Etiology, background, worldwide situation and control of Citrus Tristeza virus and its vectors

Authors: Yokomi, Raymond - Ray

Submitted to: Symposium Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 8/15/2011
Publication Date: 8/15/2011
Citation: Yokomi, R.K. 2011. Etiology, background, worldwide situation and control of Citrus Tristeza virus and its vectors. Symposium Proceedings. available: http://www.senasica.gob.mx/includes/asp/download.asp?iddocumento=24041&idu

Interpretive Summary: Citrus tristeza virus (CTV) is the most important virus disease of citrus worldwide accounting for the killing of >85 million citrus trees grown on sour orange rootstock which is extremely sensitive to severe CTV strains. In many areas, citrus must be grown on CTV-tolerant or resistant rootstocks, some of which are susceptible to other graft-transmissible citrus pathogens; however, most citrus in the Mediterranean region, Mexico, Central America, Caribbean Basin and Mexico are still grown on sour orange rootstock and remain vulnerable to CTV-decline. Quarantines, certification and clean stock programs were developed in many countries to propagate and maintain virus- and virus-like-free plants from the nursery. Diverse CTV strains range from mild to severe depending upon cultivar. These symptoms include: asymptomatic (mild); decline-inducing on sour orange rootstock; stem pitting in sweet orange and grapefruit; and seedling yellows. Stem pitting and seedling yellows are scion diseases and cannot be controlled by rootstock. CTV is reliably detected by enzyme-linked immunosorbent assay or polymerase chain reaction methods. The aphid Toxoptera citricida transmits CTV with high efficiency and feeds exclusively on citrus and citrus relatives. The principal vector in the absence of T. citricida is Aphis gossypii that has a wide host range and is less efficient in CTV transmission. Epidemics resulting from CTV transmission by T. citricida occur within 2 to 4 years; whereas those from A. gossypii can take 8-15 years. In areas of low CTV incidence with slow spread, eradication of CTV-infected trees has proved useful. Where T. citricida and stem pitting strains of CTV are prevalent and stem pitting-sensitive cultivars are grown, mild strain cross-protection is used to prolong economic life of trees. Pesticide control of vector populations is not useful in controlling CTV spread. Populations of T. citricida in Florida are maintained at low levels by generalist predators which have adapted to the aphid. Undoubtedly, these low levels also can be attributed, in part, to the rigorous spray program in citrus to control the Asian citrus psyllid. Long-term control strategies for CTV should include protection and conservation of aphid natural enemies.

Technical Abstract: Citrus tristeza virus (CTV) is readily graft-transmissible and, in nature, is spread by aphid vectors in a semi-persistent manner. CTV-decline has killed >85 million citrus trees grown on sour orange rootstock worldwide. Citrus in these areas must be grown on CTV-tolerant or resistant rootstocks. Most citrus in the Mediterranean region, Mexico, Central America, Caribbean Basin, Mexico and Texas are still grown on sour orange rootstock and remain vulnerable to CTV-decline. Quarantines, certification and clean stock programs were developed in many countries as a consequence of CTV epidemics. Thus, CTV control begins with propagation and use of virus- and virus-like-free plants from the nursery. The genome of CTV is one of the largest of known plant viruses and its genomic variation results in diverse strains which range from mild to severe depending upon cultivar. These symptoms include: asymptomatic (mild); decline-inducing on sour orange rootstock; stem pitting in sweet orange and grapefruit; and seedling yellows. Stem pitting and seedling yellows are scion diseases that cannot be controlled by rootstock. Reliable pathogen detection is by enzyme-linked immunosorbent assay or polymerase chain reaction methods. Toxoptera citricida is the most dangerous vector due to high efficiency of CTV transmission and a host range restricted to plants in the Rutaceae family. In contrast, the principal CTV vector in the absence of T. citricida is Aphis gossypii. A. gossypii has a wide host range and is less efficient in CTV transmission. These relationships result in significantly different disease dynamics. T. citricida-driven epidemics occur within a period of 2 to 4 years; whereas that from A. gossypii can take from 8-15 years. In areas of low CTV incidence with slow spread, eradication of CTV-infected trees has proved useful. Where T. citricida and SP strains of CTV are prevalent and SP-sensitive cultivars are grown, mild strain cross-protection is used to prolong economic life of trees. Pesticide control of vector populations has proven useless in controlling local CTV spread. After the establishment of T. citricida in Florida in late 1995, populations were very large until 1998 when generalist predators adapted to the aphid; subsequently were aphid populations were reduced. These low aphid levels can be attributed, in part, to the rigorous spray program for the control the Asian citrus psyllid. Long-term control strategies for CTV should include protection and conservation of aphid natural enemies.