| Citrus tristeza is found in worldwide citrus production areas, which is among the most servere virus disease. The pathogen Citrus tristeza virus (CTV) is a member of genus Closterovirus, family Closteroviridae.Now, majorly three CTV strains, including quick decline, stem pitting and mild strains have been identified based on the virus-induced symptoms on host citrus plants. Four CTV genotyps T36, VT, T3 and T30 also can be identified based on the divergence of their genome sequence. Among these genotypes, T30 is a mild strain and others belong to servere strains. In order to understand the strain and genotype constitution of CTV population from China, CTV infected samples were collected from some citrus production areas in China and analysed. The results are as followings.The efficiency of three RT-PCR methods, including a universal RT-PCR using total RNA as temple, imuno-capture RT-PCR (IC-RT-PCR) and tuber-capture RT-PCR (TC-RT-PCR), were comparied for the detection and strain differentiation of CTV.Results showed that the virus could be efficiently detected by all these three RT-PCR methods. However, the amplified products of RT-PCR using total RNA as temple had more intensive electrophoresis bands. Both IC-RT-PCR and TC-RT-PCR were not good for the strain differentiation by Bi-RT-PCR.Totally 141 citrus samples were collected and Bi-RT-PCR was used for the identificaion of CTV strains.Result showed that 107 citus samples were positive for CTV, among 75.8%.All detected CTV isolates belonged to the severe strains.The CTV CP genes were amplifed and the obtained products were subjected to Hinfâ… /RFLP. Based on the grouping rules from Gillings(1993), CTV-S45 was identifed as Hinfâ… /RFLP groupâ…¤. Samples LR1,S36, S37, S38 and S39 were mixed infected by CTV isolates belonging to Hinfâ… /RFLP groupsâ…¢andâ…¤. The CTV-S46 was a mixture of CTV isolates belonging to Hinfâ… /RFLP groupsâ…§andâ…¤.The genotypes of the CTV isolates from the above samples and 2 previously reported isolates were analysed by RT-PCR using 10 sets of primers targeted to the specific gene regions of different CTV genotypes.Results showed that only N21 and N4 were belonged a single VT genotype and T30 genotype, respectively. All others were mix-infected by CTV isolates belonging to more than two genotypes.Among these samples, B15,B16,S36,S39 and LR1 were mix-infected by CTV isolates belonging to VT,T36 and T3 genotypes, BX and S40 could be mix-infected by CTV isolates belonging to four genotypes VT, T36, T3 and T30.The isolate CTV-S45 showed the genotypes of T3 and T30. The obtained results indicates that the infection of CTV genotypes VT and T3,which are servere strains, are common in cultivted citrus in China.The amplified products usinng the primer set VT-K17 were cloned and sequenced. The phylogenectic tree constructed based on their nucleotide sequences showed that all these CTV isolates were clustered into two groups, belonging to VT and T30 genotype. Two isolates CTV-S45-2 and CTV-S45-4 from sample S45 were clustered into a branch with mild strain T30 and T385,which shared 96.6%(nt) and 94.9%(aa) identities. All other isolates were clustered into large branch with the sereve strain VT and three sub-groups were visualized in the brach due the high divergencities among these isolates. Sequence comparison also illustrated some sub-group specific nucleotide acid and amino acid sits.Forthermore, the genes CP, K17 and Pol and 5'UTR from the CTV-S45 were sequenced. Sequence analysis results showed that the deduced amino acid sequnce of CP gene had a high similarity with T30, the CP marked amino acid sites (T63, V122 and F124), which were used for the indentificatiom between CTV mild and sereve strains, were, the same as those from sereve strains. However, its K17 and Pol and 5'UTR regions showed the types from both mild and sereve strains. These results indicated that gene recombination between different genotypes might occourred or it was an isolate with specific molecular characteristics.
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