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Although tristeza is not spreading in the northern half of the country, it is of concern to the Sudan because practically all the trees in the country are grafted on two rootstocks sour orange and Balady lime - that will induce decline in the case of infection by CTV. Sour orange as a seedling tree is tolerant of CTV, but as a rootstock it yields a CTV-susceptible combination with scions of sweet orange, mandarin, grapefruit and others, except lemon. Balady lime, as a rootstock as well as a seedling tree, is highly susceptible to many strains of CTV.
Tristeza is a very serious disease because it is insect-transmitted. The vectors are several aphid species. Hence, for tristeza to be spread in nature, two conditions must be fulfilled: a vector must be present and a source of CTV inoculum has to be available for the vector to become infected.
Bové has examined many Balady lime trees in all the regions surveyed. In the case of CTV infection, Balady lime shows leaf vein clearing, a specific symptom, and stem pitting. None of these symptoms has been observed.
On the contrary, the lime trees were growing very well, except for some cases of phytophthora gummosis. Therefore, it seems quite clear that at the time of the survey (1986) natural spread of CTV was probably not occurring, or at least not on a large scale. There are probably several reasons for this fairly satisfactory situation. Toxoptera citricida, the tropical citrus aphid and the most efficient CTV vector, is probably not present in the Sudan. Childs (1964), Krezdorn (1983) and Bové in his 1986 survey saw no evidence of it. Aphis gossypii, the cotton aphid, is present in the Sudan, but it transmits only certain strains of CTV efficiently. Sources of CTV inoculum on which aphids become viruliferous are CTV-infected citrus trees. Bové did not come across trees that might have been infected by CTV. However, Childs (1964) mentions a few suspicious cases. In Shendi (Northern Province), eight-year-old navel sweet orange trees (budwood from Egypt) on Balady lime rootstock were severely stunted and had severe wood pitting on the lime rootstock. In Kassala, two grapefruit trees showed fairly strong wood pitting, though, according to Bové, these trees were probably affected by Kassala disease rather than tristeza. ELISA for CTV on budwood collected on similar trees by Bové has given negative results. At the Gezira station, Childs also observed wood pitting on sour orange rootstocks of Balady sweet orange; mild wood pitting was also seen on the orange top. These symptoms suggest gummy bark of sweet orange but not tristeza, as CTV does not produce wood pitting on sour orange (nor on most sweet orange varieties). Krezdorn (1983) mentions a stunted navel orange tree on sour orange at the Hudeiba station. In none of these cases has it been shown unambiguously that CTV infection was a fact. However, this does not mean that CTV-infected trees do not exist in the Sudan. It should be remembered that sweet orange varieties were imported many years ago from Kenya, where tristeza is endemic.
Finally, one has to be aware of strains of CTV which are symptomless in Balady lime (Bové et al., 1988). Detection of such CTV strains is only possible through ELISA.
Conclusions and recommendations
Virus and virus-like diseases
Symptoms of the following diseases were seen on Balady sweet orange trees on sour orange rootstock: gummy bark, psorosis young leaf symptoms, concave gum-blind pocket, scaly bark psorosis, popcorn and stubborn. Many of the trees are probably also infected by cachexia and exocortis, of which these trees (sweet orange on sour orange) are symptomless carriers.
A high percentage of Balady mandarin trees show severe to very severe symptoms of cachexia. They are probably also infected by exocortis, but show no symptoms, as mandarin trees on sour orange are symptomless carriers of the exocortis pathogen.
Foster pink grapefruit trees, when grafted on susceptible rootstocks (see Table 52), show symptoms of cachexia and exocortis. They were also found to be affected by a disease not reported until now: Kassala disease or bark gumming.
These results show that the commercial citrus varieties grown in the Sudan are heavily infected by infectious virus and virus-like diseases. This is not surprising as these varieties are old lines, known to be often infected by virus and virus-like pathogens. Indeed, the old mother trees in the government nurseries at Kassala and Nyala were found to be infected with gummy bark on sweet orange, cachexia on mandarin and bark gumming on grapefruit. Progeny budwood from these mother trees is bound to be infected too.
