Seasonal variation in disease development in tomato under single and mixed infection with Potato X potexvirus and Tobacco mosaic tobamovirus

Both the initial, and long term symptom responses to single and mixed infections with potato x potexvirus (PVX) and the L (wild strain) and LIIA (attenuated strain) of tobacco mosaic tobamovirus (TMV) were monitored in tomato (Lycopersicon esculentum Mill), cultivar GCR 236 (+/+) which were raised under greenhouse conditions at different seasons of the year in Japan. Symptoms expression, both in rapidly as well as in severity varied with the seasons among the treatments. Generally, disease impinged on plant growth and yield at both the cellular and macro levels and resulted in reductions in size of parenchyma and collenchyma cells from the stem, plant height, number of leaves, stem girth and weights of fresh and dry shoot and root as well as fresh fruit. There was also delayed flowering. Both singly and doubly infected plants had relatively better growth and yield during summer season. However, while there were only narrow discrepancies, in disease severity and yield loss, between summer and winter crops singly infected with TMV (L or LIIA), single infection with PVX consistently induced significantly rapid and more severe disease and consequently more growth and yield reductions in winter than in summer crops. For instance, while the percentage loss in fresh fruit weight was ca. 49% and 56% respectively for summer and winter crops under single infection with TMV-L, the loss was 33% and 57% under single infection with PVX, respectively for the two seasons. Nevertheless, simultaneous mixed infection of PVX and the TMV strains induced the most severe symptoms and also led to more growth and yield reductions than single infections with either virus, the season of growth notwithstanding. Compared to the healthy plants, about 90% reduction in total fruit weight per plant was recorded. Furthermore, cross protection was evidenced in plants that were mixed inoculated with the LIIA and the L strain of TMV as they manifested significantly delayed and milder disease compared to plants that were inoculated with the l (i.e. severe strain) alone.