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Cucurbits are among the main crops and vegetables grown in Syria, including cucumber (Cucumis sativus), melon (C. melo), watermelon (Citrullus lanatus), and various squash species (Cucurbita pepo, C. moschata, and C. maxima). Cucurbit virus diseases are responsible for significant yield and quality losses to cucurbit crops. In the current study, we investigated the etiology of cucurbit virus diseases, and assessed the molecular diversity of the causal viruses in three major regions of cucurbit production of Syria. Out of 12 viruses and virus groups investigated, zucchini yellow mosaic virus (ZYMV), watermelon mosaic virus (WMV),cucumber mosaic virus (CMV), cucurbit aphid-borne yellows virus (CABYV) and a new member of the genus Polerovirus, pepo aphid-borne yellows virus (PABYV) were detected in various cucurbit crops from plants displaying virus-like symptoms. Syrian ZYMV isolates were closely related and shared low diversity suggesting a relatively recent introduction. A diverse population of WMV with four classical and two emerging isolates of WMV were identified, which complicates the control program. Two CMV isolates were closely related to CMV isolates, reported previously from other crops in the same regions, which may indicate frequent movements of CMV among different hosts. The finding of an isolate of PABYV, a newly described species of the genus Polerovirus, reveals a wider distribution of this virus that may represent a potential emerging problem to cucurbit production.

An isolate of potato virus Y (PVY), PVY-H14, was collected on the island of Oahu, Hawaii, from tomato plants exhibiting stunting and necrotic lesions on leaves. PVY-H14 triggered the hypersensitive resistance response in potato cultivars King Edward and Maris Bard, typical of a PVYC strain, and was unable to infect systemically the four tested cultivars, Desiree, Maris Bard, King Edward, and Russet Norkotah. Phylogenetic analysis of H14 and the whole genomes of 31 PVY isolates of non-recombinant strains of PVY placed PVY-H14 in the same clade with PVYC and several unclassified PVY isolates from tomato and tobacco.

A Syrian isolate of Potato virus Y (PVY), named PVY-12, reacted to two monoclonal antibodies that are specific to PVY(O,C) and PVY(N) strains, although its coat protein (CP) belongs to the PVY(N) strain. Analysis of the CP of PVY-12 revealed that a point mutation in its N terminus switched it from PVY(N)-like to PVY(O)-like at this position. This mutation changed the second nucleotide of the codon that encodes the 29th amino acid of the CP of PVY-12 from A to G, which resulted in one amino acid substitution from Glu(29 )to Gly(29). The role of Gly(29) in the binding of PVY-12 to PVY(O,C)-specific monoclonal antibody was confirmed by gene expression in Escherichia coli. The N terminus of the CP gene of PVY-12 and another PVY isolate of the N serotype with identical CP to PVY-12 except for one amino acid substitution from Gly(29 )to Glu(29) was cloned and expressed in E. coli using a pUC18 vector. Resulting antigens showed similar reactivity to the relevant antibodies as same as the native CPs of these two isolates. Further analysis of the CP of PVY isolates showed that Gly(29) was conserved in the CP of PVY(O), PVY(C), PVY(N)W, and non-potato isolates of PVY while Gln(17) and Glu(31 )were conserved in the CP of PVY(N/NTN). Therefore, these amino acids are characteristic of the CP for these strain groups and subgroups in agreement with the serotype and phylogenetic relationships previously determined.