Control of the virus and virus-like diseases is through the use of virus-free, certified budwood. In the case of the insect-transmitted diseases of tristeza, stubborn and greening, control also involves knowledge of the nature, biology and ecology of the vectors. For tristeza, studies must be made to establish whether T. citricida is really absent in the Sudan and, if not, the geographical distribution of the aphid must be determined, especially as it is reported to be present in Ethiopia and known to be widespread in Kenya. For stubborn, a survey is needed to search for, and, if found, to establish the distribution of, the two leafhoppers, N. haematoceps and N. tenellus, which are known vectors of the disease. This is important, in the light of the knowledge that they are present in Egypt. The African vector of greening, T. erytreae, is probably unable to survive in the hot climate of the Sudan.
Phytophthora gummosis is essentially due to inadequate horticultural practices, with trees budded too low or planted too deep. Control of the disease is through good horticultural practices. Trees should be planted on a heap of soil with the crown above soil level, not below. The scion bud should be grafted at about 15-20 cm above the crown of the seedling rootstock. Flood irrigation water should be prevented from reaching the trunk, if necessary, by building soil levees around the trees (see drawings on p. 79).
Rio Grande gummosis is a serious disease in the Sudan. Prevention is the only control measure. Pruning should be reduced to a minimum and only small branches with a diameter of less than 25 mm should be cut. Pruning cuts should be disinfected and, when dry, covered with wound dressing or asphalt. In California, carbolineum with 2 percent phenols has been recommended as a disinfectant for pruning wounds and, for final coating on the disinfected wood, low melting-point asphalt mixed with an equal quantity of carbolineum. Heat and frost-injured areas are also points of entry for the Rio Grande gummosis agent. Obviously only heat injuries need be considered in the Sudan. Whitewashing of trunks and branches that might become exposed to sun (after heavy pruning) should be carried out with a zinc-copper-lime mix.
Greening and tristeza: remarks on rootstocks
Greening is likely to be absent from the Sudan, owing to the hot climate.
There may be a few tristeza-infected trees in the Sudan, but no insect transmission or spread of the tristeza virus could be detected as of November 19X6. This favourable situation may change sooner or later. For instance, in Spain, serious spread of tristeza started in 1956, following a severe frost, in trees that had been introduced in the 1930s. In the Sudan, most commercial trees are on sour orange, while some are on Balady lime rootstock. The latter should no longer be used for several reasons, including susceptibility to tristeza. Commercial citrus species or varieties (except lemon) on sour orange rootstock are susceptible to most strains of CTV. At this stage, however, it seems unwise to try to replace sour orange by tristeza-tolerant rootstocks for the following reasons: there seems to be no insect transmission of CTV; insufficient or no certified budwood of adequate citrus species is available; the nature of any replacement rootstock to be used instead of sour orange has yet to be determined; not enough experience with new rootstocks has been obtained; and sour orange is tolerant of cachexia and exocortis. Thus it seems wise to continue to use sour orange for the time being, provided that the following steps are taken. First, establish facilities for in-country detection of CTV using ELISA, as this is the tool required to monitor the situation and it must be available in the Sudan at any time; second, try out new rootstocks with certified budwood; and, third, keep the experiments to a reasonable size (see below).
A large collection of certified, virus-free citrus species and varieties was imported from California between 1967 and 1970 (see Table 50). Bové saw none of these selections in commercial orchards. The orchards harbouring this collection in Hudeiba and Nyertete are neglected, suffering from lack of water and lack of care. Non-certified citrus lines have been planted next to certified ones and this may have enabled mechanical transmission of the viroids of exocortis and cachexia to previously virus-free trees. The 1967-70 citrus programme was probably too large and ambitious in view of the facilities and human resources available, and this mistake should not be repeated if new introductions are to be made, with any new project being kept as simple and small as possible.
An appropriate strategy at this stage would be to introduce a limited number of certified citrus species and varieties from California, Spain or France (Corsica), using any information gained from the 1967-70 introductions when choosing the varieties most appropriate to the Sudan. Grapefruit and lime (Balady or even Persian) are performing well in the Sudan; mid-season or late sweet orange varieties (but not navel oranges) are also doing well. Certain mandarins are delicious. The Sudanese citrus horticulturists should have no difficulty in making the selection.
At the time of writing (1986) the orchards planted in Hudeiba and Nyertete in 1967-70 with certified budwood have to be eradicated, for the reasons indicated above. A new programme will be started, but the Sudan cannot afford to start new programmes every 20 years without having gained any benefit from previous ones. Continuity is a must, and the younger scientists must overlap with the more experienced ones and continue the work of their predecessors. In this way, the "generation gap" from which the Sudanese Citrus Programme seems to suffer today can be prevented.
