Diseases of unknown cause
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Several cases of bud-union crease were noticed: at Zafarania, on a Palestine sweet lime grafted on Troyer citrange, and at Fahama, on grafted mandarin trees.
Autumn blast refers to twigs that suddenly die and dry up in situ (see Figs 252 to 254). Gummy material is conspicuous on parts of the stem, but the dried leaves do not fall off. There is a clear-cut margin between the upper, brownish, dead bark of the stem of the dried-up twig and the lower bark that is green and still alive. The twig dieback appears on the uppermost part of the canopy but, in severe cases, lower branches also show dead twigs.
In Iraq the symptoms start around mid-October. Mandarin and Clementine trees are most susceptible, followed by sweet orange and grapefruit trees. Lemon trees are much less affected and sour orange trees never. A similar syndrome was observed in the Libyan Arab Jamahiriya in 1974 and in the Islamic Republic of Iran in November 1985. In Florida, "firing" resembles autumn blast. The problem is taken very seriously in Iraq.
The cause of autumn blast is not known, although it is probably not due to parasitic micro-organisms or pests. It seems to reflect the failure of the terminal twigs to secure sufficient water. Two factors that could be responsible for insufficient water supply in the autumn in Iraq are the large temperature difference between day and night, and the relatively low soil temperatures which contrast with high air temperatures in the day. These temperature differences might induce twig dieback, especially if the root system is inadequate because of poor cultivation, root injury, attack by fungi such as Phytophthora spp., or unbalanced water supply, with overwatering on one side of the tree and poor supply of water on the other, due to the one-furrow irrigation system.
Obviously, twig dieback is more of a problem under severe climatic conditions than in more temperate climates. It is, therefore, important to maintain adequate moisture in the soil by proper irrigation.
Shell bark was seen at Zafarania on old-line Eureka lemon trees (see Fig. 258). Young lines of lemon, free of exocortis, also develop shell bark but much later than exocortis-infected old lines. Lemon is very susceptible to phytophthora gummosis as shell bark favours Phytophthora attack through the many cracks in the bark, especially when the bud-union line is too low and close to the soil level.
Relative importance of the various diseases present in Iraq
Phytophthora gummosis and possibly phytophthora root rot and decay are probably the most destructive diseases of citrus in Iraq. By using resistant rootstocks and improving horticultural practices in the nursery and in the orchard, it should be possible to control these diseases.
Virus, viroid and mycoplasma diseases, once present, cannot be controlled. Protection against these diseases is preventive and requires the use of disease-free budwood and control of natural transmission when insect vectors are involved.
Cachexia seems to be the only important graft-transmissible disease in Iraq, affecting many mandarin and Clementine trees, on which severe symptoms of the disease can be seen. Tolerant varieties, such as sweet orange and lemon, may also carry the cachexia viroid but, being tolerant, they show no symptoms.
Cachexia has probably been spread throughout Iraq with budwood from Zafarania.
Exocortis was discovered at Zafarania on susceptible rootstocks. Very few cases of the disease were seen in the citrus farms, because the scion varieties used in Iraq are tolerant, and because no susceptible rootstocks are used.
It is, however, possible that many such trees are symptomless carriers of the exocortis viroid. This possibility is indicated by the fact that the exocortis-susceptible Troyer citrange, officially recommended in Iraq as a rootstock for several years, has given poor results.
There was surprisingly little scaly bark psorosis and concave gum-blind pocket in the sweet orange orchards that were surveyed. This is a sharp contrast to the situation in North Africa. Similarly, cristacortis and gummy bark do not seem to be much of a problem.
The majority of citrus orchards in Iraq are of the local Mahali cultivar. This cultivar has been grown for very many years and seems to be well suited to the climate. It also seems to be essentially free of the major virus and virus-like diseases which are so often seen on old-line sweet orange trees in the Mediterranean area: namely, scaly bark psorosis, concave gum-blind pocket, impietratura and cristacortis. Regarding exocortis and cachexia, nothing can be said since sweet orange, being tolerant of these diseases, does not express the symptoms. The only disease which is present in almost every orchard is stubborn. Its wide distribution is not surprising since the leafhopper vectors of S. citri, the causal agent of stubborn, are widespread in Iraq. The percentage of affected trees is relatively low, except in some orchards. There may be a higher proportion of affected trees in open orchards.
