Submitted to: Plant Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/22/2007
Publication Date: 7/26/2007
Citation: Deng, X., Chen, J., Shan, Z., Zhou, G., Li, H., Civerolo, E.L. Identification of Candidatus Liberibacter asiaticus from Foshou (Citrus medica) in China. Plant Pathology. New Disease Report, 15:Article July 2007. Available: http://www.bspp.org.uk/ndr/july2007/2007-66.asp Interpretive Summary: The Huanglongbing (HLB) pathogen was identified in Foshou (Buddha’s hand) Citrus medica var. sarcodactylus Swing, for the first time in southern China. The pathogen was characterized through molecular means as Candidatus Liberibacter asiaticus. The report is significant because Foshou is a valuable crop and infected Foshou could potentially serve as source of inoculum for HLB in other citrus crops.
Technical Abstract: In China, Foshou (Buddha’s hand), C. medica var. sarcodactylus Swing, is commonly cultured and valued for its fragrance and used medicinally as a stomach, stimulant, expectorant and tonic. Since 1999, we noted that Foshou succumbs to Huanglongbing (HLB) or yellow shoot disease with incidence up to 40%. The infected trees can also serve as a source of primary inoculum for other commercial citrus varieties. So far, HLB in Foshou is described only by symptomatology. The etiological agent, Candidatus Liberibacter sp. a noncultureable, phloem-limited alfa-proteobacterium, has not yet been identified and characterized according to recent taxonomy information. In January of 2006, we recognized 9 Foshou trees showing typical HLB symptoms (leaf mottling and yellowing in young shoots) in an orchard in Guangning city, Guangdong province, People’s Republic of China. To identify the pathogen, DNA was extracted from symptomatic leaves with the cetyltrimethylammoniumbromide (CTAB) method. Samples were tested for Ca. L. asiaticus by PCR with primer sets OI1/OI2c targeting the 16S rDNA locus and A2/J5 targeting the locus of beta-operon of ribosomal proteins. DNA samples from all symptomatic leaves were positive with both primer sets. OI1/OI2c generated a ca. 1,160 bp amplicon. Further digestion of the amplicon by XbaI yielded two DNA fragments of ca. 640 bp and 520 bp and A2/J5 generated a 703 bp amplicon, excluding Ca. L. africanus. The positive control using a sweet orange (C. sinensis) tree known to have HLB showed the same result. No DNA was amplified from the asymptomatic healthy Foshou tree. Sequences of OI1/OI2c and A2/J5 amplicons were determined and showed 99.6-100% similarity to those of Ca. L. asiaticus in the GenBank database. No DNA was amplified by PCR with primer set GB1/GB2 specific to Ca. L. americanus. These results indicate that the Foshou HLB etiological agent is Ca. L. asiaticus. This is the first report of molecular identification of Ca. L. asiaticus associated with C. medica HLB in southern China.