Bové, C., Vogel, R., Albertini, D. & Bové, J.M. 1988. Discovery of strain of tristeza virus (K) inducing no symptoms in Mexican lime. In Proc. 10th Conf: IOCV, p. 14-16. Riverside, Univ. Calif.
Childs, J.F.L. 1964. Observations on virus and other diseases of citrus in Sudan. (unpublished report)
Garnier, M., Danel, N. & Bové, J.M. 1984. The greening organism is a Gram-negative bacterium. In Proc. 9th Conf IOCV, p. 115-124. Riverside, Univ. Calif.
Godfrey, G.H. 1946. Infectious wood necrosis and gummosis of citrus. Proc. Lower Rio Grande Val. Citrus Inst., 1: 66-70.
Khairy, M.M.A., Mahgoub, B. & El Hage, A.G. 1969. Citrus culture in the Northern Province of the Sudan: survey of cultivated citrus varieties for local budwood selection. (unpublished report)
Krezdorn, A.H. 1983. Report of visit to Sudan to review fruit tree research program and related factors. WSARP Publication, No. 17, May 1983.
Pratt, R.M. 1958. Florida guide to citrus insects, diseases and nutritional disorders in color. Gainesville, Agric. Exp. Sta.
Rossetti, V. & Salibe, A.A. 1965. Incidence of different types of psorosis in citrus varieties in the State of Sao Paulo. In Proc. 3rd Conf: IOCV, p. 150-153. Gainesville, Univ. Fla. Press.
Saglio, P., Lhospital, M., Laflèche, D., Dupont, G., Bové, J.M., Tully, J.G. & Freundt, E.A. 1973. Spiroplasma citri gen. and sp. nov.: a mycoplasma-like organism associated with stubborn disease of citrus. Int. J. Syst. Bacteriol., 23: 181-204.
Salibe, A.A. & Moreira, S. 1965. Tahiti lime bark disease is caused by exocortis virus. In Proc. 3rd Conf. IOCV, p. 143-147. Gainesville, Univ. Fla. Press.
Chapter 21: The Syrian Arab Republic
Virus and virus-like diseases
Citriculture has flourished in the Syrian Arab Republic for centuries and occupies an important position among crops grown commercially in the country. Plantings are primarily along the Mediterranean coast (see Map 3, p. 71), which has a mild climate. Citrus fruits are highly appreciated by the Syrian people and, in spite of the fairly large local production, additional imports are made from neighbouring countries to meet consumer demand.
The total plantings of citrus in Syria were estimated at around I 000 ha in 1959. Since then, new orchards have been planted and the total area is now nearly 8 000 ha. Plantings could be further expanded, provided the water supply is increased through irrigation projects.
Production of citrus fruit could be increased, without expanding the growing area a great deal, by using modern cultural techniques in existing orchards and healthy, genetically superior propagative material in new plantings. Average yield has already shown an increase in recent years as a result of modern production methods, such as balanced fertilization, better irrigation, appropriate pruning and efficient pest-control measures. However, large production increases in the older orchards would be difficult to achieve, not only because unselected budlings were used in the plantings, but chiefly because virus and virus-like diseases are widespread. Also, older orchards are closely spaced and not uniform.
Citrus growers in Syria have been using the same propagative material for generations to establish their orchards. This has resulted in a decrease in the vigour and productive capacity of trees, owing to the ageing of clones and to infection by debilitating pathogenic organisms. Many orchards in Syria have trees affected by infectious agents, such as psorosis, stubborn, cachexia-xyloporosis, gummy bark and other pathogens causing tree deterioration. The production and selection of genetically superior, healthy material and its multiplication for use in new orchards would result in more vigorous trees with a higher productive capacity.
Some destructive diseases affecting citrus trees in certain areas of the world, such as tristeza, greening and blight, apparently do not occur in Syria. Tristeza, a virus disease, has caused considerable tree death in South America and the United States of America, and now threatens all Mediterranean countries whose citrus industries are based on the sour orange rootstock. Greening, a disease induced by a phloem-restricted bacterium, is spreading in the Southeast Asia region and in South Africa, causing extensive tree decline. It has now reached the Arabian Peninsula and, it is feared, may also reach the Mediterranean basin. Both the tristeza and greening pathogens have efficient insect vectors.
