Photographer: Elizabeth Grafton-Cardwell Affiliation: University of California Source: http://www.idtools.org/id/citrus/diseases/ Copyright: Public Domain
CTV is in the genus Closterovirus and it is limited to the phloem tissues of its host. It is introduced to the plants by the brown citrus aphid (Toxoptera citricida). CTV is a single strand of (+)-sense RNA enclosed by two types of capsid proteins. Farmers in Brazil and other South American countries gave it the name "tristeza", meaning sadness in Portuguese and Spanish, because of the devastation this disease has caused. Symptoms of CTV infestation can be highly variable and are affected by environment, species of Citrus and the aggressiveness of the CTV isolates. Overall symptoms include: quick decline (pictured left), stem/trunk pitting, seedling yellowing, poor fruit quality (pictured right) and loss of chlorophyll in leaves.
Host Plant: All variations (hybrids, etc.) of Citrus species. In general, Mandarins are mainly tolerant of CTV infection but Sour Oranges and Alemow are highly susceptible.
CTV is transmitted most efficiently by the invasive brown citrus aphid and infects all species, cultivars and hybrids of citrus. Some strains can affect a tree quickly and show severe symptoms while, some strains can be asymptomatic and can remain undetected. Grapefruits, limes and sweet oranges are most susceptible to stem pitting strains and quickly produce declining fruits (grapefruits, pictured right). Once CTV is present, it severely reduces crop yield and has a major economic impact on the citrus industry and its local farmers. Unfortunately, humans can quickly spread citrus tristeza virus faster and further than any aphid. One pickup truck of infested nursery stock can spread the virus several hundred miles in a few hours from nursery sites to grove plantings.
For more information on the brown citrus aphid click here
This closterovirus penetrates the host cell and proceeds to release the viral genomic RNA into the cell’s cytoplasm. Once the viral genomic and sub-genomic RNA is translated it is integrated into the plant it causes the expression of genes that are normally suppressed. The change in gene-suppression affects the phloem and causes the symptoms that destroy citrus crops. Severity of infection depends on how well viral gene products interact with the specific Citrus hosts and CTV infections in nature seem to be populations of different CTV strains.
In the 1940s, a devastating wilt and decline disease of sweet orange trees and sour orange rootstock was discovered in citrus-growing areas of the world. At the same time, it was observed that aphids collected from the declining trees transmitted stem-pitting disease to grapefruit and lime seedlings. It was later realized that the cause of the quick decline and stem pitting of citrus species was the tristeza closterovirus.
Besides the brown citrus aphid, CTV can be spread by several other aphid species including Aphis gossypii and sometimes by Aphis spiraecola. Even though A. spiraecola is less efficient, it tends to have higher populations and can transfer the virus fairly well. Due to several vectors CTV has been able to spread from Florida, all the way west to Texas and California. However, with more severe strains and the presence of the brown citrus aphid, its threat is of great importance in Florida.
Native Origin: possibly Asia and Mediterranean
U.S. Habitat: Throughout citrus-growing areas.
U.S. Present: AZ, CA, FL, HI, LA and TX
CTV can be managed by the use of CTV-tolerant rootstocks. However, there are still CTV strains that are far more resilient and can still cause severe symptoms in tristeza-tolerant plants.
Preventing the spread of the brown citrus aphid can also help prevent CTV spreading farther; but in the end humans can spread this virus much more quickly than aphids. The most successful way to prevent the spread of CTV is to prevent the establishment of the disease by focusing on cultural controls. Firstly, budwood and nursery stock are must be free of disease. In Florida, the department's budwood protection program periodically tests all sources of propagating material for CTV annually and, where sources testing positive for severe strains of CTV will no longer be permitted to be used for propagation. Having a known clean pool of propagating material and protecting the gene pool will help ensure that humans are not spreading the pathogens farther.
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