Nineteen interspecific hybrid breeding lines were tested for resistance to a TSWV isolate using enzyme-linked immunosorbent assay (ELISA) to check for presence of the virus after inoculation. These lines were all BC1F6 lines derived from L. esculentum crosses with seven L. chilense accessions. All of these lines had been selected for high tolerance/resistance to tomato mottle virus (ToMoV), a geminivirus [Scott et al., Bemisia 1995: Taxonomy, Biology, Damage Control and Management 30: 357–367 (1996)]. The initial TSWV screening indicated that eight of the 19 original lines had “possible” TSWV resistance. Seed from these selected eight lines were then planted and inoculated with TSWV ≈3 weeks after emergence. Three weeks later, ELISA results indicated that all plants from all lines were infected with TSWV. However, none of the plants from Y118 (derived from the LA 1938 cross) showed visual TSWV symptoms. The Y118-derived plants were allowed to grow for several months, and at no time developed significant visual symptoms of the virus. The consistent lack of TSWV symptoms prompted a second ELISA test on the Y118 plants, and the results indicated the plants were completely free of TSWV. Further tests were then initiated with F2 (L. esculentum × Y118) seed, and results indicate a single dominant gene is responsible for TSWV resistance. Data from this segregating population, including a molecular marker study which screened 800 randomly amplified polymorphic DNA (RAPD) primers, will be presented. Approximately two to five RAPD primers are possibly linked to TSWV resistance.
M.A. Canady, M.R. Stevens, and J.W. Scott
J.W. Scott, B.K. Harbaugh, and E.A. Baldwin
N. Georgelis, J.W. Scott, and E.A. Baldwin
Small-fruited cherry tomato accession PI 270248 (Lycopersicon esculentum Mill. var. cerasiforme Dunal) with high fruit sugars was crossed to large-fruited inbred line Fla.7833-1-1-1 (7833) that had normal (low) fruit sugar. Sugars in the F2 were positively correlated with soluble solids, glucose, fructose, pH, and titratable acidity, and inversely correlated with fruit size. Earliness was not significantly correlated with sugars but was negatively correlated with fruit size. Thus, the lack of a sugar-earliness correlation indirectly indicates a trend for early tomato plants to be lower in sugars than later maturing plants. Sugars were not correlated with yield or pedicel type. Fruit from indeterminate plants had significantly more sugars than from determinate plants. Six random amplified polymorphic DNA (RAPD) markers linked to high sugars were found, five dominant (OPAE 4, UBC 731, UBC 744, UBC 489, UBC 290) and one co-dominant (UBC 269). Five of the markers were also linked to small fruit size and one of these also was linked to low yield (UBC 290). The sixth marker (UBC 269) was linked to indeterminate plant habit. UBC 731, UBC 489, and possibly OPAE 4 were in one linkage group, while UBC 744 and UBC 290 were in another linkage group. Combinations of all the markers together explained 35% of the sugar variation in the F2 grown in Spring 2002.
Samuel F. Hutton, Jay W. Scott, and David J. Schuster
Tomato yellow leaf curl virus (TYLCV), a Begomovirus in the family Geminiviridae, is an important disease of cultivated tomato (Solanum lycopersicum L.) in many parts of the world. Disease is managed primarily by chemical control of the vector, the sweetpotato whitefly, Bemisia tabaci (Genn.), and by growing resistant varieties. Resistance derived from the cultivar Tyking is being used in many breeding programs, but the location of resistance factors has not been reported. The breeding lines Fla. 8753 and Fla. 344 both have high levels of resistance to TYLCV derived from ‘Tyking’ and from S. chilense accession LA 1938, but none of their parent lines contain any of the known genes Ty-1 to Ty-4. An additional resistance locus, Ty-5, was recently identified, and to determine if this locus controls TYLCV resistance in Fla. 8753 and Fla. 344, appropriate segregating populations were analyzed using the Ty-5 marker, SlNAC1. Results show that SlNAC1 cosegregates with a recessive allele derived from ‘Tyking’. We suggest the gene symbol ty-5 be used to describe this gene. Mean disease severity of progeny homozygous for either the resistant or susceptible alleles did not equal parental levels of resistance and susceptibility, respectively, suggesting the involvement of an additional gene that is likely derived from LA1938.
