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. 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. Diagnosis CTV is classically diagnosed by graft-inoculating a Mexican lime (Citrus aurantifolia) with tissue from a diseased plant. The Mexican lime will develop highly predictable symptoms. Symptoms on the leaves begin as clear veins that turn corky, which is then followed by chlorosis and cupping of the leaf. Particularly severe strains may result in stunting, and stem-pitting may also occur. Alternatively CTV can be indicated by the presence of aggregates of cross-banded inclusion bodies in the phloem of the diseased plant. Other diagnostic procedures include electron microscopy, double-antibody sandwich enzyme-linked immunosorbent assay (DAS-ELISA), tissue-print ELISA, and PCR-based assays. These methods identify signs of the virus, such as its microscopic structure (electron microscopy), the presence of its proteins’ antigens (ELISA) or the presence of its RNA (PCR). Epidemiology 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. In Florida, it has been shown to be from six to twenty five times as efficient as Aphis gossypii, the most efficient vector found in the state before the introduction of the brown citrus aphid prior to 1995. This efficiency is enhanced by the narrow host range of the brown citrus aphid and its tendency to produce winged forms in order to colonize new growth. A. gossypii has a much wider host range, including hundreds of plant species in Florida, and the transmission of the virus is blocked when it feeds on a different host. Environment Aphids are the main vector by which CTV is transmitted. Initially the United States had as vectors only Aphis gossypii (melon-and-cotton aphid), A.spiraecola (green citrus aphid), and the black (or brown) citrus aphid Toxoptera aurantii. These aphids transmit the virus much less efficiently than Toxoptera