Citrus Tristeza Virus: the most destructive and the biggest threats to citrus production 
By M. Azher Nawaz1, M. Mumtaz Khan2 and M. Mithal Jiskani3

1. Agriculture Officer, Dhanote, Lodhran.
2. Professor, Institute of Horticultural Sciences, University of Agriculture, Faisalabad.
3. Assistant Professor, Department of Plant Pathology, Sindh Agriculture University, Tandojam.

Sweet oranges, blood oranges, sour oranges, mandarins, lemons, mediterranean lemons, limettas, limes, grapefruits, kumquats and citrons are world known members of a fruit genus citrus, therefore, the ‘Citrus fruit’ is alone in technical name but is multiple in type species. Every citrus member species have number of varieties, which varies in taste as well as in nutritional and medicinal properties. Lemons, limes and citrons are most sensitive to frost, while sweet oranges, grapefruit, tangerines and calamondins are intermediate. Kumquats and Owari Mandarin Satsuma are the most frost-tolerant, tolerating temperatures in the low twenties.

At all, the citrus stands as the second most important fruit worldwide after grapes in terms of area and production. At present the world’s citrus yield is 105 million tones per annum. Pakistan occupies 13th position among the eminent citrus producing countries, where citrus fruit comprise about 40% of the total fruits produced in Pakistan with an area of about 200,000 hectares (2.25% of the world), and a yield of two million tones of the fruit per year. In this way, citrus also stands on top among the 30 fruits grown throughout the country with Punjab dominating in production, where, about 59 per cent of the total area and 64% of the production among citrus is captured by kinnow alone.

According to reports, 22.57 million tones of citrus was produced in Pakistan, earns Rs 5,394 million as foreign exchange in 2006. Russia imposed ban in 2006-end, but due to strict quarantine measures conforming to Sanitary and Phyto-Sanitary requirements, Russia, again is enthusiastically engaged in importing Pakistani kinnow this season following the lifting of ban, therefore, as compared to last year, exports of kinnow are likely to fetch $100 million, thereby exceeding last year`s exports by 30 to 40%. Last year, it had exported 122,000 tonnes, but this year the export figures may hit over 200,000 tonnes.

The increase in export is due to quality production of citrus fruit, but it is also true that citrus are attacked by more than 30 virus and virus-like diseases in the world, but the citrus tristeza virus (CTV) is arguably the most destructive and represents one of the biggest threats to citrus production around the globe. In 1970, it was reported that citrus decline on sour orange rootstock was presumably caused by a tristeza virus. In 1981, the total world loss attributable to this disease was estimated at 50 million trees. Tree decline (Fig.) continues to be a problem today in groves that still have plantation on sour orange rootstock.

The studies are carried out to understand the CTV situation in Pakistan through a research project supported by Ministry of Science and Technology, Pakistan, implemented at the Institute of Horticultural Sciences, University of Agriculture Faisalabad. To check the status of plants against CTV and to produce virus free citrus plants were major objectives of the project. The survey was conducted to determine the severity of the problem in the citrus belt of the country. The samples were collected from Sahiwal, Renala Khurd, Bhalwal, Sargodha, Toba Tek Singh, Faisalabad, Peshawar, Mardan, Islamabad, Rawalpindi, Hyderabad, Mirpur Khas, Jamshoroo, Nawab Shah and different areas of Balochistan also. All the samples were tested against the virus at the Institute of Horticultural Sciences, University of Agriculture, Faisalabad. The results indicated the presence of infection in plant samples collected from some surveyed areas. Therefore, it is the need of the time that government as well as private sector should take it serious and more and more projects should be started as early as possible to save the citrus industry of Pakistan, otherwise, like many other countries, we may also have to scarify citrus grooves.

This may also be remembered that CTV is caused by Citrus tristeza closterovirus, comprised of many strains or isolates, which are considered from mild to severe, causing little damage to severe decline, especially on trees grafted on sour orange rootstock.