Three types of symptoms can be used to detect stubborn in Iraq:
Note that uniformly green palmate leaves are the result of excessive heat, not stubborn. In the absence of tristeza and greening, stubborn is the disease of most concern, especially if modern citriculture is to be developed.
Decline due to poor horticultural practices Many more trees decline because of bad horticultural practices than because of disease. Phytophthora disease is a direct consequence of these practices. The remedy is simple: trees should be grafted on phytophthora-resistant rootstocks; budded at the correct height (25 to 30 cm from soil level); planted in the field at the correct depth and properly irrigated - the archaic single furrow technique should be abandoned in favour of open, regular orchards. In Iran, orchards are always of the open type even in areas where the climate is as extreme as in Iraq.
Autumn blast is probably related to marked temperature differences between roots in the soil and twigs in the air. However, poor cultural practices may favour twig dieback. Indeed, trees with inadequate irrigation and poor root systems are more susceptible. Even though autumn blast occurs in open orchards in Iran and Iraq (Al-Beldawi, personal communication), the one-furrow irrigation system used in Iraq is certainly not the most appropriate method for ensuring an adequate water supply. Again, changing to open orchards with irrigation systems controlled by tensiometer readings is probably the answer to autumn blast.
Aziz, S. & Al-Ali, M.S. 1977. Phytophagous and entomophagous insects and mites of Iraq. In A. D. Niazi, M.S. Abdul-Rassoul & M. Shamsuddin, eds. Nat. Hist. Res. Cent. Publ. No. 33. Baghdad, Al-Zahra Press. 142 pp.
Bové, J.M., Saillard, C., Vignault, J.C. & Fos, A. 1984. Citrus stubborn disease in Iraq and Syria: correlation between symptom expression and detection of Spiroplasma citri by culture and ELISA. In Proc. 9th Conf: IOCV, p. 145-152. Riverside, Univ. Calif.
Fos, A., Bové, J.M., Lallemand, J., Saillard, C., Vignault, J.C., Ali, Y., Brun, P. & Vogel, R. 1986. La cicadelle Neoaliturus haematoceps (Mulsant et Rey) est vecteur de Spiroplasma citri en Méditerranée. Ann. Inst. Pasteur/Microbiol., 137 A: 97-107.
Reuther, W., Nauer, E.M. & Roistacher, C.N. 1979. Some high temperature effects on citrus growth. J. Am. Soc. Hort. Sci., 104(4): 353-356.
Chapter 13: Jordan
Virus and virus-like diseases in Jordan
Citrus plantings in Jordan have no parallel. Citrus is primarily cultivated in the Jordan Valley, an area ranging from 300 to 400 m below sea level and enjoying a special climate with mild wet winters and extremely hot dry summers. Soil is heavy and calcareous and the water used for irrigation is salty. Trees grow very fast and vigorously, and fruits are of excellent quality.
At present there is rapid development in Jordan in all sectors of social and economic life. Citrus production is no exception, and extensive new orchards confirm the increase in the area dedicated to this crop - which is currently estimated at about 4 000 ha. Each year some citrus is exported, but importation is necessary outside the picking season, to meet local demand. The government is particularly interested in the growth and improvement of the citrus industry and a well-founded research programme has recently been initiated. Crop protection is also receiving close attention, as in the state campaign for the control of pests (scales and mites) in the South Jordan Valley.
Superior, healthy budwood of major commercial citrus varieties has been introduced in recent years. The aim of these introductions was to provide clean propagative material to growers. Seeds of new' potentially better rootstocks were also imported. The performance of all these imported clean stocks is being tested under the particular conditions prevailing in the Jordan Valley. Several rootstock experiments were recently established in agricultural experiment stations in the area.
Virus and virus-like diseases significantly limit fruit productivity in the orchards of the Jordan Valley. Diseases such as psorosis and cachexia-xyloporosis are debilitating many of the older orchards. Certainly, the use of virus-free budwood will be an important factor in the improvement of production in the new orchards. Moreover, citrus growers in Jordan are very concerned about the possible introduction into the country of two particularly damaging diseases - namely tristeza and greening - which are constantly spreading. Tristeza is caused by an insect-transmitted virus, and greening by a sieve-tube restricted bacterium, also insect-transmitted.