Blight, or young tree decline (of unknown cause), is destroying trees in Argentina, Brazil, the United States of America (Florida), Uruguay and a few other countries. It is suspected to be of viral origin. Proper action should be taken to prevent the introduction of these and other destructive diseases into the flourishing citrus areas of Syria.
Virus and virus-like diseases
Stubborn disease is not included here. In view of the extensive work devoted to stubborn and its transmission in Syria, a special chapter has been prepared on it - Chapter 7.
Diseases inducing psorosis young leaf symptoms
Psorosis young leaf symptoms (see Figs 91 to 96) are associated with various diseases, and are frequently observed throughout the citrus orchards of Syria (Fig. 275).
Scaly bark psorosis (psorosis A). Salibe found no psorosis symptoms in orange orchards, but saw severe scaling in a high percentage of trees in one orchard of Balady mandarin. Bové also noted severe psorosis A symptoms on Mandalina mandarin trees (Fig. 276).
Concave gum-blind pocket. Typical symptoms of the disease were seen by Bové on scions of Washington navel sweet orange at Meterkie (Figs 277 and 278); Balady sweet orange (these trees also had symptoms of gummy bark and cristacortis) and Mandalina mandarin (these trees also had symptoms of cachexia) at Ugarit; and satsuma and Clementine (these trees also had cachexia symptoms) at Mamlaka.
Cristacortis. Bové observed cristacortis symptoms on both the scion and rootstocks of Balady sweet orange trees on sour orange (Hourie orchard, Ugarit). The sweet orange scions also showed gummy bark and concave gum symptoms. Cristacortis and cachexia symptoms were seen on Clementine trees on sour orange at Ashrafie (Figs 280 and 281), on Algerian Clementine trees on sour orange rootstock at the Tartus Centre of Agriculture, and on Mandalina mandarin trees on sour orange at Ashrafie (Figs 282 to 284). On the latter, the cachexia bark-gumming was mild, but the cristacortis stem pitting was severe, not only at the bud-union line (Fig. 283) but also higher up on the trunk (Fig. 284).
Impietratura. Bové observed severe symptoms of impietratura on Washington navel sweet orange at Mamlaka, grapefruits at Besnada (Fig. 279) and Mandalina mandarins at Hmaymim.
Viroid and viroid-like diseases
Cachexia. Salibe observed symptoms of cachexia on Balady mandarin and Clementine trees. Bové also noted cachexia symptoms on many Balady mandarin scions at El Annadeh (Sidawi orchard), Ashrafie and Besnada (Shreita orchard) (Figs 273 and 274); on Mandalina mandarin scions at Ugarit (sib Vitali orchard) and Ashrafie (Figs 282 to 284); on Clementine scions at Ashrafie (Figs 280 and 281) and Mamlaka; and on the C. macrophylla rootstocks of declining lemon trees at Jable (Figs 56 and 57). Some of the trees showing cachexia symptoms were also affected by cristacortis (Figs 280 to 284), concave gum, or both.
Gummy bark. Bové observed very severe symptoms of gummy bark on Balady sweet orange trees on sour orange at Latakia (Awarnia orchard) (Fig. 66) and Ugarit (Hourie orchard). The trees at Ugarit also showed cristacortis symptoms.
Exacortis. Salibe noted typical symptoms of exocortis on the Rangpur lime rootstocks of four-year-old Fortuna mandarin trees in the El Annadeh Government nursery. Some citron plants of a local variety also showed stunting and yellow blotches on branches, which are characteristic of exocortis. Bové observed exocortis symptoms on all the citrange rootstocks of sweet orange trees in certain rows of the Haroun orchard at Tartus, while trees in other rows were free of symptoms. It was found that the trees with exocortis symptoms were propagated from a local old-line mother tree, while the trees without symptoms were budded with certified virus-free plant material from Corsica. Since the exocortis viroid is mechanically transmissible by tools such as pruning knives or scissors, it is to be expected that some of the exocortis-free trees will become infected with the viroid. The old-line mother tree showed no symptoms of exocortis even though it was infected with the viroid, because both the sweet orange scion and the sour orange rootstock are tolerant of exocortis. Infection is revealed by grafting buds of the infected (but symptomless) tree on susceptible citrange rootstocks. In Syria many sweet orange, mandarin, grapefruit and lemon trees on sour orange are tolerant of exocortis, but are probably infected with the viroid. Similarly, sweet orange, grapefruit and lemon trees (but not mandarin trees) on sour orange are tolerant of cachexia, and may well be infected with the viroid.
