Isolation,Identification,Whole Genome Sequencing And Bactericides Screening Of Xanthomonas Citri Subsp.Citri

Citrus bacterial canker is one of the most important quarantine diseases of citrus,mainly caused by Xanthomonas citri subsp.citri（Xcc）in China.In order to identify the pathogenic type of Xcc in Zhanjiang area and screen out effective bactericides,the pathogenic bacteria was isolated and identified from lemon leaves with suspected infection by Xcc.The whole-genome of the pathogen was sequenced through Nanopore three-generation sequencing platform.endophytic bacteria were isolated and identified from lemon leaves infected by Xcc based on the method of plate dilution,morphological observation and 16S sequence analysis.Plate confrontation test was conducted to detect the interaction between endophytic bacteria and Xcc,then the inhibitory effect of several commonly used bactericides on Xcc were examined by inhibiting zone method.1.The stain zlm1908 was isolated from the lemon leaves with suspected infection by Xcc.The stain zlm1908 was identified as Xcc by morphological observation,physiological and biochemical characteristics and molecular identification.The typical symtoms of citrus canker were showed on lemon and Hongjiang orange leaves by in vitro inoculation of stain zlm1908.2.Whole genome sequencing results showed that the genome size of stain zlm1908was 5.19 Mb,which contains a circular chromosome with 5,192,880 bp and two circular plasmids with 34,099 bp and 119,114 bp.The genome GC content is 64.67%.The number of CRISPR,gene islands,t RNA,prephages and plasmids in gemone of strain zlm1908were significant different from strain jx-6 from Jiangxi province by comparative genomic analysis.3.42 endophytic bacteria strains were isolated from lemon leaves infected by Xcc,and all strains were divided into 14 different types.The strain of Bacillus amyloliquefaciens,which had a strong inhibition effect of Xcc was screened and identified from endophytic bacteria by the plate antagonism test.The strains BL,SN and KC preserved in the laboratory showed strong inhibition effect on the strain zlm1908.The average diameters of bacteriostatic circle of BA,BL,SN and KC to strain zlm1908 weremm,mm,mm andmm,respectively.4.The inhibition results of different bactericides on strain zlm1908 showed that the EC₅₀of 46%Copper hydroxide WG,27.12%Copper dihydroxosulphate SC,Bacillus subtilis WP,12%Zhongshengmycin WP and 50%Chlorobromoisocy-anurate PX were18.45 g/L,14.99 g/L,5.28 g/L,14.60 g/L,32.81 g/L,respectively.According to the analysis of EC₅₀va Iue,the strongest toxic effect to Xcc was Bacillus subtilis WP,followed by 12%Zhongshengmycin WP,27.12%Copper dihydroxosulphate SC,46%Copper hydroxide WG and 50%Chlorobromoisocy-anurate PX.In conclusion,the pathogen was identified by morphological observation and molecular biology,the whole genome was sequenced and the screening of effective bactericides and antagonistic strains was carried out.The aim of this study was to identify the type of citrus canker pathogen in Zhanjiang area and to provide theoretical basis for field control of citrus canker disease.