Characterization of a Watermelon mosaic virus Isolate Inducing a Severe Disease in Watermelon in Saudi Arabia

In one of the field visits to cucurbits-growing areas in vicinity of Riyadh city during 2013-2015, severe virus disease-like symptoms were observed on watermelon in Al-Ammariyah area. Mechanical inoculation of the different plant species used in the host range study, from the collected symptomatic watermelon samples, produced mosaic symptoms on Citrullus lanatus, Cucumis sativus, Cucurbita pepo, C. melo, C. melo subsp. melo and Nicotiana benthamiana, but chlorotic local lesions on Chenopodium amaranticolor. No symptoms were observed on the rest of the other inoculated plant species. The virus was transmitted by Aphis gossypii and A. craccivora in a non-persistent manner. Transmission electron microscopic examination of watermelon samples using the leaf dip method revealed only microscopic filamentous shaped virus particles measuring 750 nm in length and 12 nm in diameter in average. ELISA revealed positive results only to Watermelon mosaic virus (WMV) and negative to Zucchini yellow mosaic virus (ZYMV), Papaya ringspot virus (PRSV), Cucumber mosaic virus (CMV) and Squash mosaic virus (SqMV). Specific bands of approximately 825 bp were formed on agarose gel following electrophoresis of the reverse transcriptase-polymerase chain reaction (RT-PCR) products of each of the naturally infected C. lanatus, and artificially infected C. lanatus, C. pepo, C. sativus, and N. benthamiana. The homology tree that was constructed from multiple sequence alignments of the detected Saudi Arabian isolate of WMV (WMV-SA) with 18 other isolates of WMV from nine different countries indicated close relationships between them. Two isolates from Spain and two other isolates from Iran were more closely related to the WMV-SA whereas the isolate from Poland was the least.