Stem pitting of Citrus volkameriana rootstocks: an unidentified disorder
In 1979 Bové was shown, at the Citrus Experiment Station in Jable, stunted, yellowish mandarin trees on C. volkameriana. The rootstocks showed severe stem pitting and wood staining (Fig. 293). Seedling trees of C. volkameriana, on which a mandarin bud has been grafted but stayed dormant, showed similar symptoms.
Bové found the same symptom on C. volkameriana rootstocks of lemon trees at the Tartus Centre of Agriculture (Fig. 294). Uninoculated C. volkameriana seedling trees showed no symptoms. The cause of the disorder remains unknown. Whether it is related to the agent inducing bud-union crease on Parson's Special mandarin grafted on C. volkameriana (Vogel and Bové, 1988) remains to be seen. It has been pointed out that C. volkameriana could be susceptible to cachexia. Whether the disorder in question is cachexia or not can only be determined by experimentation.
Mal secco. This is widespread throughout the lemon orchards of Syria (see Figs 232 to 235) and causes severe damage. Control lies in the use of more tolerant or resistant lemon varieties such as Interdonato, Quattrocchi and Santa Teresa. An important collection of lemon cultivars affected by mal secco is at Siano Experiment Station near Jable and should yield interesting information regarding cultivar tolerance of or susceptibility to mal secco.
Phytopithora gummosis. This is not an important problem in Syria, where rootstock seedlings are grafted high enough and trees are planted with bud-unions well above soil level so as to avoid infection of the susceptible scions by spores of soil-borne Phytophthora spp.
Symptoms of autumn blast (see Figs 252 to 254) were seen by Bové on Washington navel sweet orange trees at Bestan-AI Bacha (Al Eskif orchard).
Diseases not present in Syria
Tristeza, greening and citrus canker have not been detected in Syria. The two psyllid vectors of greening and the most efficient aphid vector of tristeza, Toxoptera citricida, are not present in the Mediterranean basin. In Israel, Aphis gossypii has been found to be an efficient vector of certain strains of tristeza virus. In spite of an eradication programme, tristeza could not be controlled in Israel, and in certain areas (south of Tel Aviv) the virus has swept through the orchards as badly as in South America. The tristeza situation in Israel is a threat to all its neighbouring countries.
The situation regarding virus and virus-like diseases of citrus in Syria reflects the general situation in the Mediterranean citrus-growing countries. As most trees are propagated from old-line mother trees, they show the usual symptoms: cachexia on mandarin trees, psorosis A and gummy bark on sweet orange, impietratura on grapefruit and exocortis on susceptible rootstocks. In many cases, several diseases were found on the same trees: cachexia with cristacortis or concave gum on mandarin trees; and gummy bark and cristacortis with or without concave gum on sweet orange trees. The exocortis viroid is probably also present in several of these trees.
The need for virus-free, certified budwood has long been recognized in Syria. The FAO nursery at El Annadeh was set up in 1978 to satisfy this need. Unfortunately, the sweet orange and grapefruit mother tree blocks became heavily infected with Spiroplasma citri, the causal agent of stubborn disease, as a result of natural transmission of the spiroplasma.
Chapter 7, on stubborn and its transmission in Syria, describes the work that was carried out on the problem by the Bordeaux group and Syrian colleagues from 1981 to 1986 in Syria. This work, supported by FAO and the French Ministry of Foreign Affairs, resulted in the identification of the leafhopper Neoaliturus haematoceps (see Fig. 34) as the vector of S. citri in Syria and other Mediterranean and Near East countries, and the identification of the preferred host plant of the leafhopper, namely Salsola kali (Chenopodiaceae) (see Figs 36, 170 to 174 and 322). The presence of this weed in and around the FAO nursery (see Fig. 174) explains the high rates of S. citri transmission found in the sweet orange and grapefruit mother tree blocks.
By 1986, practically all the trees in these blocks had been indexed for stubborn. The trees found infected were pulled out; those free of S. citri were carefully marked. The chances that they will become reinfected by S. citri are small, as they were seven years old in 1986, and it is now known that natural transmission of S. citri usually occurs only on young trees.
