National Center for Vegetable Crops Research (CNPH), Brasilia (DF), Brazil
Tomato spotted wilt virus (TSWV) is the causal agent of one of the most important diseases of tomatoes in Brazil. TSWV can cause severe economic losses in all varieties presently grown in the country. The identification of sources and incorporation of resistance into commercial cultivars is one of the most promising methods of control.
Screening trials for TSWV resistance were conducted under greenhouse conditions at National Center for Vegetable Crops Research (CNPH), Brasilia (DF), Brazil. Minimum temperature was 18 deg.C (at night) and maximum temperature was 30 deg. C during the day. Photoperiod was 12 hr during the course of the experiments. Young Nicotiana rustica plants were inoculated with a TSWV isolate (TSWV-SP) recovered from naturally infected tomato plants in Sao Paulo State, Brazil. This isolate was chosen due to its high virulence to all commercial tomato genotypes. Sap of upper leaves of N. rustica plants showing typical TSWV symptoms (about 10 days after inoculation) were used as inoculum.
Inoculation was made by griding leaf extracts in 0.1 M potassium phosphate buffer (pH 7.0) containing 0.01 M sodium sulfite and rubbing extracts on leaves of 15 days-old Lycopersicon spp plants. All leaves were previously dusted with 600-mesh Carborundum. Twelve to fifteen plants of each introduction were inoculated with the virus. A mixture of buffer and crude sap from virus-free N. rustica was applied to a set of plants from each introduction. These plants were used as uninfected controls. The inoculated leaves were rinsed with distilled water immediately after inoculations. All plots were reinoculated 1 week later by using the same procedure to ensure infection. Plants were scored visually for TSWV symptoms 4 weeks after inoculation.
The results were presented in Table 1. All the three susceptible checks (IAC Santa Clara, Rutgers, and Angela I-5100) were highly infected by TSWV-SP verifying the virulence of the isolate. LA 444-1 (an L. peruvianum introduction) and CNPH Tx 405 (a F5 population derived from a cross between cv. Stevens and cv. Rodade) presented higher level of resistance (no infection) to TSWV-SP. The F5 (Stevens x Rodade) was previously reported as resistant to TSWV isolates from Arkansas, Hawaii and Texas (Stevens et al., 1991). Reasonable levels of resistance to TSWV were found in the PI 134417 (a L. hirsutum sp. f. glabratum introduction), PI 203230 and in the cvs. Platanse/INTA and Quil-Quil/INTA from Argentina. Except for LA 444-1, all the resistant genotypes are being combined in a population with the aim of increasing the general resistance to TSWV through a recurrent selection breeding program.
Table 1 Reaction of Lycopersicon spp. cultivars and accessions evaluated for resistance to TSWV-SP isolate of tomato spotted wilt virus.
_____________________________________________________________ Cultivar or Accession Species % of plants with symptoms _____________________________________________________________ IAC Santa Clara L. esculentum 100.0 Rutgers L. esculentum 100.0 Angela I-5100 L. esculentum 91.7 Quil-Quil INTA L. esculentum 38.5 Plantense INTA L. esculentum 25.0 PI 203230 L. esculentum 16.6 PI 134417 L. hirsutum ssp. f. glabratum 13.3 LA 444-1 L. peruvianum 0.0 CNPH Tx 405 L. esculentum 0.0 _____________________________________________________________Acknowledgements
The authors would like to thank Mr. A.N. Dusi for his support in the screening trials and by laboratory facilities and to William P. Dutra for his skillful technical assistance. The authors are also grateful to Dr. M.R. Stevens from University of Arkansas for provide seeds from the TSWV resistant population.
Literature Cited
Stevens, M.R., S.J. Scott, and R.C. Gergerich 1991. Inheritance of resistance to tomato spotted wilt virus in a Lycopersicon esculentum cultivar. HortScience 26(6):781.