CITRUS, the second most important fruit worldwide after grapes in terms of area and production, is attacked by more than 30 viruses and virus-like diseases in the world. Among them Citrus tristeza virus (CTV) is one of the biggest threats to this fruit around the globe. In 1981, the total world loss due to this disease was estimated at 50 million trees.

Studies were carried out to understand the CTV situation in Pakistan through a research project supported by the ministry of science and technology, implemented at the Institute of Horticultural Sciences, University of Agriculture, Faisalabad.

The objective of the project was to check the status of plants against CTV and to produce virus-free citrus plants. The survey was to determine the severity of the problem in the citrus belt of the country. Samples were collected from Sahiwal, Renala Khurd, Bhalwal, Sargodha, Toba Tek Singh, Faisalabad, Peshawar, Mardan, Islamabad, Rawalpindi, Hyderabad, Mirpur Khas, Jamshoro, Nawab Shah and different areas of Balochistan.

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 areas. Therefore, it is the need of the time that the public and the private sectors should take it seriously and more projects should be started at the earliest to save the citrus tree.

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

The virus (CTV) is transmitted by aphid species. Brown citrus aphid (Toxoptera citricida) is the most efficient vector, which is widespread in western counties and other citrus-growing countries, 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.

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 decay 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.

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

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

The condition of every citrus tree should be strictly observed, 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.

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.

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.

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.

CTV management practices: More and more projects may be initiated to define the severity and complexity of CTV strains 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 citrus industry. However, the following valuable measures will also be fruitful:

The certification of bud wood and the use of resistant rootstock are the primary counter-measures in controlling the disease. CTV-free trees of important commercial cultivars can be maintained in an aphid-proof screen house. Strict control by quarantine must be continued in order to prevent the introduction of more severe strains. It is also necessary to spray nursery plants and young trees with environment-friendly insecticide, to control occasionally observed aphids. This should prevent any re-infection with the virus.