For these reasons, and in spite of the early stubborn problem, in 1986-87 the FAO nursery offered the best plant material available in Syria. Regarding nursery trees, Bové recommended that they be produced in leafhopper-free tunnels (Fig. 290) or that the nursery be kept clean of the primary host of the leafhopper, the weed S. kali.
Vogel, R. & Bové, J.M. 1988. Graft transmission from kumquat of an agent inducing bud-union crease in Parson's Special mandarin grafted on Volkamer lemon rootstock. In Proc. 10th Conf: IOCV, p. 367-369. Riverside, Univ. Calif.
Note: See also Bibliography, Chapter 7, p. 109.
Chapter 22: Tunisia
Citrus growing in Tunisia
Virus and virus-like diseases
Citrus fruits are highly appreciated in Tunisia and are practically the only fresh fruits available in large quantities during the winter and spring seasons. Average annual per caput consumption of citrus fruit is nearly 30 kg, placing Tunisia in the same category of high citrus consumption as the highly industrialized countries.
Tunisia produces citrus for local consumption and also for export. Oranges are of fine, high quality and increasing volumes are being exported every year. So citrus fruits are becoming a major source of foreign revenue, among other exports, the most important of which are olive oil, wine, grains, phosphates, petroleum and iron ore. In 1983/84, exports of citrus amounted to about 32 000 tonnes out of a total production estimated at around 220 000 tonnes. Increases in output may be expected, provided that orchard productivity is increased and new plantings are made.
Total citrus plantations at present cover 14 500 ha, with average yields ranging from 12 to 15 tonnes per hectare. This is a fairly modest productivity level. A good yield is considered to be around 50 tonnes per hectare. In a number of citrus areas, such as Florida, Sao Paulo (Brazil) and some others, the average yield of adult orchards ranges from 30 to 35 tonnes per hectare, although some orchards are known to produce higher yields, from 50 or 60 up to 80 or 90 tonnes per hectare. Individual trees (40-70 years old) with exceptionally good health are known to produce up to 1 200 kg of fruit per year. The cost of production obviously decreases as the level of productivity rises.
Productivity of citrus orchards in Tunisia is said to have increased in the last few years as a result of improved cultural practices and greater availability of water for irrigation. High crop yields result from a number of interrelated factors, among which the most important are: appropriate and balanced fertilization, good soil management, proper irrigation, a modern pest control programme and, unquestionably, the use of superior, healthy propagation material when establishing the orchards. The bud-eye gives the young citrus tree its horticultural and genetic characteristics; it establishes the potential vigour and productivity and is all-important in the spread or control of virus and virus-like diseases. There is as yet no known method of curing orchard trees of virus infection, and diseased trees are poor bearers that die early. The use of bud-eyes only from healthy mother trees is the only possible control against virus and virus-like diseases.
Citrus growing in Tunisia
Citrus plantings in Tunisia, as in most other countries in North Africa, are concentrated in regions near the Mediterranean coast, which enjoy a mild climate and where rainfall is higher. The total area of citrus orchards in
Tunisia is estimated at around 14 500 ha, of which the major part (11 000 ha) is in the Cap Bon area. The regional distribution of orchards is as follows: Nabeul, 11 000 ha; Tunis, Ariana and Ben Arous, 1 800 ha; Bizerte, 800 ha; Jendouba, 250 ha; Beja, 150 ha; Kairouan, 200 ha; and others, 300 ha (Groupement Interprofessionel des Agrumes et des Fruits [GIAF], 1976).
The average area of each citrus property is only 1.87 ha for private owners and 9.95 ha for government-owned farms. State farms, 111 in number, represent 1 103 ha or about 8 percent of the citrus area. Private farms number about 6 300.
According to the results of a survey conducted in 1975, there were 3 271 000 citrus trees in Tunisia. Varieties grown were: Maltaise orange (53 percent); Clementine mandarin (16 percent); acidless orange (7 percent); lemons (6 percent); mandarins (Willowleaf and Wilking) (6 percent); navel orange (Thompson and Washington navel) (2 percent); Valencia late orange (2.5 percent); sour orange (3.5 percent); and other varieties (2.5 percent).
