Plants of a common tomato ( Lycopersicon esculentum
Mill.) cultivar i.e. Fukuju # 2, were raised under greenhouse conditions at different times in Japan. Both the initial and long-term symptom responses to single and mixed infections with potato X potexvirus (PVX) and the L (wild strain) of tobacco mosaic tobamovirus (TMV) were monitored. Symptoms expression, both in rapidity as well as in severity varied among the treatments and were influenced, not only by virus strains but also by the inoculation regime. The reduction in sizes of parenchyma and collenchyma cells from the stem, plant height, number of leaves, stem girth and yield of fresh and dry shoot and root as well as fresh fruit that were recorded in all infected plants, showed strong correlation with observed symptom severity which in turn was found to be a function of the concentration, as measured by enzyme linked immunosorbent assay (ELISA) of the accumulated viral particles in the host during the acute stage of infection. Simultaneous mixed inoculation of PVX and TMV-L, which induced the most severe symptoms also led to relatively more growth reduction than sequential mixed inoculation with either virus, 4 days before the other, and single infections with PVX or TMV. Significantly delayed flowering and a reduction of at least 80% in fresh shoot weight and fruit yield were recorded for all mixed infected plants regardless of sequence of infection.