Ultrastructural Aspects of the Mixed Infections of Watermelon Mosaic Potyvirus and Cucumber Green Mottle Mosaic Tobamovirus Isolated from Watermelon

Symptoms on 4 varieties of watermelons inoculated with watermelon mosaic potyvirus II isolated from watermelon (WMV-W) were severe mosaic and leaf malformation while those inoculated with cucumber green mottle mosaic tobamovirus from watermelon (CGMMV-W) were mild mosaic and chlorotic spots. Inoculation of the mixture of WMV-W and CGMMV-W produced extremely severe mosaic along with necrotic spots and general necrosis. Doubly infected plants were also stunted. Cells infected with WMV-W or CGMMV-W alone exhibited the intrinsically ultra-structural properties of each virus infection. WMV-W induced potyvirus-characteristic cylindrical inclusions in the cytosol. Virus particles were orderly aligned along the tonoplasts. CGMMV-W induced tobamovirus-characteristic stacked crystalline arrays of virus particles in the cytosol. Cells infected doubly with WMV-W and CGMMV-W contained striking cytopathic effects that were not present in single infection of each virus. The unique ring structure, nonagon, was that a single potyvirus particle was surrounded by 9 CGMMV-W tobamovirus particles.