Occurrence Dynamics Of Citrus Huanglongbing In P.R. China And Population Differentiation Of ’candidatus Liberibacter Asiaticus’

Citrus Huanglongbing （HLB） is a highly destructive disease for citrus industry, and has been reported in nearly50countries or regions located in Asia, Africa, Oceania, North America and South America. In China,11provinces have suffered damage from HLB among the19citrus producing areas. HLB has become one of the key factors to restrict the sustainable development of citrus industry. In the past decade, the distribution area of HLB insect vector is gradually enlarged with climate change of the global warming. Meanwhile, along with the transportation of nursery trees or budwood without restrictive guarantine procedure, the damage caused by HLB is becoming serious. The earliest record of HLB symptom was in India central regions in the18th century, and Chaoshan district in the year1919in Guangdong, China. The first report of citrus psyllid was mentioned in1927in Punjab, and it’s not untill1970s that people began to relize the relationship between citrus psyllid and HLB in China.The pathogen of HLB belongs to ’Candidatus Liberibacter’, which is divided into three species-’Ca. L. asiaticus’,’Ca. L. africanus’, and ’Ca. L. americanus’, according to the sequence differences on16S rDNA and β-operon.’Ca. L. asiaticus’ and ’Ca. L. americanus’ are spread by Diaphorina citri, while’Ca. L. africanus’ by Trioza erytreae. Because of the inability to successfully culture the pathogen in vitro, the biological, physiological and biochemical characteristics are poorly understood till now. Furthermore, due to the lack of effective chemicals and disease-resistant cultivars, planting disease-free citrus trees, removing HLB-infected trees in time, and strictly controlling citrus psyllid, are mainly used in the field to prevent and control HLB.Researches on the occurrence dynamics of HLB will help to understand the origin and epidemiology of the disease, as well as the population characteristics of the pathogen. Therefore, it’s very necessary to realize the history of HLB in each endemic province. As the information about the disease progress is currently limited, this study has preliminarily figured out the dynamics of HLB in the11provinces which suffered from HLB, and also pointed out the possible cause in each of them. Meanwhile, populations diversity was investigated on3loci which have been reported on the genome of’Ca. L. asiaticus’. This help to further understand the origin and epidemiology of HLB in gene level. This research was supported by MOA’s Public Benefit Research Foundation of China （201003067-02;200903004-06）. The main results are as follows:1. Based on the literatures collected, the occurrence dynamics of HLB in the endemic provinces were preliminarily approached. Also the possible causes were analyzed, according to which, these endemic provinces were roughly divided into two types. One is due to seedlings/nursery trees/budwood transportation, and the other is for these areas where might be origin centers themselves. The reason of this kind of classification was explained, and a scientific point that more than one HLB origin center may exist in China were proposed.2. Two phage-related polymorphic loci （CLIBASIA₀5538and CLIBASIA₀5620） were used to evaluate the intra-specific differentiations of China ’Ca. L. asiaticus’ populations,232HLB strains/isolates have been studied. Conventional PCR amplification statistics indicated that in China, amplification frequency of the’Ca. L. asiaticus’population from a high altitude region was different from that of ’Ca. L. asiaticus’ population from a low altitude region. Sequence alignment and phylogenetic tree showed that high and low altitude population were respectively related to two Liberibacter Phage SCI and SC2recently reported, variation also existed within one population, especially in Yunnan and Sichuan. Consequently,1Ca. L. asiaticus’ population from a high altitude region seemed to be more varied and special than that from a low altitude region.3. One mosaic locus of prophage origin （CLIBASIA₀5640-CLIBASIA₀5650） in the genome of’Ca. L. asiaticus’, which was recently reported, was selected to study the polymorphism,207HLB strains/isolates have been addressed. Based on the characteristics of PCR amplifications, three electrophoretic sub-types, defined as E-type, were found. Sequence analyses indicated that each amplicon had the same framework, and the genomic region studied was a prophage origin and included nucleotide insertions/deletions. Sequence alignment and the distribution of E-type indicated that the prophage was more active and strains were also more complex in high altitude regions, like Yunnan and Sichuan, than that in low altitude regions. In summary, this research was combined with the order of historical documents and technology of molecular biology, showed a highly diversity of Chinese’Ca. L. asiaticus’. More than one HLB origin center may exist in China, or differentiation was generated among the different geographical origins or habitats in the process of spread.