Abstract: Jincheng sweet orange seedlings were pre-inoculated with CTV mild isolate CT11 by grafting，and seven months later，challenged by severe CTV isolate CT14 with Toxoptera citricida. To identify and quantitate the mild and severe CTV isolates within the same plant，a real-time RT-PCR technique was modified and housekeeping genes 18S rRNAF/R and nad5F/R were approached for rectification. The results showed that CT14 was not detectable in any challenge-inoculated plants three months after CT14 attacked through aphids. The titer of CT11 in most samples of the plants challenge-inoculated via 25 or 50 aphids per plant was higher than that in the pre-immunized control plants on the 45th or 10th day，respectively，then decreased to the same level of the pre-immunized control，whereas the titer of the mild isolate in the pre-immunized control plants was much stable.

Key words: Citrus, Citrus tristeza virus, real-time RT-PCR, mild strain cross protection