Aliya Momotaz, John W. Scott, and David J. Schuster
Cultivated tomato (Solanum lycopersicum L.) accessions have been susceptible to the whitefly-transmitted begomoviruses Tomato yellow leaf curl virus (TYLCV) and Tomato mottle virus (ToMoV) that can cause serious crop damage. S. habrochaites accession LA1777 has been reported to be resistant to TYLCV. To locate putative virus resistance genes, 89 recombinant inbred lines (RILs) previously developed from LA1777 in a tomato background, LA1777 and the susceptible RIL parent E6203, were screened against the begomoviruses TYLCV and ToMoV. An initial study showed 18 RILs had less disease severity to TYLCV or ToMoV. Eight RILs had S. habrochaites alleles at TG27 (restriction fragment length polymorphism marker) on chromosome 1, three RILs had S. habrochaites alleles at TG202 on chromosome 7, and one RIL had S. habrochaites alleles at both marker loci. The RILs with these regions were intercrossed in 10 different cross combinations and F2 seeds were then obtained. The F2 progenies were inoculated separately with both viruses and then evaluated in the field. The F2 plants with less disease severity were selected, but most did not have the markers from the hypothetical resistance regions. The F3 progenies were then inoculated and rated for disease severity to both viruses. None of the F3s demonstrated any increased level of resistance, even if derived from F2s homozygous for the target regions from both chromosomes. All plants from every cross combination were susceptible for both TYLCV and ToMoV, suggesting that there is no begomovirus resistance in the LA1777 RIL population. Some limitations of capturing all genes in an RIL population derived from an outcrossing accession are discussed.
J. C. Bouwkamp, L. E. Scott, and W. L. Harris
A procedure and device is described involving a small jet of water under pressure directed onto the surface of a sweetpotato (Ipomoea batatas L.) root. The time required for the stream to abrade the skin is taken as a measure of the “skin-toughness.” The time required for abrasion of 18 cultivars was correlated with their condition scores after mechanical harvesting. Skin was more resistant to abrasion 1 day after digging than when freshly dug and curing further increased skin-toughness. Relative skin-toughness of cultivars changes during curing and storage.
S.M. Shakya and J.W. Scott
Male-sterile, male-fertile, exserted stigma, and exserted stigma with positional sterile (ps) genotypes of tomato (Lycopersicon esculentum Mill.) were pollinated at daily intervals from one day before to 2 days after anthesis (A-1, A0, A+1, A+2, respectively) in 2 field experiments and one greenhouse experiment. A male-sterile with gene ms-10 35 had greater seeds per pollination than other genotypes in all 3 experiments. Seeds per pollination produced in the greenhouse was greater than that of a similar field experiment at all pollination stages except A-1, where seed production was similar. Seeds per pollination was less at A-1 than at later pollination stages. This was due to significant differences in seeds per fruit in all experiments and percentage of fruit set in 2 of 3 experiments. There was no selfing in the 2 male-sterile genotypes in any experiment and no selfing for exserted stigma with ps in the field experiments. There was about 2% selfing with ps at A+2 in the greenhouse. Selfing with stigma exsertion alone ranged from 2% to 22% in the field and from less than 1% to about 5% in the greenhouse. The relationship of selfing with pollination stage was not clear for the exserted stigma genotype.
J. W. Scott and William L. George Jr.
Two inbreds of tomato (Lycopersicon esculentum Mill.) with exserted stigmas, one without and the other with positional sterility (ps), were crossed without emasculation, at 3 stages of maturity under 3 environmental conditions. Seed production was maximal when flowers at anthesis were pollinated during cloudy weather with relative humidities (RH) of about 70% and temperatures about 24°C. Seed production was poor when flowers were pollinated 3 days before anthesis during hot (32°C), clear, dry weather (RH - 48%). The ps inbred had less than 1% selfing at all stages of flower development and environments. Selfing contamination for the ps + inbred was less than 4% per line except in some cases when flowers were crossed before anthesis. Selfing of 35% occurred when flowers were crossed 3 days before anthesis during favorable pollinating weather.
J. W. Scott and James D. Farley
‘Ohio CR-6’ is a pink-fruited, tomato (Lycopersicon esculentum Mill.) hybrid adapted to greenhouse culture with resistance to Fusarium crown and root rot (Fusarium oxys-porum f. sp. lycopersici radieus). It was released by the Ohio Agricultural Research and Development Center in Aug. 1982.
G. Cameron Somodi, J.B. Jones, J.W. Scott, and J.P. Jones
A `spray-inoculation seedling screening procedure was developed for detecting resistance to Xanthomonas campestris pv. vesicatoria (Doidge) Dye, causal agent of bacterial spot of tomato (Lycopersicon esculentum Mill.). Two-week-old transplants were preconditioned under 95% humidity for 16 hours before spray inoculation and then rated for bacterial spot 2 weeks later. Resistant plants could also be distinguished from susceptible genotypes using a modified bacterial speck [Pseudomonas syringae pv. tomato (Okabe) Young, Dye, and Wilkie] screening procedure (cotyledon-dip technique). When results of both screening methods were compared to field ratings from three previous seasons, significant correlations were more frequently observed for the spray-inoculation method. In Summer 1991, individual plants were evaluated by the spray-inoculation technique and then were placed in the field to determine susceptibility under field conditions. Correlations (r = 0.28 to 0.34) between spray-inoculation seedling screening ratings and field ratings, although low, were significant (P ≤ 0.0001). More than 90% of susceptible plants could be eliminated, saving labor, space, and time.