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{{DISPLAYTITLE:''Citrus L.'' (PROSEA)}}<big>''[[Citrus L.]]'' L.</big>
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== Vernacular names ==
*General: *citrus (En). *Agrumes (Fr)
*Indonesia: jeruk
*Malaysia: limau
== 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 ==
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.
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.
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.
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!
== 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.
* 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 ==