Virus and virus-like diseases in Jordan
As virus and virus-like pathogens produce visible symptoms in susceptible hosts, they can be detected by careful inspection of orchard trees during field surveys conducted by trained virologists. More common symptoms of virus and virus-like diseases include tree stunting, malformation of branches, trunk and branch bark scaling, trunk and stem pitting of various kinds, gum pockets, gum impregnation and exudation, leaf deformation, leaf chlorosis of several types, vein clearings, ringspots and fleckings and fruit abnormalities. Field surveys are very useful to determine the incidence of various kinds of such disorders in the orchards.
However, some scion-rootstock combinations can harbour viruses without exhibiting specific symptoms, thus acting as symptomless carriers. Detection of viruses and other internal pathogens in these cases requires appropriate indexing procedures.
During Salibe's visit to Jordan, a number of orchards were inspected in the Jordan Valley and the presence of symptoms of virus and virus-like diseases was recorded.
Cachexia-xyloporosis. This is a bud-transmissible disease inducing wood pitting and gumming in mandarins, tangelos, satsumas, sweet limes, mandarin limes, Citrus macrophylla and some other citrus types. Symptoms range from the severe in Orlando, Wekiwa and Seminole tangelos, Clementine,
Mandalina and Parson's Special mandarins, Murcott and Ellendale tangors, to the undetectable in sweet orange, sour orange, grapefruit and trifoliate orange.
Xyloporosis was first described (Reichert and Perlberger, 1934) as a disease affecting Shamouti orange trees on sweet lime rootstock in the country then known as Palestine. In 1950, a disease with symptoms resembling xyloporosis, but affecting Orlando tangelo, was described under the name of cachexia in Florida (Childs, 1950) and shown to have an infectious nature. Cachexia and xyloporosis were considered to be synonymous and the disorder was frequently referred to as cachexia-xyloporosis. This view has recently been challenged (Roistacher, 1988).
Cachexia-xyloporosis appears to be wide spread in the citrus orchards of the Jordan Valley. Most Clementine and Mandalina mandarin trees inspected showed typical trunk symptoms above the bud-union (gum streaks in the bark and wood pitting). However, a number of old trees of these varieties were found to be free from symptoms, indicating that both healthy and cachexia-xyloporosis-infected clones are being propagated by nursery workers and growers. Symptoms of the disease were also found in a few trees of satsuma mandarin at the Baqura Station.
An indexing survey is necessary to determine the incidence of cachexia-xyloporosis in trees of lemon, orange and grapefruit budded on sour orange rootstock, the combinations most extensively grown in the Jordan Valley. Since the cachexia-xyloporosis viroid is perpetuated by the use of budwood from infected trees, it can be excluded from new orchards by the use of virus-free propagative material.
Stubborn disease. Stubborn is a very serious disease found in most countries growing citrus under desert or semi-desert conditions. It has been reported from the Arabian Peninsula, Arizona and California, Egypt, Israel, Lebanon, Morocco, Peru, the Syrian Arab Republic, Turkey and several other countries. Symptoms of stubborn include tree stunting, an abnormally bushy aspect, small leaves and short internodes, small, lopsided and acorn-shaped fruits, with curved columella and inverse or pale coloration and sometimes blue albedo. In seedy varieties, excessive seed abortion frequently occurs. Pinholing of sour orange bark below the bud-union is found in severe cases (Caravan and Carpenter, 1965).
It is considered that practically all citrus varieties and combinations are susceptible to the stubborn pathogen, the helical mycoplasma Spiroplasma citri. These include essentially navels and other sweet oranges, grapefruits, mandarins, tangelos, sour oranges and others. Several non-rutaceous hosts are known, including Catharanthus roseus (periwinkle), Trifolium repens, Trifolium pratense, Vicia faba and Pisum sativum. The stubborn organism is transmitted through tissue grafts and buds, and is also spread by leafhoppers.
