A portable recombinase polymerase amplification assay for the rapid detection of cucurbit leaf crumple virus in watermelon leaves and fruits

Cucurbit leaf crumple virus (CuLCrV), a single-stranded DNA virus belonging to the species Cucurbit leaf crumple virus in the genus Begomovirus , causes cucurbit leaf crumple disease in many species of the Cucurbitaceae family. This report describes the development and standardization of field-based isothermal recombinase polymerase amplification (RPA) and exonuclease RPA (exo-RPA) assays for the detection and diagnosis of CuLCrV in the leaves of watermelon, squash and pumpkin, and the fruits of watermelon. The oligonucleotide primers were made to target a portion of the BL1 movement protein. The primer set used in the RPA was highly specific for CuLCrV and did not show a cross-reaction with other cucurbit viruses nor in tissues with co-infections of CuLCrV with cucurbit yellow stunting disorder virus (CYSDV) and/or squash vein yellowing virus (SqVYV). These assays were sensitive, producing a positive signal with 1 fg/uL of purified total DNA and at a dilution of 1 × 10 –5 of crude extract from CuLCrV-infected watermelon plants. Incorporation of a 6-fluorescein amidite (6-FAM) probe into the exo-RPA assay enabled a total sample processing and detection time of 30 min and positive reactions were detectable in different tissues including watermelon leaves, fruits, and pips (seeds lacking an embryo). The exo-RPA was efficiently used in the detection of CuLCrV in 21 cucurbit plants growing in different regions of southeastern U.S. A sensitive, rapid and easy field-based method for the detection and diagnosis of CuLCrV in watermelon was developed.