Literature cited: Britten, R.J., Graham, D. E., and Neufeld, B.R. (1974) Meth. Enzymology 29: 363-405 Felsenfeld, G. (1971) pp. 233-244, In: Cantoni, G.L and Davies, D.R. (eds) Procedures in nucleic acid research, vol 2. Harper and Row, New York. Ingle, J., Pearson, G.G., and Sinclair, J. (1973) Nature New Biol. 242: 193-197   Mandel, M., Schildkraut, C.L., and Marmur, J. (1968) Meth. Enzymology 12: 184-195. Response of tomato cultivars partially resistant to TYLCV in Spain.   Pico, B., Diez, M.J., and Nuez, F. Department of Biotechnology, Genetics and Breeding. Polytechnic University of Valencia, E-46022 Valencia, Spain. The tomato yellow leaf curl geminivirus (TYLCV), transmitted by the whitefly Bemisia tabaci, was first detected in Spain in 1992. Today it seems well established in the Southeastern coast, becoming a serious problem to tomato production. Recently, it has spread dangerously to more northerly areas, Alicante and Valencia, where disease pressure is still low (,lords, 1996; Pico et al., 1996). To date no resistance has been found in Lycopersicon esculentum. Only some tomato varieties have been reported to be less susceptible than others during severe epidemics. At present, there are some partially resistant hybrids commercially available. The sources of resistance of these hybrids have not been revealed by the Seed Companies (Laterrot, 1993), except for the L. peruvianum origin of the TY hybrids (Pilowsky and Cohen, 1995). In spring 1996, twelve of these varieties and hybrids were screened in the field, in Valencia, to evaluate their response against TYLCV-Spanish isolates. The disease incidence was monitored from transplanting, in late April, to early august. TAS-ELISA and PCR techniques were used to detect TYLCV in infected plants. Symptoms severity was visually scored per plant. Field infection pressure was low. First TYLCV symptoms appeared between 30-45 days post transplanting in the susceptible control NE-1. A differential response was observed in the cultivars assayed (Table 1). Columbia, Roza, and Rowpack were almost as susceptible as the control. These varieties are no so resistant to TYLCV in Spanish conditions as they have proved to be in some African and Asian countries (Laterrot, 1993). The TY hybrids developed moderate symptoms and the fruit set was not severely affected. Tomato spotted wilt virus (TSWV) infection was severe in these hybrids, making more difficult their evaluation under field conditions. F3524, Tydal, and Jackal reached 100% of plants TYLCV-infected, but developed only slight symptoms at the end of the assay. Disease incidence was lower (<80%) in the most resistant hybrids F3522, Fiona, and Tyking. These hybrids developed no symptoms or very slight ones. The effect on flowering and fruit set was irrelevant. These results point to the last six hybrids, mainly F3522, Fiona, and Tyking, as the most appropriate to spring-summer plantings, when the TYLCV infection is less severe in the Mediterranean coast of Spain. We are going to evaluate the response of these hybrids in the higher inoculum pressure conditions of the summer-autumn growing season, when the environmental conditions are more favorable for Bemisia tabaci. ACKNOWLEDGMENTS   This research was supported by the project N° PB 94-0530-003-02 of the DGICYT program.

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