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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.
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