title: Development and systematic validation of qPCR assays for rapid and reliable differentiation of Xylella fastidiosa strains causing citrus variegated chlorosis.
abstract: The xylem-limited, Gram-negative, fastidious plant bacterium Xylella fastidiosa is the causal agent of citrus variegated chlorosis (CVC), a destructive disease affecting approximately half of the citrus plantations in the State of São Paulo, Brazil. The disease was recently found in Central America and is threatening the multi-billion U.S. citrus industry. Many strains of X. fastidiosa are pathogens or endophytes in various plants growing in the U.S., and some strains cross infect several host plants. In this study, a TaqMan-based assay targeting the 16S rDNA signature region was developed for the identification of X. fastidiosa at the species level. Another TaqMan-based assay was developed for the specific identification of the CVC strains. Both new assays have been systematically validated in comparison with the primer/probe sets from four previously published assays on one platform and under similar PCR conditions, and shown to be superior. The species specific assay detected all X. fastidiosa strains and did not amplify any other citrus pathogen or endophyte tested. The CVC-specific assay detected all CVC strains but did not amplify any non-CVC X. fastidiosa nor any other citrus pathogen or endophyte evaluated. Both sets were multiplexed with a reliable internal control assay targeting host plant DNA, and their diagnostic specificity and sensitivity remained unchanged. This internal control provides quality assurance for DNA extraction, performance of PCR reagents, platforms and operators. The limit of detection for both assays was equivalent to 2 to 10 cells of X. fastidiosa per reaction for field citrus samples. Petioles and midribs of symptomatic leaves of sweet orange harbored the highest populations of X. fastidiosa, providing the best materials for detection of the pathogen. These new species specific assay will be invaluable for molecular identification of X. fastidiosa at the species level, and the CVC specific assay will be very powerful for the specific identification of X. fastidiosa strains that cause citrus variegated chlorosis.
authors.affiliation: USDA-APHIS-PPQ-CPHST National Plant Germplasm and Biotechnology Laboratory, Beltsville, MD 20705, United States. Wenbin.Li@aphis.usda.gov
authors.forename: Wenbin
authors.initials: W
authors.lastname: Li
authors.forename: Diva C
authors.initials: DC
authors.lastname: Teixeira
authors.forename: John S
authors.initials: JS
authors.lastname: Hartung
authors.forename: Qi
authors.initials: Q
authors.lastname: Huang
authors.forename: Yongping
authors.initials: Y
authors.lastname: Duan
authors.forename: Lijuan
authors.initials: L
authors.lastname: Zhou
authors.forename: Jianchi
authors.initials: J
authors.lastname: Chen
authors.forename: Hong
authors.initials: H
authors.lastname: Lin
authors.forename: Silvio
authors.initials: S
authors.lastname: Lopes
authors.forename: A Juliano
authors.initials: AJ
authors.lastname: Ayres
authors.forename: Laurene
authors.initials: L
authors.lastname: Levy
mesh_terms: D001431:Bacteriological Techniques / Q000379:methods* / Q000592:standards; D001938:Brazil; D002489:Central America; D002957:Citrus / Q000382:microbiology*; D017931:DNA Primers / Q000235:genetics; D015345:Oligonucleotide Probes / Q000235:genetics; D010935:Plant Diseases / Q000382:microbiology*; D012329:RNA, Bacterial / Q000235:genetics; D012336:RNA, Ribosomal, 16S / Q000235:genetics; D060888:Real-Time Polymerase Chain Reaction / Q000379:methods* / Q000592:standards; D012015:Reference Standards; D012680:Sensitivity and Specificity; D014481:United States; D044167:Xylella / Q000145:classification* / Q000235:genetics / Q000302:isolation & purification*
chemical_list: D017931:DNA Primers; D015345:Oligonucleotide Probes; D012329:RNA, Bacterial; D012336:RNA, Ribosomal, 16S