Citrus - Tristeza

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Tristeza (citrus tristeza virus)Thumbnail of Tisteza

The virus infects most types of citrus and it is transmitted by certain aphids or by the use infected budwood. The symptoms produced depend upon the variety, environmental conditions, and the virus strain. The virus can cause a decline [Picture ] or death of the tree [Picture]. Mild strains were identified in Texas in 1991, although tristeza was also detected in the 1950's. There are several serious strains that are not known to occur in Texas. Three strains are of particular concern: one causes the decline of sweet orange on sour orange by killing phloem, resulting in girdling at the bud union; and the stem pitting strains (one on grapefruit, the other on sweet orange). The stem pitting strains causes limbs of trees to become brittle, forms deep pits on the underside of bark, and a roping appearance of the trunk that results in a decline of the tree. Although the tree doesn't die, there is reduced set, size and quality of fruit.

The virus is inefficiently vectored by three species of aphids present in Texas: Aphis gossypii, A. spiraeola, and Toxoptera aurantii. It is very efficiently vectored by the brown citrus aphid, Toxoptera citricida, which is present in Florida, but not in Texas. If the brown citrus aphid arrives in Texas, it could move severe strains of tristeza suspected to be present on symptomless hosts in dooryard (i.e. non-commercial) trees to commercial orchards, potentially destroying the Texas industry. This is because sour orange rootstock, which is predominantly used in Texas, is susceptible to severe strains.

Control efforts in Texas are mainly directed toward keeping infected budwood out of the Lower Rio Grande Valley by quarantine measures and a shoot tip grafting and budwood certification program to clean up infected material already in Texas and place it in commercial orchards. Should these measures fail, a proposed alternative control is cross-protection of trees with mild strains of the virus and/or alternative rootstocks which may tolerate the disease.