| Hunan is one of the most important citrus producers in China, but its citriculture suffers from several virus diseases such as Citrus Tristeza Virus (CTV), Citrus Tatter Leaf Virus (CTLV), Citrus Exocortis Viroid (CEVd), Satsuma Dwarf Virus (SDV) etc. Due to widely utilization of CTV-resistant trifoliate rootstock, up to now CTV has not caused severe damages to the citrus industry, even though CTV is quite popular in the province. However, as the strong CTV strain's infection, some economic losses in citrus have been demonstrated. Meanwhile, trifoliate is sensible to CEVd and CTLV, which will be more dangerous to citricultrue. The only effective way to control citrus viral diseases is to establish virus free system. A quick and reliable method for virus identification is essential for such system. Present study aims at the establishment of a quick and reliable technique for virus identification, to sequence various viral isolates in order to isolate local mild CTV strains.The main results of the thesis were as follow:1. A quick and reliable method was established for the identification of CTV, CEVd, and CTLVThe leaves were sampled from the index plants showing typical symptoms of CTV and CEVd and from plants indicating typical CTLV symptom to isolate RNA and RT-PCR method was set up for CTV, CEVd, CTLV identification.By using this RT-PCR technique, samples from plants showing typical symptoms in the citrus orchard and from the virus free cultivars were checked for the presence of CTV, CEVd and CTLV. The results confirmed the reliability and reproducibility of the molecular identification system.2. Sequence analysisA fragment of the p33 gene of CTV was sequenced and the sequence blast analysis of 21 CTV isolates indicated 15 sequence variants, which were divided into four clusters-A,B,C and D in the phylogenetic tree.Sequence analysis of two CEV isolates showed that the isolate CEVd-HN was identical to the isolate CEVd-HB, while the isolate CEVd-YN had 97.85% of similarity...
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