Stubborn is present in many citrus orchards in Jordan, but only in scattered trees. Affected trees found during the survey were mainly of imported varieties such as Valencia and Abu Surra (Washington navel) oranges and Marsh seedless grapefruit. Stubborn was also found in a few Mandalina mandarin trees in one orchard, and in the South Jordan Valley. There, the more affected Mandalina mandarin trees showed advanced decline and typical crease at the bud-union with the sour orange rootstock, indicative of an extremely severe form of the disease.
Stubborn-like symptoms were also observed in five trees of Balady orange on sour orange rootstock at the Wadi El Yabi Experiment Station and in two trees of shaddock at the Wadi Shuaib Station. See also the section below on the influence of high soil pH on stubborn.
No stubborn was found in commercial orchards with Shamouti orange and Clementine mandarin as scion. Several trees of Demble and French acidless oranges were examined and also found free from stubborn symptoms.
Scaly bark psorosis and concave gum-blind pocket. Typical bark scaling symptoms of psorosis were rare and only seen in a few trees of Shamouti orange. Blind pocket psorosis was encountered in one tree of the same variety. Branch flattening and bark splitting were common in trees of Shamouti orange, but it was not possible to determine the relationship of these abnormalities to the psorosis agent.
Tristeza. Tristeza is a very destructive disease caused by a virus that affects trees of sweet orange, grapefruit and mandarin budded on sour orange and certain other rootstocks. The virus and its efficient vector, the aphid Toxoptera citricida Kirk, are spreading all over the world and have now been reported from Argentina, Australia, Brazil, Paraguay, Peru, South Africa, Uruguay, Venezuela and most countries of Southeast Asia. Spread of tristeza is also occurring in areas where T. citricida is not present but where other aphids are abundant, such as California, Florida, Israel and Spain. The disease represents a tremendous threat to the citrus industries of all Mediterranean and neighbouring countries that use the sour orange rootstock extensively.
Symptoms of tristeza disease on susceptible scion-rootstock combinations vary from sudden wilting (defoliation and death of the entire tree within a few months after infection -the so-called "quick decline" type) to a slow dieback during which trees develop an overgrowth of the scion-trunk bud-union and starch depletion in the rootstock portion of the tree and die within two or three years. An additional symptom of tristeza in the slow decline type is pinholing or honeycombing of the sour orange trunk just below the bud-union (see Fig. 129). This symptom is also shown by stubborn-affected trees on sour orange rootstock.
The recent spread of tristeza virus in Israel makes it possible that tristeza may invade the orchards of the Jordan Valley in the near future. A programme for tristeza indexing, aimed at rapid identification of the virus and subsequent elimination of all infected trees, is urgently needed in Jordan.
Symptoms resembling tristeza, including tree dieback and trunk overgrowth and honeycombing, were found in certain orchards of the South Jordan Valley. The number of trees with tristeza-like symptoms was small in each orchard, which may indicate a very inefficient insect transmission. Scions were Valencia and Shamouti oranges and Marsh seedless grapefruit on sour orange rootstock. Indexing of declining trees with the use of appropriate indicator plants or by ELISA will conclusively demonstrate whether or not tristeza is present in the Jordan Valley.
Exocortis. Exocortis has been reported from practically all citrus-growing areas of the world. The disease complex affects Poncirus trifoliata and most of its hybrids, Rangpur lime, Palestine sweet lime and a number of other citrus varieties. It is carried symptom-lessly in other varieties, such as sweet and sour oranges, grapefruits, mandarins and rough lemon and, therefore, its effects are only manifested when budwood taken from symptomless carriers is used for propagation on intolerant rootstocks. Exocortis is caused by one or several viroids. No insect vector is known, but the viroid is sap-transmissible to citrus and non-citrus herbaceous hosts and is quite resistant to heat, drying and some chemicals (Garnsey and Weathers, 1972). Decontaminating budding knives and other tools with household bleach or other solutions (5 25 percent sodium hypochlorite) is recommended to prevent the spread of exocortis in nurseries and commercial orchards.
