The polymorphism of Citrus tristeza virus in Chongqing,China

Citrus tristeza virus (CTV) exists as a large number of distinct strains differing in biological characters in citrus.The control strategies such as mild strains cross protection (MSCP),require clearly understanding the constitution of CTV strains,and segregating the single strain from the mixture of strains by single aphid transmission.Restriction fragment length polymorphism (RFLP) and single-strand con- formation polymorphism (SSCP) assays on coat protein gene of 168 CTV isolates collected from 10 differ- ent places in Chongqing and two aphid transmission subisolates,demonstrated that the mixture of CTV strains often occur in the field trees.In this study,about 90% of the isolates analyzed contain those strains with CP/Hinf Ⅰ RFLP groups 1,3 and 6 which were classified as severe strains.Sweet orange samples contain more CP/Hinf Ⅰ RFLP groups than others.The efficacy of CTV by single brown aphid, Toxoptera citricida Kirkaldy,transmission from pummelo to sweet orange was very low based on our exper- iment (less than 1% ).And brown aphid may change the constitution of CTV isolate.