CITRUS TRISTEZA DISEASE

PAT401

1. COLLEGE OF AGRICULTURAL TECHNOLOGY (Affiliated to Tamil Nadu Agricultural University, Coimbatore-3) Kullapuram (Po),Via VaigaiDam , Theni-625 562 CITRUS TRISTEZA DISEASE STUDENT COURSE TEACHER S.DHIVYA Dr.S.Parthasarathy 2015021031 Assistant Professor (Plant Pathology)

2. PATHOGEN Citrus tristeza virus transmitted by BROWN CITRUS APHID- Toxoptera citricida

3. HISTORY The disease has led to the death of millions of  Citrus  trees all over the world and has rendered millions of others useless for production. Farmers in Brazil and other South American countries gave it the name " tristeza ", meaning sadness in Portuguese and Spanish, referring to the devastation produced by the disease in the 1930s. The virus is transmitted most efficiently by the brown citrus aphid.

4. ECONOMIC IMPORTANCE CTV is the most economically important and damaging virus of citrus trees. It has killed more than 80 million trees worldwide. With the spreading of  T. citricida  the severity and impact has increased dramatically in Central America and the U.S. In Spain there has been a progressive decline in production from over 40 million sweet orange and mandarin trees.

5. HOSTS Sour orange Lime Mandarin

6. SYMPTOMS The three most common groupings of symptoms are decline (quick and slow), stem-pitting, and seedling yellows. DECLINE Decline is generally exhibited with sweet orange, mandarin, or grapefruit when they are grafted on infected sour orange rootstock. This decline includes chlorotic leaves and general dieback of the infected tree. In this case the infected tree will also show a bulge above the bud union and honeycombing on the inner face of the original sour orange root stock bark.

7. STEM PITTING Stem-pitting is another symptom of CTV that manifests in most host types under the proper conditions, and especially in  Citrus  trees grafted onto sour orange rootstock. The host will develop pits in the trunk and stem. This results is decreased tree vigor and reduced fruit yield. This is typically caused by the more virulent strains of CTV.

8. YELLOW OF SEEDLING The third major symptom of CTV infection is seedling yellows. This tends to occur on sour orange,  Natsudaidai , lemon and  buntan . Symptoms include yellowing of foliage and general dieback.

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10. VIRUS CLASSIFICATION Group:Group IV ((+) ssRNA ) Family: Closteroviridae Genus: Closterovirus Species: Citrus tristeza virus

11. VECTOR CTV is a virus that is limited to the phloem tissues of its host. It is transmitted semi-persistently by vectors that penetrate the phloem to extract sap, mostly the aphid species that colonize the crop.  The brown citrus aphid is considerably more efficient at transmitting the virus than are other aphids that infest citrus.

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13. MODE OF SPREAD The aphids require at least 30 to 60 minutes of feeding to acquire the virus, and remain viruliferous for at least 24 hours.  T. citricida  is much more efficient than the other aphids, and it can transmit CTV strains causing severe stem pitting or decline that the other aphids cannot vector. The main cultural practice that increases the severity of the CTV is when the citrus trees are grafted onto the sour orange rootstock. Using CTV infected budwood for grafting can transfer the CTV from the original tree to the new one.

14. PATHOGEN CHARACTERS CTV is a flexuous rod virus with dimensions of 2000nm long and 12nm in diameter. The CTV genome is typically between 19.2 and 19.3 kb long and consists of a single strand of RNA enclosed by two types of capsid proteins. The size of genome makes CTV one of the largest RNA viruses known. The CTV genome consists 12 open reading frames which could encode atleast 17 proteins

15. MANAGEMENT-PREVENTIVE MEASURES Check quarantine regulation in the area to avoid transmission of the disease. Use resistant rootstock of e.g. trifoliate orange, Sunki , and Shiikuwasha have been used for many years. Some hybrids, such as Troyer citrange or Swingle citromelo , show promise as resistant root-stock. Keep nurseries and greenhouses free of insect vectors. Do not transport suspicious citrus material to other farms.

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17. BIOLOGICAL CONTROL: Some field experiments with parasitoid wasps or gall midges are ongoing, which could naturally control some of the aphids in citrus grooves. Use commercially available formulations (natural pyrethrum, fatty acids), insecticidal soaps or horticultural oils (plant or fish oils) to control populations. Aphids can also be wiped out by spraying the leaves of the plant with a mild solution of water and a few drops of detergent.

18. REFERENCES Tripathi . D.P., Introductory Plant Virology , Kalyani Publishers, New Delhi.pg:351-353 Thind . T.S., Diseases of fruits and vegetables and their management , Kalyani Publishers , New Delhi.pg:70-82