Experienced growers have been observed differences in the incidence and progress of the spotted wilt disease among susceptible tomato cultivars. According to them, some cultivars, even though susceptible, are able to endure or slow down the epidemic of this disease more efficiently than other susceptible cultivars under similar field conditions. In this work, we report a comparative study on the differ ential level of field resistance to the spotted wilt disease of seven tomato lines including two traditional susceptible Brazilian cultivars ('IAC Santa Clara' and 'Angela I-5100') and two breeding lines carrying the resistance gene Sw-5 (Stevens et al., 1992).
The trail was conducted under field conditions at CNPH/EMBRAPA in Brasilia (DF), Brazil. The experiment was established using transplants (in a vegetative stage of four true leaves) in June 26, 1992. This experiment was installed nearby an old thrips-infested onion planting. In the vicinities of the experimental field and among its plots was allowed to grow several solanaceous weeds previously reported as hosts of Tospovirus 'species'. Those procedures were employed as a mean of ensure a high and uniform pressure of viruliferous thrips. The experiment was conducted in a randomized design with four replications. Plots were single 4.0 m long rows (with 10 plants each) in a row-to-row distance of 1.0 m. Disease ratings were made in September 18, 1992 by three observers. A disease score was given to each individual plant according to the following qualitative scale: 1 = no disease; 2 = weak apical leaf curl; 3 = top distortion plus chlorosis/purplish coloration of the apical leaves; 4 = severe top distortion and necrosis of the apical leaves; and 5 = severe stunting, and general necrosis. Reading were converted to a disease severity index (DSI) using the following procedure: the number of plants in each symptom category was multiplied by the corresponding numerical grade and the products were added and then divided by the total number of plants. Disease incidence (DI) was also calculated (as the proportion of plants exhibiting Tospovirus symptoms) and included as an additional evaluation criterion. In order to confirm the absence of spotted wilt infection in asymptomatic plants, a pool of leaves within each plot was tested against a TSWV-polyclonal antiserum using ELISA.
First symptoms of the spotted wilt desease were observed about 30 days after transplanting. Inoculum pressure was adequate as indicated by a severe and nonclustered pattern of infected plants within the experimental field. The significant differences observed among the seven tested lines (Table 1) indicated that exist genetic variation for field resistance to a spotted wilt. Disease incidence and severity were higher in the cultivar 'IAC Santa Clara', followed by 'Zeneth' (a processing F1 tomato hybrid). DI and DSI were slightly lower in the cultivars 'Angela I-5100' and 'Plantense'. A good level of field resistance was observed in the cultivar Quil-Quil. Our results are in agreement withobservations of some Brazilian growers that 'IAC Santa Clara' has a constant ranking of lower level of field resistance to spotted wilt when compared with 'Angela I-5100'. This behavior has been observed regardless which geographical region or phenotypicvariant of the virus is being used for testing. Until the present, no vertical resistance breeding programme for tomato spotted wilt has been done in Brazil. Thus, the significative differences observed between Brazilian cultivars may only be conditioned by different levels of horizontal or field resistance. Two breeding lines (CNPH Tx 405 #6 and CNPH Tx 405 #10) carrying the Sw-5 gene (Stevens et al., 1992) presented 'field immunity' response to the disease as demonstrated by visual analysis (Table 1) and ELISA (data not shown). This result indicated that the Sw-5 gene was also operative against the natural prevalent population of the 'tomato spotted wilt disease complex' at CNPH/EMBRAPA in Brasilia (DF).
The resistant reaction conditioned by the Sw-5 gene has been also observed after mechanical inoculation with a serologically distinct isolate of TSWV (Nagata et al., 1992) obtained from Sao Pulo State (Boiteux and Giordano, 1992). In Brazil, the resistant response of those two lines carrying Sw-5 gene has been recently confirmed against other 18 distinct Tospovirus isolates (Nagata et al., in preparation). Breeding lines carrying the Sw-5 gene have been already identified as having resistance against three TSWV isolates from the USA (Stevens et al., 1 992). All those aforementioned data strongly suggest (as previously discussed by Stevens et al., 1992) that Sw-5 gene may possess a large-spectrum of action against several phenotypic or genotypic variants ('species') that comprise the genus Tospovirus. Additional studies are necessary to identify for which of the already identified Tospovirus 'species' (de Avila, 1992) the Sw-5 gene is operative.
The genetic background of Tx 405 #6 and #10 lines has a great contribution of 'Flora-Dade', a high field sus ceptible material (Boiteux et al., unpublished data). The combination of single vertical gene in a background with low level of field (horizontal) resistance to the tomato spotted may represent a potential risk of a severe outbreak of this disease followi ng the breakdown of this vertical resistance gene. An adequated breeding programme for spotted wilt resistance should take into account the inclusion of parental lines that also possesses good residual levels of field or rate-reducing resistance to this disease.
Investigations are being conducted to explain if the observed differences in the disease incidence could be related to differential preference of thrips vectors. Additional studies are necessary to establish which parameters (e.g. differences of incubation and latent periods) are involved with the differences observed also in the severity index. Once defined, such parameters may be used in screening for horizontal resistance to the spotted wilt disease.
Boiteux, L.S. and Giordano, L de B. Screening lycopersicon germplasm for resistance to a Brazilian isolate of tomato spotted wilt virus (TSWV). TGC Report 42:13-14, 1992.de Avila, A.C. Diversity of the Tospoviruses. PhD Thesis. Wageningen Agricultural University, Wageningen, The Netherlands, 1992.
Nagata, T., L.S. Boiteux, N. Iizuka, and A.N. Dusi. Hospedeiros diferenciais e serologia na identificacao de grupos do vira-cabeca do tomateiro em tomate e pimentao no Brasil. Fitopatologia brasileira 17:187, 1992.
Stevens, M.R., S.J. Scott and R.C. Gergerich. Inheritance of a gene for resistance to tomato spotted wilt virus (TSWV) from Lycopersicon peruvianum Mill. Euphytica 59:9-17, 1992.
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Lines % of Disease
infected plants severity index
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IAC Santa Clara 17.2 a* 3.34 a
Zeneth F1 62.1 b 2.94 b
Angela I-5100 41.8 c 2.31 c
Platense 32.0 d 1.44 d
Quil-Quil 14.0 e 0.39 e
CNPH Tx 405 line #6 0.0 f 0.00 f
CNPH Tx 405 line #10 0.0 f 0.00 f
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* = Means not sharing a letter in common differ significantly
(P = 0.01) according to Duncan's multiple range test.