Citrus tristeza virus (CTV) is a viral species of the genus Closterovirus that causes the most economically damaging disease to its namesake plant genus, Citrus. 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. The pathogen CTV is a flexuous rod virus with dimensions of 2000 nm long and 12 nm in diameter. The CTV genome is typically between 19.2 and 19.3 kb long and consists of a single strand of (+)-sense RNA enclosed by two types of capsid proteins. The size of its genome makes CTV one of the largest RNA viruses known. The CTV genome contains 12 open reading frames, which could encode at least 17 proteins. Hosts and symptoms CTV infects several species of the plant genus, Citrus, including sour orange (Citrus aurantium) and any Citrus grafted onto sour orange rootstock, key lime and Seville orange (C. × aurantifolia), Hassaku orange and sweet orange (C. × sinensis), grapefruit (C. × paradisi), and mandarin (C. reticulata). CTV is also known to infect Aeglopsis chevalieri, Afraegle paniculata and Pamburus missionis of the citrus subfamily Aurantioideae, as well as Passiflora gracilis which belongs to an entirely different lineage of rosid plants. CTV is distributed worldwide and can be found wherever citrus trees grow. Symptoms of CTV infection are highly variable and depend on several factors including host, virulence of the particular virus strain, and environmental conditions. The three most common groupings of symptoms are decline (quick and slow), stem-pitting, and seedling yellows. 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. Decline may be slow, lasting several months to years after the first symptoms are noticed. 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. The decline may also be quick, resulting in host death just days after the first symptoms are noticed. Stem-pitting is another symptom of CTV that manifests