Citrus is a major fruit tree crop in the world, and its
100 million tons annual production represents 25%
of the world fruit production (FAO 2001). Citrus
tristeza virus (CTV), a member of genus Closterovirus,
is the causal agent of the most economically important
viral disease of this crop. CTV produces two
main field diseases depending on the isolates: common
isolates cause decline and death of most scion
varieties grafted on sour orange (Citrus aurantium L.)
rootstock, whereas highly virulent isolates additionally
cause stem pitting on different scion varieties regardless
the rootstock, resulting in reduced vigor,
yield and fruit quality. CTV is considered a most serious
threat to the citrus industry world-wide (Bar-Joseph
et al. 1989).