Identification of watermelon mosaic virus in pumpkin (Cucurbita pepo L.) in Virginia

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Virginia Polytechnic Institute and State University


Pumpkin plants (Cucurbita pepo L. 'Howden') with severely deformed leaves and fruits showing tumour-like protrusions and warty nodules were observed in a field in Carroll County, southwestern Virginia. In addition to deformity, the leaves exhibited puckered mosaic patternings. Mechanical inoculation of a leaf sap extract from the infected Howden vines to 'Small Sugar' pumpkin seedlings resulted in the transmission of a virus. Leaf dip extracts were examined in a transmission electron microscope and flexuous rod-shaped virus particles were observed. The host range included several cucurbit species and suggested the virus to be watermelon mosaic virus (WMV). Luffa cylindrica which is susceptible to WMV-1 was systemically infected by WMV from Virginia (designated WMV-V). Two non-cucurbit species Chenopodium amaranticolor and C. quinoa were also susceptible and, like WMV-2 infection, developed local lesions in response to inoculation with WMV-V. No legumes tested were susceptible to this isolate of WMV and no natural source of inoculum was found. Electron micrographs of ultrathin sections revealed the presence of cytoplasmic inclusion bodies, of pinwheel, scroll and tubular bundle formation, characteristic of WMV-1 infection, in addition to the amorphous inclusion bodies characteristic of WMV-2 infection. These cytoplasmic inclusion bodies were only found in the relatively light-green and not in the abnormally dark-green areas of infected leaf tissue. No nuclear inclusion bodies were observed. In SDS immunodiffusion tests the pumpkin virus reacted with antisera to three isolates of WMV-2 and an isolate of WMV from Egypt, but not with antisera to WMV-1.