Isolate-Specific Detection of Grapevine fanleaf virus from Xiphinema index Through DNA-Based Molecular Probes

Tests with a real-time reverse transcription-polymerase chain reaction (RT-PCR) were performed on specimens of Xiphinema index collected from the rhizosphere of Grapevine fanleaf virus (GFLV)-infected grapevines at Palagiano, Italy. A 1,157-bp fragment of the GFLV RNA-2 coat protein (CP) gene was amplified and sequenced. A fluorescent Scorpion probe was designed to detect a highly conserved CP region. A second region with isolate-specific multiple nucleotide polymorphisms was used to detect GFLV isolates using molecular beacons (MB). The Scorpion probe allowed quantitative estimation of GFLV RNA-2 in single nematodes, using a dilution series of a 692-nucleotide transcript of the CP gene. The assay allowed detection of GFLV RNA-2 in individual X. index, with a minimum template threshold of 800 fg or 2.8 × 10 6 RNA-2 molecules per nematode. The CP fragment used for GFLV detection with the Scorpion probe appeared highly conserved among isolates. The probes were tested against other GFLV isolates, which were recognized by the species-specific Scorpion probe and by the corresponding MB specific to the particular isolate. Both tests appeared useful as diagnostic tools or for studies on GFLV in acquisition, retention, and transmission experiments.