A THREAT TO CITRUS PRODUCTION IN PAKISTAN
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Citrus has been cultivated as one of the important fruit crops providing local people with valuable nutrition, and as an indispensable cash crop for both domestic consumption and export. The citrus fruit is consumed worldwide as fresh fruit and processed product such as fruit juice.
The role of citrus fruits in providing nutrients and medicinal value has been recognized since ancient times. Citrus fruits, belonging to the genus Citrus of the family Rutaceae, are well known for their refreshing fragrance, thirst-quenching ability, and providing adequate vitamin C. In addition to ascorbic acid, these fruits contain several phytochemicals, which play the role of neutraceuticals, such as carotenoids (lycopene and β-carotene), limonoids, flavanones (naringins and rutinoside), and vitamin B complex and related nutrients (thiamine, riboflavin, nicotinic acid/niacin, pantothenic acid, pyridoxine, folic acid, biotin, choline, and inositol).
There is no doubt that the genetic pool of the citrus plants originated in South-Eastern Asia. Citrus fruits spread from the Asian areas to other regions following the paths of civilization (Dugo and Giacomo, 2002). The genus Citrus is native to Southeast Asia, occurring from northern India to China and Extending to South Malaysia, the East Indies, and the Philippines and from the Himalayas South to Indonesia or Australia.
The genus comprises of large number of species which are small shrubs and evergreen trees, usually with spines on trunks and branches. Records of citrus domestication go back to about 500 BC. The oldest citrus species is known as Citron (Citrus medica L.). The common edible and important commercial fruits in this group are: mandarin (Citrus reticulata Blanco), king mandarin (C. nobilis Andews Non. Lour), sweet orange-Malata and Musambi (C. sinensis L. Obsk), grapefruit (C. paradisi Macf.), sweet lime (C. limettioides Tanaka), acid lime (C. aurantifolia Christm, Swingle), lemon (C. limon L. Burm), pomelo (C. grandis L. Osb), sweet lemon (C. limetta Wight and Arn.) and Tahiti lime (C. latifolia L.). The main root stock species are: sour orange (C. aurantium L), rough lemon (C. jambhiri Lush.), Rangpur lime (C. limonia Osb.), Karna khatta (C. karna Raf.) and Troyers citrange (C. sinensis x Poncirus trifoliata).
A number of citrus cultivars are grown in tropical or sub-tropical countries depending upon favorable climate and soil conditions. The contribution of Brazil, USA and China in the total world citrus production is about 47%, whereas Mexico, Spain, Italy, Egypt and Pakistan contribute another 25%. Citrus is very dominating fruit of Pakistan, being grown almost in all provinces of the country. It ranks sixth in production and is the largest producer of Kinnow mandarin in the world. According to rough estimates approximately 95 % of the total Kinow produced all over the world comes from Pakistan and Punjab alone produces over 95% of the crop. Major citrus growing districts are: Sargodha, Sahiwal, Okara, Faisalabad, Toba Tek Sing, Jhang and Multan in the Punjab and Mardan, Peshawar, Swat, Swabi, Noshera and Hazzara in Khyber Pukhtoonkhwa. Small contribution of 2.5% also comes from Sindh (Sukhar, Khairpur and Nawabshah) and Balochistan (Mekran, Sibi and Kech).
The CTV a member of the genus Closterovirus is widely distributed in most of the citrus growing countries of the world. During the last century, over 50 million trees have been killed by CTV worldwide decline, especially affecting the species grafted on sour orange. Therefore CTV is the most serious and economically important disease all over the world. It ranked as one of the most important citrus diseases for the last sixty years (Bar Joseph et al., 1989) and yet viral genetic basis of CTV disease is poorly understood. It is not yet possible to attribute particular symptoms on a particular citrus cultivar to specific viral sequences, nor is it known which sequence influence transmissibility by different aphid species.
The average yield of citrus in Pakistan is very low as compared to other countries as Brazil, USA and Egypt. This may be due to the prevalence of number of pests and diseases and poor management especially. Among them the major virus and virus like diseases reported in Pakistan are Citrus Tristeza Closterovirus (CTV), Citrus greening may be associated with citrus decline. The CTV a member of the genus Closterovirus is widely distributed in most of the citrus growing countries of the world. During the last century, over 50 million trees have been killed by CTV worldwide decline, especially affecting the species grafted on sour orange. Therefore CTV is the most serious and economically important disease all over the world. It ranked as one of the most important citrus diseases for the last sixty years and yet viral genetic basis of CTV disease is poorly understood.
CTV is naturally transmitted and spread by several aphid species in a semipersistant manner, with no latent period; acquisition and inoculation periods being at least 30 minutes in some cases of which brown aphid (BrCA) Toxoptera citricida Kirk is the most efficient vector, and green peach aphid Myzus persicae, cotton aphids Aphis gossypii and the spirea or green citrus aphid A. spiraecola have also been incriminated as vectors of CTV on citrus (Stoetzel, 1994). There is also one unconfirmed report of a mealy bug (Ferrisia virgata) transmitting CTV after 30hrs feeding on a CTV-infected plant. The virus has a phloem tropism in citrus plants and is spread long distances generally by use and movement of CTV-infected nursery propagation materials. CTV is limited to the phloem and causes various symptoms according to the rootstock/scion combination and the virus strain typically as vein clearing in lime, stem pitting and honey combing. Aphid dispersion of the virus is important within citrus growing regions.
In Pakistan more than 30 virus and virus like diseases of citrus are known to exist. Survey reports confirmed the presence of several virus and virus-like diseases in Pakistan including the serious threat by CTV and greening diseases. CTV was found to be common in citrus orchards and nurseries with average incidence of 27%. On grafting and mechanical inoculation symptoms of vein clearing and chlorosis in young leaves of C. aurantium, C. limon cv Eureka and C. sinensis has been observed. In Pakistan, the presence of CTV was confirmed through electron microscopy and. It was reported to be widely distributed and a serious threat to citrus production in the Punjab and in Khyber Pukhtoonkhwa. Recently large number of samples collected from the citrus orchards in Punjab were tested and found to be ELISA positive indicating that CTV is widely distributed in the major citrus growing areas. The highest incidence of CTV was found in Khushab (71%) in the Punjab.
New molecular tools were introduced for sensitive detection of CTV and polymerase chain reaction assay (PCR) provides more sensitivity. The introduction of molecular tools for successfully, sensitive and reliable detection will reduce the dispersal of CTV in citrus and may increase the income of citrus growers. The yield of citrus will also play a vital role in the Gross domestic product (GDP) of Pakistan.