Photo 2. Yellowing of leaves, general dieback and decline of orange trees affected by Citrus tristeza virus.
Citrus quick decline virus, grapefruit stem pitting; tristeza
Citrus tristeza virus, Citrus tristeza closterovirus. The abbreviation is CTV. The word "tristeza" is from Portuguese meaning "sadness".
Worldwide. Africa, Asia, North, South and Central America, Caribbean, Europe, Oceania. CTV is common in orchards in Southeast Asia, southern Africa, India, Japan, South America, North America (Florida and California) and Oceania. It is recorded from Australia, Cook Islands, Fiji, French Polynesia, Niue, Northern Mariana Islands, Papua New Guinea, Samoa, Tonga, Wallis & Futuna.
Citrus species and hybrids.
CTV is spread by aphids; those that are most important are Toxoptera citricida (brown citrus aphid), Toxoptera aurantii (black citrus aphid), Aphis gossypii (cotton or melon aphid) and Aphis spiraecola (green citrus aphid). Among the four, the brown citrus aphid is the most efficient, as it can spread strains that cause stem pitting that the others cannot. The aphids acquire the virus as they feed on the sap of infected plants; this takes 30-60 minutes. The ability to transmit is lost within 24-48 hours.
There are many strains; some show no symptoms, while others are extremely serious and result in death. Symptoms are divided into three types: (i) slow and quick decline; and (ii) stem pitting, and (iii) seedling yellows:
Symptoms of CTV become more obvious in the hotter summer months when water needs are greater and cannot be met by poor root systems.
Usually, aphids disperse only a few kilometres, either by flight or on the wind. However, long-distance spread of aphids is possible by tropical storms and cyclones.
Long-distance spread also occurs with the movement of nursery plants infected with CTV, or infested with CTV-infected aphids.
CTV is the most economically important disease of citrus worldwide, and responsible for enormous losses. Damage is worse for sweet orange, mandarin and grapefruit when grafted on to sour orange rootstocks. It is estimated that in Brazil and Argentina, 16 million citrus trees on sour orange rootstocks were killed by CTV after the introduction of the citrus brown aphid that spreads the virus. Major epidemics have also occurred in Peru and Venezuela, as well as in California and Florida. Citrus is commonly grafted onto sour orange, and there may be some 200 million trees still vulnerable to CTV worldwide.
Bark flaps cut across the graft union show small holes (honeycombing) on the inside face of the bark flap from the rootstock side of the union. Quick decline trees may only have a yellow-brown stain at the bud union, without honeycombing.
There are three methods of detection used:
There are many strains of CTV and they are not found in all countries; this is particularly so for stem-pitting strains. Therefore, it is important that introductions of citrus follow the FAO/IBPGR Technical Guidelines for the Safe Movement of Citrus Germplasm (http://www.bioversityinternational.org/uploads/tx_news/FAO_lBPGR_technical_ guidelines_for_the_safe_movement_of_citrus_germplasm_501.pdf). Where possible, introductions should be made as seed and, if vegetative natural is needed, it should be treated to remove possible virus infections, and then tested to make sure the treatments have been successful.
Certification schemes are used to control CTV. Nurseries, whether private or public, provide plants that have been established from budwood-source trees that have been screened for viruses, and grafted onto tolerant rootstocks.
Only trees on sour orange rootstock are affected by tristeza decline. Sweet orange is usually more affected than grapefruit, whereas lemons on sour orange rootstock are not affected by tristeza decline.
Poncirus trifoliata (trifoliate orange) is commonly used as a rootstock for citrus decline. Other CTV-tolerant root stocks are Rangpur lime, rough lemon and sweet orange. However, CTV is not the only consideration; there are other important diseases and environmental conditions that influence the choice of rootstock. For instance, if Phytophthora citropthora is a concern, trifoliata would be a good choice, but Rangpur lime, rough lemon, and sweet orange are susceptible. In addition, trifoliata has poor tolerance to salt, highly acid and alkaline soils and to drought.
As for scions, tangerines are generally tolerant of stem-pitting strains, and so are most varieties of mandarin, including Satsuma. However, there are severe strains that cause stem pitting, stunting, poor quality fruit, even on tolerant rootstocks. There are no CTV-tolerant limes.
Chemical control of aphids is not likely to stop the spread of CTV; this is because spread of the virus is fast, and occurs before the insecticide kills the aphid. However, chemical control of aphids may be beneficial in nurseries, and also to protect trees used as sources of budwood. (See Fact Sheet no. 38 - Melon aphid, for recommendations).
AUTHOR Grahame Jackson
Information from CABI (2015) Citrus tristeza virus (grapefruit stem pitting) Crop Protection Compendium. (http://www.cabi.org.cpc/). Photo 1 Richard Davis Northern Australia Quarantine Strategy, Department of Agriculture, Queensland, Australia.
Produced with support from the Australian Centre for International Agricultural Research under project PC/2010/090: Strengthening integrated crop management research in the Pacific Islands in support of sustainable intensification of high-value crop production, implemented by the University of Queensland and the Secretariat of the Pacific Community.
This fact sheet is a part of the app Pacific Pests and Pathogens
The mobile application is available from the Google Play Store and Apple iTunes.