Symptoms of exocortis include stunting of trees, bark splitting and scaling of the rootstock portion of the tree. On trees budded on trifoliate or Rangpur lime rootstocks, bark scaling usually appears after four to eight years. Symptoms of exocortis also show up when intolerant varieties used as scions are infected, and these include yellowing of the bark of young branches, splitting and limited shelling. Certain selections of Etrog citron develop leaf epinasty and are used as indicators for exocortis.
No symptoms indicative of the presence of exocortis viroid were observed in the commercial orchards visited by Salibe. Some old Shamouti orange trees budded on sweet limes were examined and found healthy. Typical exocortis symptoms were encountered in a few citron trees in the citrus collection at the Wadi El Yabis Experiment Station in the form of scion bark yellowing and splitting. Exocortis viroid causes a certain degree of debilitation in the so-called tolerant scion-rootstock combinations and future indexing for the viroid is recommended, especially if rootstock diversification in the country becomes necessary.
An apparently new disease: stubborn as influenced by high soil pH. An abnormality of apparently unknown nature was found affecting orange and mandarin trees in various experiments at the Deir Alla Experiment Station in the Jordan Valley. Affected trees showed the following symptoms: slight to severe stunting; bushy aspect; and yellowish leaves with intense vein clearing, cupping and in abnormally upright positions. The trunk showed abnormal thickening just above the bud-union, with severe wood pitting and also bark honeycombing and gum impregnation. Fruits were smaller than normal. Most affected trees were from five to 11 years old.
Scion-rootstock combinations affected by the disorder included: Washington navel and Shamouti oranges budded on Volkamer lemon, Cleopatra mandarin, Troyer citrange and Citrus macrophylla; Washington navel on sour orange rootstock; Valencia orange on Cleopatra mandarin; and Clementine mandarin on Cleopatra mandarin rootstock. Affected declining trees were in between apparently healthy trees of the same stionic combinations. Marsh seedless grapefruit, Eureka lemon and Mandalina mandarin trees in the same orchard were apparently not affected by the problem.
It may be that the disorder was caused by an infectious agent that spread from the Washington navel orange trees into neighbouring trees of other varieties. The origin of the budwood of the Washington navel orange was said to be unknown. It was suggested by local research workers that the problem could be named Deir Alla disease.
The abnormality has many symptoms resembling those of stubborn and greening diseases. However, the general vein clearing of all leaves on all affected trees is not common to these diseases. The disorder should therefore be regarded as a new problem until proper research demonstrates its true nature. If it should spread, it would represent a serious threat to the citrus industry of Jordan. Therefore it must be considered a serious problem that deserves immediate attention from scientific authorities.
At Salibe's request, Bové was asked to visit the Deir Alla Experiment Station in December 1983.
As pointed out by Salibe, most if not all trees had yellowish leaves (Figs 164 and 295), irrespective of the varieties involved. These symptoms are very probably due to the high pH of the soil (8.5) which the Deir Alla scientists are trying to lower by applying acidifying nitrogenous fertilizers such as ammonium sulphate. Also, the available calcium content of the soil is high, explaining why all trees on Troyer citrange rootstock are severely stunted.
In addition to leaf yellowing, Salibe noted that the trees had a bushy aspect, with cupped leaves in an abnormally upright position. These symptoms are typical of stubborn and indeed S. citri, the causal agent of stubborn, was cultured from plant samples collected at the Deir Alla Station on symptomatic trees and carried immediately to Bordeaux. These included fruit axes from a Valencia late sweet orange tree on Cleopatra mandarin rootstock (Fig. 2951, mottled leaves from a second Valencia late orange tree on Cleopatra mandarin rootstock, and fruit axes from a Thompson pink grapefruit tree on Cleopatra mandarin rootstock (Fig. 164).
On the basis of these observations and results, it is clear that the symptoms observed by Salibe at the Deir Alla Station are due, at least in part, to the combined effects of high pH and infection by the stubborn pathogen. No evidence for greening and its psyllid vectors could be seen.
Salibe also observed wood pitting, bark honeycombing (pinholing) and gum impregnation of the bark above the bud-union of affected trees. These symptoms are those of cachexia-xyloporosis on mandarin scions (Fig. 296) or gummy bark on sweet orange scions. In fairly severe cases of stubborn-affected trees on sour orange, pinholing is seen below the bud-union on the sour orange bark. Several Washington navel sweet orange trees on sour orange rootstock at the Deir Alla Station showed this stubborn-related pinholing.
