars (e.g. orange dog on the leaves; fruit- piercing moths) and the maggots of fruit flies should also be mentioned. The mere risk of maggots being present makes the export of fruit to certain countries impossible.

Whereas integrated pest control has been adopted with great success in several subtropical areas, commercial growers in South-East Asia still rely on frequent spraying. A small survey in Indonesia showed that "marginal" growers applied insecticides 0-6 times, "good" growers 17-44 times per year! The interval between treatments of mandarin in Thailand's central delta ranges from 3 days during a flush to a fortnight during quiescent periods, adding up to similarly high frequencies. Nevertheless, control is inadequate, partly because the natural balance between pests and predators is disturbed.

Crop management to ensure synchronous flushing, flowering and harvesting would force pathogens to synchronize their life cycle with the crop cycle. This would reduce the number of treatments and facilitate proper timing and choice of the most suitable product. Coupled with improvements in the spraying technique, this approach should open the way to successful integrated pest control for the benefit of producers, consumers and the environment. In the present situation certified planting material cannot be protected against rapid reinfection in the orchard; this stresses the urgency of a new approach in citrus crop protection.

Harvesting

Citrus fruit is harvested by clipping (with shears) or twisting off by hand. In some subtropical regions fruit for processing is harvested mechanically.

Fruit should be harvested dry to limit the spread of moulds. As maturity progresses the fruit changes colour and becomes sweeter but also less acidic and hence more insipid; the juice content rises to a maximum and falls again. Since shelf life is shortened as the fruit matures, sweet fruits require more speedy marketing. Juice content and the ratio of total soluble solids (measured in degrees Brix) to the percentage water-free citric acid in the juice can be used as indications of ripening. With a prolonged harvest season in the tropics it is not easy to pick only fruit which has attained the desired stage of maturity.

Yield

Citrus yields can be quite high; for several species in the order of 50 t/ha per year. To sustain such yields control over the growing conditions is essential. In South-East Asia yield is mainly limited by diseases and pests and the asynchronous growth rhythm. Commercial growers aim to get 25 t/ha per year within 4-6 years from planting and battle against progressing tree decline and death to keep up that yield level for a few years. Mean yields are much lower, in the order of 5 t/ha in Indonesia.

Handling after harvest

Wooden boxes or PVC (polyvinyl chloride) crates (Thailand) are used to pick fruit for distant markets. Pregrading in the orchard greatly improves the uniformity of the produce.

In large packing houses an extensive range of post-harvest treatments may be applied: degreening, washing, brushing with soap, disinfection, drying, "colour-adding", waxing, grading and sizing, and packing. Only grading and sizing - by hand - are common practice in South-East Asia. Most citrus fruits can be kept in cool storage for one month or longer.

Genetic resources and breeding

Under the Citrus rehabilitation programmes in South-East Asia germplasm collections are being extended and evaluated to enable proper choice of cultivars. The University of Malaya (Kuala Lumpur) maintains a collection with emphasis on plants gathered in the wild state. Very extensive germplasm collections are maintained in the major citrus-producing countries.

Nearly all important cultivars have resulted from selection rather than breeding work. Early breeding programmes in Indonesia (disease-resistant rootstocks) and the Philippines were interrupted. Insight gained in the heritability of characters such as nucellar embryony, dwarf tree size, tolerance to foot rot, cold-hardiness and fruit acidity puts breeding work on a stronger footing. Interspecific and even intergeneric hybridization widen the scope for recombination of genes. At the same time mutations and graft-transmissible diseases require constant vigilance to safeguard the identity and health of the established cultivars.

Prospects

The prospects for citrus in South-East Asia greatly depend on the success of programmes to provide certified planting material and to maintain tree health in the field. If successful, these programmes will have profound effects on commercial citrus growing and home gardeners will benefit as well.

Owing to the infestation with diseases, citrus production in the region is much too low to satisfy local demand. High yields and low production costs of orchards with healthy trees allow for a drop in prices; this creates room for a several-fold increase in production and enhances the chances for export.

Literature

• Aubert, B., Garnier, M., Guillaumin, D., Herbagyandono, B., Setiabudi, L. & Nurhadi, F., 1985. Greening, a serious threat for citrus production of the Indonesian Archipelago. Fruits 40: 549-563.
• CIBA-GEIGY, 1975. Citrus. Technical Monograph 4. Basle. 88 pp.
• Hume, H., 1957. Citrus fruits. Macmillan, New York. 444 pp.
• Kranz, J., Schmutterer, H. & Koch, W. (Editors), 1977. Diseases, pests and weeds in tropical crops. Paul Parey, Berlin. 666 pp.
• Monselise, S.P., 1986. Citrus. In : Monselise, S.P. (Editor): Handbook of fruit set and development. CRC Press, Florida. pp. 87-108.
• Reuther, W., Webber, H.J. & Batchelor, L.D. (Editors). The citrus industry. Revised Edition, Vol. 1, 1967, Vol. II, 1968, Vol. III, 1973, Vol. IV, 1978. University of California, Division of Agricultural Sciences, Berkeley.
• Samson, J.A., 1970. Rootstocks for tropical fruit trees. Tropical Abstracts 25: 145-151.
• Samson, J.A., 1977. Problems of citrus cultivation in the tropics. Span 20: 127-129.
• Samson, J.A., 1986. Tropical fruits. 2nd ed. Longman, London. pp. 73-138. | 10 | Samson, J.A. & Bink, J.P.M., 1976. Citrus budding in the tropics: towards an explanation of the favourable results of lopping. Proceedings 7th IOCV Conference. pp. 213-216.

Authors

E.W.M. Verheij & B.C. Stone