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.

Behavior of Watermelon Cultivars after Inoculation with Watermelon mosaic virus in Saudi Arabia

All seven watermelon cultivars that were screened for their reactions to a severe Saudi Arabian isolate of Watermelon mosaic virus(WMV-SA)that was found inducing a severe disease in watermelon in Riyadh region,were found to be susceptible and showed different virus-like symptoms upon mechanical inoculation.Sugar Baby,Crimson Sweet 1 and Crimson Sweet 2 cultivars showed milder symptoms and,therefore,got lower grand mean of weekly symptom ratings than Charleston Gray No.502,Jubilee,Black Diamond and Charleston Gray No.133 in both first and second experiments.Artificial inoculation with this isolate significantly reduced the plant height,fresh and dry weights of the tested cultivars.The reduction percentages in plant height of Sugar Baby and Crimson Sweet 2 were significantly lower than those of Crimson Sweet 1,Charleston Gray No.502 and Charleston Gray No.133 in both experiments.Also the reduction in percentages of fresh weights of Sugar Baby,Crimson Sweet 2 and Jubilee were significantly lower than reduction percentages of Crimson Sweet 1,Black Diamond and Charleston Gray No.133.The dry weight reduction percentages of Jubilee,Sugar Baby and Crimson Sweet 2 were lower than those of Charleston Gray No.502,Charleston Gray No.133,Black Diamond and Crimson Sweet 1 in both experiments.No correlation existed between the virus titer in the infected cultivars and their performances.In general,Sugar Baby and Crimson Sweet 2 performed better than the other cultivars as they had the lowest symptom severity ratings,the lowest percentages of plant height,fresh and dry weight reductions compared to the other tested cultivars.