Up to date, breeding programs in citrus species
have shown poor results mainly because of their complex
reproductive biology, with many cases of apomixis
and cross- and self-incompatibility, their high
heterozigosity, and their long juvenile periods. Also
the mode of inheritance of most important agronomic
characters is largely unknown. Regarding CTV resistance,
only the tolerant rootstocks Troyer and Carrizo
citrange (C. sinensis (L.) Osb. x Poncirus trifoliata
(L.) Raf.), obtained in 1909 in a cross made to incorporate
cold tolerance to citrus fruits (Van Vuuren et
al. 1993), have reached wide use. They have practically
substituted sour orange rootstocks in certain citrus
areas, as California and Spain, where citrange
rootstocks perform well and CTV isolates are relatively
mild.