Abstract: To understand the spread characteristics of HLB pathogen in citrus at early infection stage，the populations of the Candidatus Liberibacter asiaticus（Las）pathogen，the symptom development in the susceptible‘Jincheng’Orange（Citrus sinensis Osbeck）were investigated using leafdisc grafting during the first eighty-four days of infection. Quantitative real-time PCR（qPCR）analysis showed that the presence of Las was firstly detected in the proximal midrib 49 days after grafting，and until 77 days after grafting，the pathogen was found in the distal midrib，petiole and the marginal tissue around the leafdisc. Using the proximal midrib as the primary infection tissue，the early spread of Las pathogen in the leafdisc-grafted leaf can be grouped into three phases from one to eighty-four days after Las inoculation：lag phase（1–42days），logarithmic phase（43–70 days）and stationary phase（71–84 days）. The results showed that the pathogen spreads mainly along the midrib from the proximal tissue to the distal tissue in the grafted leaves，which was in accordance with HLB symptoms spread out from the proximal midrib. The study also displayed that leaf-disc grafting was a powerful tool for the transmission of HLB pathogen in citrus.

Key words: citrus HLB, early spread, leafdisc grafting, qPCR