Present status of citrus Huanglongbin and genetic diversity of Candidatus Liberibacter asiaticus in Guangxi

【Objective】To investigate the hazards of Huanglongbing（HLB）and the genetic diversity of Candidatus Liberibacter asiaticus（CLas）in citrus producing areas of Guangxi，so as to provide reference for field diagnosis and scientific prevention and control of Huanglongbing.【Method】A two-year（2020 and 2021）follow-up survey was carried out in representative citrus orchards in 8 cities in the main citrus producing areas of Guangxi，and 1048 citrus leaf samples were randomly collected. The collected samples were detected for CLas by qPCR technology，and the amplified products of the hypervariable regions of the genome of CLas strains in HLB positive samples were sequenced to analyze the genetic diversity of the CLas strains.【Result】Compared with the citrus samples collected in 2020，the detection rate of CLas in citrus samples collected from Yulin City，Guilin City，Hechi City，Wuzhou City，Nanning City，Hezhou City and Laibin City showed an upward trend in 2021，among which the detection rate of CLas in the citrus samples collected from Wuzhou City was the highest（52.94%），while the detection rate of CLas in the citrus samples from Baise City remained unchanged（14.29%）in the 2 years. There were great differences in the detection rate of CLas among different citrus varieties. The detection rates of CLas in sweet orange，pomelo，hetero citrus and kumquat samples in 2021 was higher than those in 2020，and the detection rate of CLas in mandarin orange in the two years remained unchanged（14.29%）. There were also great differences in the detection rate of CLas in citrus samples with different symptom types. Among them，the detection rate of CLas in chlorosis deficient samples collected in 2021 was the highest（61.64%），followed by uniform chlorosis（24.00%）and chlorosis mottled（20.00%），and the detection rate of CLas in asymptomatic samples was also as high as 11.54%，all of them were higher than those in 2020. By sequencing the PCR products of the hypervariable regions of the CLas genome，it was found that CLas strains had rich genetic diversity. The 37 CLas strains could be clustered into 7 groups，which were related to the species of the host citrus，but they had no obvious correlation with symptom types and geographical origin.【Conclusion】The detection rate of CLas varies greatly in different years，geographical sources， citrus species and symptom types，and the incidence rate has an increasing trend year by year. The plants with element deficiency chlorosis in the field are most likely to be diagnosed with HLB，followed by uniform yellowing and chlorotic mottle. Asymptomatic plants also have a high probability of being infected with CLas. CLas strains have rich genetic diversity. It is related to the host citrus varieties at certain degrees，but there is no obvious correlation with the symptom types and geographical origin.