Mechanical transmission is the method most frequently used to evaluate the response of infected materials
in order to look for genetic resistance (Paterson et al., 1989).
TSWV is naturally spread by thrips, but it is possible to obtain different results if the infection of plant
material takes place though mechanical transmission. These differences have been stated by Krishna et al. (1993)
in some accessions of Lycopersicon and by Nuez et al. (1994) in others belonging to Capsicum genus. Materials
screened in natural conditions versus artificial infection (including mechanical and thrips transmission) have also
shown different results. The phenomenon known as field resistance or tolerance has been observed in
Lycopersicon (Costa et al., 1994; smith and Gardner, 1993, Krishna et al., 1993 and Boiteux at al., 1993).
Taking this fact into account, we have evaluated some varieties of L. esculentum and different accessions
of L. pimpinellifolium and L. peruvianum by means of mechanical and thrips transmission and field conditions
(Costa et al., 1994). UPV-101 and UPV-200 lines are carriers of Sw1 and Sw2 genes respectively, and RDD line
has been developed from Stevens.
Seedlings of accessions were exposed to viruliferous thrips maintained on susceptible tomato (cvs.
Muchamiel and Marmande) and pepper (cvs. Buketar, Negral and Sonar) previously infected. Trials were carried
out in a semiclimatic room at 22-24°C, 45-50% of relative humidity during the day and 85-100% of relative humidity
at night, photoperiod of 14 hours and 3500 lux of luminosity. Plants were introduced at 4-6 developed leaves stage
and kept there for enough time for systemic symptoms to appear. Plants that did not show any symptom, remained
in the room for three months. Samples of each plant were taken and analyzed by serological technique ELISA with
antiserum TSWV-L (Loewe). Plants free of symptoms were transplanted to fields under natural infection conditions.
Plant samples and analysis were carried out each 15 days.
First symptoms on susceptible accessions appeared 12-15 days after its contact with viruliferous thrips.
Table 1 shows results of the three methods of inoculation
We would like to point out two aspects. First, Argentinean materials (Quilquil, Nuco, Uco Plata and Pitihue)
were susceptible when thrips transmission was utilized as well as with mechanical transmission (Quilquil and
Nuco). Nevertheless, level of infection under field conditions were very low. This field resistance was reported
previously by Boiteux et al. (1993) for this type of material. Similar results have been obtained for L.
pimpinellifolium CIAPAN-20, which did not show any symptoms under field conditions.
On the other hand, resistance was observed in RDD and UPV-101 when inoculated mechanically with
different isolates (Jorda at al., 1994), whereas many plants showed systemic symptoms when thrips transmission
was used as well as in field conditions. Plants RDD that did not show symptoms by thrips transmission were
transplanted to the field where they became infected at the end of the crop period. PVY was detected in infected
plant by means of electronic microscopy, as was TSWV. Therefore, we can not attribute the death of the plants to
TSWV alone.
Slight local lesions appeared in L. peruvianum PI-126944 when plants were exposed to viruliferous thrips.
Systemic symptoms did not appear. We can consider this accession resistant to TSWV.
We conclude that it is convenient to test materials by using infected thrips under controlled conditions, as
interactions between host and vector have to be taken into account. The same conclusion has been drawn by
Krishna et al. (1993). We would also like to emphasize that materials showing susceptibility to artificial transmission
can be
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