Influence of the Host Cultivar on Disease and Viral Accumulation Dynamics in Tomato under Mixed Infection with Potato virus X and Tomato mosaic virus

The primary leaves of seedlings of tomato (Lycopersicon esculentum Mill.) cultivar Fukuju No. 2 (a common Japanese cultivar that is susceptible to Tobacco mosaic virus (TMV, genus Tobamovirus) were inoculated at the five-true leaf stage with the O strain of Potato virus X (PVX, genus Potexvirus) and with a mixture of that strain plus Tomato mosaic virus (ToMV, genus Tobamovirus). Inoculation resulted in varying degrees of disease manifestation. During the acute stage of the resulting severe disease (between 5 and 12 days postinoculation), PVX and ToMV levels rose considerably in both the inoculated and the systematically infected leaves. Furthermore, levels of PVX in the systemically infected upper leaves (positions 5 to 7) of plants with a mixed infection were three to six times as high as in plants given the single infection, as determined by direct double antibody sandwich-enzyme linked immunosorbent assay (DAS-ELISA). In tomato cv. GCR 236 (+/+), symptom manifestation and the accumulation of both PVX and ToMV closely followed the pattern recorded for cv. Fukuju No. 2. In cv. GCR 237 (Tm-1) plants, however, only PVX accumulated while ToMV whether inoculated singly or mixed with PVX was detected neither in the inoculated nor in the systemically infected leaves even 14 days after inoculation. In contrast to other cultivars, SDS-PAGE, Western blot and Northern blot hybridization did not reveal any enhancement of the coat protein and genomic RNA of PVX in such systemically infected leaves. Consequently, the characteristic severe symptoms normally associated with mixed infection in TMV-susceptible cultivars were absent.