Detection of Xylophilus ampelinus in grapevine cuttings using a nested polymerase chain reaction

A sensitive and specific assay was developed to detect bacterial blight of grapevine caused by Xylophilus ampelinus (Panagopoulos, 1969) comb. nov. in grapevine cuttings. The 16S−23S rDNA intergenic spacer region of X. ampelinus was sequenced and pathogen-specific primers were designed from a region in the spacer between the tRNA (Ala) and the 23S genes. A nested PCR (n-PCR) reaction was applied with a first-stage PCR using universal primers within the ends of the 16S and 23S genes, followed by a second-stage PCR with nested primers specific to the X. ampelinus spacer region. A 277-bp fragment was amplified from 38 Xylophilus strains tested, but not from saprophytes associated with grapevine or phylogenetically related phytobacteria. The 277-bp product was shown to be derived from the X. ampelinus spacer region by restriction with Dra I, Sau 3AI, Taq I and Msp I, Southern hybridization and genomic DNA dot blots. When the (n-PCR) procedure was applied in the absence of nontarget DNA, the limit of detection was less than 10 colony-forming units (CFU) per µ L. The same number of  X. ampelinus CFU could be detected in the presence of 1·5 × 10⁵ CFU  µ L⁻¹ of Erwinia herbicola cells using the n-PCR procedure.