The effect that Tomato yellow leaf curl virus (TYLCV)-infected resistant tomato plants may have on virus epidemiology was studied. Four tomato genotypes that exhibit different levels of viral resistance, ranging from fully susceptible to highly resistant, served as TYLCV-infected source plants. Viral acquisition and transmission rates by white-flies following feeding on the different source plants were evaluated. TYLCV transmission rate by whiteflies that had fed on infected source plants 21 days postinoculation (DPI), shortly after the appearance of TYLCV symptoms, was negatively correlated with the level of resistance displayed by the source plant. Therefore, the higher the resistance, the lower the transmission rate. In addition, TYLCV DNA accumulation was shown to be lower in the resistant source plants compared with the susceptible plants. Whitefly survival rate, following feeding on source plants 21 DPI, was similar for all the cultivars tested. Significant differences in whitefly survival were found, however, following feeding on the infected source plants at 35 DPI; here, whitefly survival rate increased with higher levels of resistance displayed by the source plant. At 35 DPI, the susceptible plants had developed severe TYLCV disease symptoms, and transmission rates from these plants were the lowest, presumably due to the poor condition of these plants. Transmission rates from source plants displaying a medium level of resistance level were highest, with rates declining following feeding on source plants displaying higher levels of TYLCV resistance. TYLCV DNA accumulation in whiteflies following feeding on infected source plants at both 21 and 35 DPI was directly correlated with viral DNA accumulation in source plants. Results show that, in essence, the higher the resistance expressed, the less suitable the plant was as a viral source. Consequently, following acquisition from a highly resistant plant, TYLCV transmission by whiteflies will be less efficient.

The effect of tomato yellow leaf curl virus (TYLCV) on total yield and yield components of various resistant F1 tomato cultivars and new breeding lines was evaluated in the field. Seeds of the F1 hybrids 8484, 3761, Fiona, and Tyking and the new breeding lines TY172 and TY197 were sown in an insect-proof greenhouse. When the seedlings reached the first-leaf stage, they were inoculated with TYLCV by means of the whitefly vector Bemisia tabaci. Noninoculated plants of the same cultivar or line which were exposed to nonviruliferous whiteflies served as controls. After a short recovery period, the plants were transplanted to the field. The inoculated plants of each cultivar or line were compared with their respective control, noninoculated plants, in terms of total yield, average fruit weight and number, and plant fresh weight. Disease symptom development and virus accumulation in the inoculated plants were monitored. There were substantial differences among the different entries tested in the extent of yield loss relative to the corresponding noninoculated control plants as well as viral DNA accumulation levels. Plants of TY172 and TY197 suffered the least relative yield loss and contained the lowest level of viral DNA. Therefore, these two lines exhibited the highest level of resistance. These results clearly demonstrate the ability of the various resistant tomato cultivars and lines to inhibit the effects of the virus, even following inoculation at a very early stage of plant development.

Tomato yellow leaf curl virus (TYLCV), transmitted by the tobacco whitefy (Bemisia tabaci Genn.), can be devastating to tomato (Lycopersicon esculentum L.) crops in tropical and subtropical regions. The development of resistant cultivars is the best option for control of TYLCV. However, all the available resistant commercial cultivars tested at the Volcani Center, when inoculated with TYLCV, developed different levels of disease symptoms. In this study, we report the development of a breeding line, TY172, which is a symptomless carrier of TYLCV. Line TY172, whether infected in the greenhouse with viruliferous whiteflies, or when grown in the field under natural infection, showed no symptoms of the disease. Viral DNA was detected in infected TY172 plants, albeit at much lower levels than a susceptible infected control. In addition, grafting experiments using infected susceptible scions grafted onto TY172 stocks, showed that even when exposed continuously to very high levels of virus, line TY172 did not develop disease symptoms, nor did it accumulate high levels of the virus. When TY172 was crossed with susceptible lines, the hybrids exhibited milder symptoms and lower viral content than the susceptible parent, yet higher than that of TY172, suggesting a partial dominance for the TY172 resistance. Upon inoculation of F2 populations, the amount of symptomless individuals appeared in a ratio of≈7:64. This suggests that at least three genes may account for the resistance.

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