Finally, one severely stunted Washington navel tree on sour orange rootstock (Fig. 297) showed pinholing below and above the bud-union line, but in addition there was a bud-union crease so severe that, when a piece of bark was removed, it broke at the bud-union line (Fig. 298). Bud-union crease is not a symptom of stubborn and S. citri could not be cultured from leaf samples of the tree. Tristeza-affected trees on sour orange may show bark pinholing on the sour orange rootstock, but the tree indexed negative for tristeza virus by ELISA. The above bud-union crease, whatever the cause, could be partly responsible for the yellow foliage of the tree.
Other virus and virus-like disorders. A number of minor isolated problems were observed during the field survey. Trees on Shamouti orange were found showing small pimply eruptions in the trunk and larger limbs, suggesting popcorn psorosis. Others had longitudinal splitting with some scaling in the margins and still others had flat branches. No definite cause was found for these abnormalities, which could result from some form of psorosis virus or be determined by local environmental conditions.
Bud-union crease, characterized by an intermittent or continuous depression in the wood at the bud-union, with corresponding projection from the inner face of bark, was found in a few diseased trees. Scion-rootstock combinations with bud-union crease were Abu Surra (Washington navel) orange and Mandalina mandarin on sour orange rootstock, in commercial orchards. A similar type of bud-union crease was observed on Shamouti orange trees of about five years old budded on Volkamer lemon, Troyer citrange and Cleopatra mandarin rootstocks at Deir Alla Experiment Station. This was thought to be a result of genetic incompatibility.
Shell bark and some abnormal flat branches were seen on a number of lemon trees in commercial orchards.
It should be emphasized that nutritional disorders, especially iron deficiency due to high pH, are such major problems in the citrus orchards of the Jordan Valley that they overwhelm all other existing factors that constrain production.
None of the following virus and virus-like diseases was observed during the visit to citrus orchards: impietratura, gum pocket, gummy bark, vein enation-woody gall, satsuma dwarf, leaf curl, citrus tatterleaf, infectious variegation, leprosis, rumple, yellow vein, citrus blight and citrus decline. Local plant pathologists should make themselves familiar with the symptoms of these and other unwanted diseases in order to be able to recognize them and promptly eradicate any tree suspected of harbouring their pathogenic agents. Good descriptions of disease problems are available (Childs et al., 1968; Bové and Vogel, 1975; Wallace, 1978).
The Wadi Shuaib Station. The Wadi Shuaib Station is a small government farm located in the Shuaib Valley. It maintains a very valuable bank of healthy germplasm of citrus and produces superior nursery trees of virus-free material. The mother block is composed of trees produced with nucellar budwood imported from California in 1977, including Washington navel and Valencia oranges, Willow-leaf (Balady) mandarin and Redblush and Marsh seedless grapefruits.
A careful inspection made by Salibe and accompanying plant pathologists revealed the presence of malformed, lopsided fruits, with inverse colouring in a few trees of Marsh seedless grapefruit and Willowleaf mandarin, suggesting stubborn disease. Indexing of these trees and eradication of those found positive for the stubborn pathogen is recommended.
Among old-line citrus varieties maintained at the station were trees of Clementine mandarin, French sweet orange, shaddock and lemon. The general appearance of most trees was excellent and the nursery was well managed.
The Deir Alla Experiment Station. Located in the Jordan Valley, the Deir Alla Experiment Station is part of a chain of research centres and stations established in this fertile region of Jordan. Several trials have been planted at this station to study the performance of trees of commercial citrus varieties budded on various rootstocks. Salibe visited the trials of Shamouti and Washington navel oranges, Mandalina mandarin and Eureka and Lisbon lemon scions. Rootstocks under test included Brazilian sour orange, Kenya sour orange, Palestine sour orange, Cleopatra mandarin, Volkamer lemon and C. macrophylla. Two kinds of disorder were observed to affect trees in experimental orchards: stubborn disease, mainly on Washington navel orange trees, and bud-union crease symptoms on various scion-rootstock combinations (not reported before in the literature). All these problems deserve attention from local plant pathologists.