The causal virus is a flexuous and thread-like rod, 2,000 nm in length and 15 nm in width. The virus particles are located in the phloem of the host plant, and disturb its transportation systems. The optimum temperature for virus infection and multiplication is 20-30°C.

CTV is transmitted in nature by aphid species. Brown citrus aphid (Toxoptera citricida) is the most efficient vector, which is widespread in the western counties and other citrus growing countries of the world, but fortunately not in Pakistan. The other aphid species include the green citrus aphid (T. armentia); melon aphid and cotton aphid (Aphis gossypii) have been reported as a vector with little efficiency. The high population of these aphids especially A. gossypii may be a potent source of spread of the disease in Pakistan in near future.

The most important symptom is the quick decline or death (Fig. 1) of orange, grapefruit and mandarin trees. Typical symptoms include: honeycombing or stem pitting, decline, wilting, dieback, quick decline, leaf chlorosis and curling, heavy fruit set, bud union staining, scion overgrowth, thickened bark at bud union, starch depletion in roots, and loss of feeder roots. Tree decline results from the necrosis and death (blockage) of the conductive tissue (phloem) at the bud union, resulting depletion of sugars/starch in the feeder roots that ultimately are decayed leading to the death of the tree (Fig. 1).

The rootstocks of sour orange are frequently and successfully used for citrus orchards, because it has many horticultural advantages, including blight tolerance, adaptation to many soil conditions, tolerance to cold and foot rot, as well as producing fruit of good quality and high yields; but due to its susceptibility to CTV, the viruses continue to spread, hence use of sour orange as a rootstock now has been reduced worldwide; even is no longer in use world over. The rootstocks, that offer resistance to tristeza decline include the citranges (C-35, Carrizo, Troyer), Citrumellos (Swingle), mandarins (Cleopatra, Sun Chu Sha, Sunki), mandarin hybrids (Rangpur), lemons (Volkamer, Rough), Poncirus trifoliata, Smooth Flat Seville, Kinkoji and Gou Tou.

The future of citrus in Pakistan could be saved through field and Lab. Diagnoses as well as integrated management of CTV that only could be achieved by training of the citrus growers. However, the following preliminary strategies are proposed with reference to present situation of CTV in country:

1. The rootstock should be determined on top priority, because the rootstocks other than sour orange and bittersweet are not affected by CTV.

2. The condition of every citrus tree should also be observed strictly, because CTV affected trees exhibit an overall decline with the absence of vegetative shoots on the large scaffold branches or limbs near the trunk of the tree.

3. The tree trunk must be examined carefully for any damage, because CTV limits the tree growth, over all health and in many cases, foot rot lesions on the tree trunk also causes tree decline.

4. The bud union should be located on the tree trunk and a patch of bark be removed from across the bud union to examine the bark patch for numerous small pinholes below the bud union or honeycombing areas in the inner face of the bark or a brown discoloration at the bud union, all that corresponds to the disease.

5. In addition to the above field diagnoses, an enzyme-linked immunosorbant assay (ELISA) or immune tissue blot test may also be done in a laboratory to detect the presence of mild or severe strains of CTV in the plant tissue.

Integrated CTV Management Practices
More and more projects may be initiated to define the severity and complexity of CTV strains in Pakistan and to seek potential mild strains, which could be used as cross protection. A calm but proactive approach to CTV should be taken by the Pakistan citrus industry. However, following valuable measures will also be fruitful:

1. The certification of bud wood and the use of resistant rootstock are the primary counter-measures in controlling the disease.

2. Continuing the maintenance of CTV-free trees of important commercial cultivars in an aphid-proof screen house.

3. Strict control by quarantine must be continued in order to prevent the introduction of more severe strains.

4. It is also necessary to spray nursery plants and young trees with eco-environment friendly insecticide, to control occasionally observed aphids. This should prevent any re-infection with the virus.

5. Ensuring that we are maintaining technical awareness according to the CTV research developments of world.