New citrus plantings made after the official survey (GIAF, 1976) are estimated to have included about 300 000 trees, equivalent to about 8 ha. The primary limitation preventing expansion is lack of water for irrigation. Density of planting is around 270 trees per hectare, the spacing being normally 6 x 6 m. However, higher densities are frequent, up to 400 trees per hectare.
Table 54 shows fruit production for the four seasons from 1980-84, and estimates for 1984/85, according to data supplied to the consultant by the Institut National de la Recherche Agronomique de Tunisie (INRAT).
Exports are mainly to France and practically only of orange varieties, predominantly Maltaise orange. Amounts exported are shown in Table 55. Export normally starts in January each year, and fruits exported are of an exceptionally high quality.
Clementine mandarin is grown only for local consumption and the season starts as early as November. Because orchards are small and generally contain mixed varieties, it is normal to find fruits with one, two or more seeds. This variety is said to have been discovered in Algeria in 1902, in an orphanage of a priest, Father Clement. It was promoted by Trabut, and is believed to be a natural hybrid of a common mandarin (Willowleaf) and sour orange. The best clones of Clementine mandarin grown are named Cassar and Cadoux.
The orange named Maltaise Demi-Sanguine, also known as Portugaise Demi-Sanguine, is the variety most widely grown in Tunisia. It is round to oblong in shape and is normally seedless and of incomparable quality. Nucellar clones are available for propagation. Trees have some characteristics which resemble Shamouti or Palestine Jaffa oranges and the Pera orange of Brazil. However, when ripe, the fruits have a strong orange colour, almost red peel and some blood colour in the pulp. Some other selected clones of Maltaise Demi-Sanguine orange are known: early Maltaise or Boukhobza-Sakesly orange, late Maltaise or Maltaise Ballerin, and also an acidless clone. The origin of the Maltaise Demi-Sanguine orange is unknown.
Other varieties grown include Washington and Thompson navel oranges (Frost nucellar clones introduced from California); Willow-leaf mandarin, locally known as Youssef Effendi or common mandarin; Wilking mandarin (a hybrid King mandarin x Willowleaf mandarin, made by Frost in California); Eureka lemon; Valencia late orange (nucellar from California); and Marsh seedless grapefruit (INRAT, 1981). Also grown is an acidless orange, known locally as Meski orange.
TABLE 54 Citrus production in Tunisia
|1982/3||76 000||10 750||8 600||21 650||20 200||138 000|
|1983/4||128 700||24 500||13 000||23 800||30 000||220 000|
|1984/5||114 500||19 000||10 900||21 600||26 100||193 000|
TABLE 55 Tunisian exports of citrus
Sour orange, locally named Bigaradier, is the rootstock commonly used in the country. Also used as rootstock, but on a very limited scale, are Troyer citrange and Cleopatra mandarin.
The area where citrus is grown in Tunisia has a rainfall ranging from 300 to 500 mm per year. Artificial irrigation is used to supplement this low rainfall in most orchards. Because of limited availability of water for irrigation, growers are being urged to change their irrigation systems from flow to drip irrigation. Some problems of salinization have occurred in certain areas.
The topography of the citrus-growing areas is flat and exceptionally good, without erosion problems. Soil is generally sandy and alkaline. According to the 1975 survey (GIAF, 1976), the use of fertilization, by surface area, in citrus orchards was: fertilization well balanced 29 percent; fertilization plus ammonium nitrate - 49 percent; fertilization not balanced -18 percent; and not fertilized - 4 percent.
A positive correlation generally occurs between water deficiency and nutritional deficiency, one factor inducing the other. A programme for leaf analysis, carried out jointly by GIAF and INRAT and started in 1976/77, has shown that nitrogen is lacking in 25 percent of cases, phosphorus in 35 percent and potash in 65 percent. Deficiency of minor elements (oligo-elements), mainly zinc and manganese, was found in more than 80 percent of all leaf samples analysed.
Pruning of trees is a common practice in all orchards and, while beneficial in certain cases, it is exaggerated in most orchards.
Health problems observed in citrus orchards by local specialists are fungal diseases (phytophthora root and trunk rot caused by soil-borne Phytophthora spp., mal secco disease caused by Phoma tracheiphila, and Fusarium spp.); bacterial disease (caused by Phytomonas syringae); virus diseases (psorosis and cachexia); mycoplasmal disease (stubborn); and insect and mite problems (fruit-flies, scales, aphids and mites).
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