The Wadi El Yabi Agricultural Research Station. Several citrus experiments are being carried out at this modern research farm in the Jordan Valley. A large experiment to study the behaviour of four rootstocks with several scion varieties was established in 1979. The rootstocks were sour orange, Volkamer lemon, Cleopatra mandarin and C. macrophylla. Scions included Valencia, Salustiana, Hamlin, Shamouti, Marsh, Washington navel, Tarocco and Pineapple oranges, kumquat, Fairchild mandarin and others.
A large collection of citrus varieties, imported mainly from the United States of America, was also planted in the station. Among outstanding varieties observed by Salibe were Pixie, Freemont, Bowen, Ponkan and Frost Dancy mandarins. The great value of this citrus collection is emphasized by the small number of varieties currently grown commercially in Jordan. Careful studies on fruit quality and time of ripening may reveal new varieties of potential value to the country's citrus industry.
Twelve-year-old mother seedling trees of rootstock varieties in the station included Volkamer lemon, Cleopatra mandarin and sour orange.
In the citrus collection, symptoms typical of stubborn disease were observed on Washington navel orange trees: namely, bushy aspect, off-season bloom, small-sized leaves in abnormally upright positions, small malformed and lopsided fruits, columella, etc. Similar, but less evident, symptoms of stubborn were also found on a few Balady orange trees. Exocortis was seen on citron, Citrus medica L.
Baqura Station. Baqura Station is a government farm, mainly dedicated to producing nursery plants of citrus and to date propagation. A small collection of citrus trees of about 16 years of age was examined for symptoms of virus diseases. Those of satsuma mandarin and shaddock were found to be apparently healthy, while the Clementine mandarin showed severe cachexia-xyloporosis. Lemon trees showed trunk scaling typical of shell bark.
Virus indexing facilities. The Research Department of the Ministry of Agriculture does not have the facilities to conduct the virus indexing that is urgently needed for a citrus health and improvement programme. There is a pressing need to provide laboratory and greenhouse facilities to this department to enable personnel to conduct virus indexing by serology and tissue-grafting methods. Further training on virus and virus-like disease identification and on indexing procedures would help local plant pathologists in the establishment and implementation of a phytosanitation project.
The Department of Plant Pathology of the University of Jordan is well equipped, with a modern virus laboratory and greenhouse facilities. However, the work there is concentrated on virus diseases affecting vegetable crops.
Varieties and rootstocks. The citrus industry of Jordan is based on a very small number of varieties. Commercial orchards consist mainly of seven varieties, Clementine mandarin, Mandalina mandarin, Abu Surra (Washington navel) orange, Shamouti orange, Valencia orange, Marsh seedless grapefruit and Eureka lemon.
Clementine mandarin is an early ripening mandarin variety, generally seedless, and the trees are heavy bearers. Most trees carry the xyloporosis viroid that causes a certain degree of stunting, and reduces yield and fruit size. Mandalina mandarin is a mid-season to late ripening variety, closely resembling Ladu mandarin from the Philippines and Ladoo mandarin from India. It also has some characteristics of the Dancy tangerine largely grown in Florida. A large majority of the Mandalina mandarin trees examined in the Jordan Valley were found to bear only light crops. Cachexia-xyloporosis is also widespread on trees of this variety.
The Valencia orange orchards visited had heavy yields, but those of Shamouti and Washington navel oranges were consistently light bearers. There are selections of Washington navel orange, such as Baianinha orange from Brazil, that give higher yields than the old parent variety. Shamouti orange is also known to be a light bearer. This variety has many characteristics resembling those of Verna and Verna Peret oranges from Spain, Lamb Summer orange of Florida and Pera orange of Brazil. Performance experiments among these varieties and selections (both old-line and nucellar clones) of Shamouti may disclose superior clones with higher returns in Jordan.
Sour orange is practically the only rootstock variety used in the citrus orchards, with only a few small plots budded on Palestine sweet lime. Since there are several selections of sour orange (at least ones considered to have some tolerance of the tristeza virus), they should also be tested in competition experiments under Jordanian environmental conditions.
