General resistance to CTV has been found in Poncirus
trifoliata (L.) Raf., and strain-specific resistance
in Citrus grandis (L.) Osb. and in some citrus relatives,
as Fortunella crassifolia Swing. and Severinia
buxifolia (Poir.) Tenore among others. A resistance
gene from P. trifoliata has been characterized and
mapped (Gmitter et al. 1996; Mestre et al. 1997a;
Fang et al. 1998), however, because of the complex
genetics of citrus, it is extremely difficult to introgress
this resistance gene into citrus varieties. On the other
hand, cloning of this gene is under way in several
laboratories (Deng et al. 2000; Yang et al. 2001).