Bové, J.M. 1966. Citrus virus diseases in the Mediterranean area. Report presented at the meeting on Phytiatry and Phytopharmacy, Marseilles (France), 1965, updated for the 4th Conf. IOCV. 44 pp. (mimeo)
Bové, J.M. & Vogel, R. 1975. Description and illustration of virus and virus-like diseases of citrus. an IOCV project. Institut Français de Recherches Fruitières Outre-Mer. Paris, Editions SETCO.
Calavan, E.C. & Carpenter, J.B. 1965. Stubborn disease of citrus trees retards growth, impairs quality and decreases yields. Calif. Citrog., 50(3): 86-87, 96, 98-99.
Childs, J.F.L. 1950. The cachexia disease of Orlando tangelo. Plant Dis. Rep., 34: 295298.
Childs, J.F.L., Bové, J.M., Calavan, E.C., Fraser, L.R., Knorr, L.C., Nour-Eldin, F., Salibe, A.A., Tanaka, S. & Weathers, L.G. eds. 1968. Indexing procedures for 15 virus diseases of citrus trees. Washington DC, USDA/ARS Agriculture Handbook 333.
Garnsey, S.M. & Weathers, L.G. 1972. Factors affecting mechanical spread of exocortis virus. In Proc. 5th Conf, IOCV,p. 105-111. Gainesville, Univ. Fla. Press.
Reichert, J. & Perlberger, J. 1934. Xyloporosis, the new citrus disease. J. Agency for Palestine Agric. Exp. Sta. (Rehovot) Bull., 12: 1 -50.
Roistacher, C.N. 1988. The cachexia and xyloporosis diseases of citrus - a review. In Proc. 10th Conf. IOCV, p. 116-124. Riverside, Univ. Calif.
Wallace, J.M. 1978. Virus and virus-like diseases. In The citrus industry. vol. IV, p. 67-184. Berkeley, Div. Agric. Sci., Univ. Calif.
Chapter 14: The Libyan Arab Jamahiriya
Citrus growing in the Libyan Arab Jamahiriya
Virus and virus-like diseases
Citrus growing in the Libyan Arab Jamahiriya
Citrus growing in the Libyan Arab Jamahiriya is concentrated mainly around the coastal areas of the Mediterranean Sea where the climate is mild and rainfall abundant. The main citrus area extends from Surman (40 km west of Tripoli) to Gharabulli 1120 km east of Tripoli) and into the interior (40 km south to Azizia). Some orchards are also planted in the Benghazi area (I 050 km east of Tripoli) and around Fueihat (15 km south of Benghazi).
The total citrus area is currently estimated to be more than 7 000 ha, as compared with about 3 500 ha in 1974-75. Statistics available for 1980 on the number of trees and on fruit production for the more important citrus species are shown in Table 35.
The major varieties of sweet orange that are cultivated in Libya are Demi blood (similar to Maltaise orange), Moro, Sukkari, Tarocco, Shamouti, Abu Surra (local name for Washington navel orange), Balady and Valencia.
Among mandarins, the most widely grown variety is Kinya, a selection of Youssef Effendi or Willowleaf mandarin. Clementine mandarin is also grown in limited quantities and a few trees of Mandalina mandarin were also observed.
Lemons are represented mainly by Femminello from Sicily and Lunario, a less popular variety. Limes, Citrus aurantifolia Swingle (Mexican lime type), and Citrus latifolia Tanaka (Tahiti lime) are practically unknown in the country. Grapefruit is not popular and the few existing trees are budded on Naring sour orange.
Citrus fruit is of a high quality thanks to a combination of favourable factors, mainly related to soil and climate. Soils are mostly sandy, neutral or slightly alkaline. The climate in the citrus belt is mild, with temperatures never low enough to cause frost damage, but with a day-night fluctuation high enough to secure very good fruit quality.
All citrus produced is consumed locally, with no exports. On the other hand, no citrus is imported. The citrus season usually lasts from November until March. An industry to process oranges and lemons for juice and marmalade production has been established.
The main problems affecting citrus production are: Mediterranean fruit-fly (Ceratitis capitata) and a few other insect pests, virus diseases, dieback of twigs, autumn leaf drop, deficiency of trace elements' gummosis, water quality and salinity.
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