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Samuel F. Hutton, Jay W. Scott, and Jeffrey B. Jones

Resistance of tomato (Solanum lycopersicum) to bacterial spot race T4 (Xanthomonas perforans) was characterized by generation means analysis (GMA) in three advanced breeding lines: Fla. 8326, Fla. 8233, and Fla. 8517. GMA of Fla. 8326 for two of three seasons (Fall 2006 and Summer 2007) indicated that resistance is mostly dominant with significant additive and epistatic effects. GMA of Fla. 8233 in Spring 2007 and of Fla. 8517 in Summer 2007 also showed dominance to be the main effect in addition to additive and epistatic effects. Duplicate dominance or recessive suppressor type epistasis was indicated in each breeding line. Transgressive segregation was not clearly observed in F2 populations of crosses between resistant parents, suggesting that these lines have quantitative trait loci in common.

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J.W. Scott, J.B. Jones, and G.C. Somodi

Hawaii 7981 tomato (Lycopersicon esculentum Mill.), resistant to race T3 of the bacterial spot pathogen [Xanthomonas campestris pv. vesicatoria (Doidge) Dye], was crossed to the susceptible tomato inbred, Fla. 7060, and subsequently F2 and backcross seed were obtained. These generations were planted in the field, inoculated with the race T3 pathogen and evaluated for disease severity over two summer seasons. Data were tested for goodness-of-fit to a model based on control by the incompletely dominant gene Xv3 that confers hypersensitivity. The F1 was intermediate in disease severity to the parents for both seasons. When data were combined over both seasons, the backcrosses fit the expected 1:1 ratios although each deviated from the expected ratio in one of the 2 years tested. The F2 did not fit the expected 1:2:1 ratio in either year or when data from the two years were combined due to a deficiency of resistant plants. Thirty-three F2 plants representing an array of disease severities and hypersensitivity reactions were selected in the second season and their F3 progeny were inoculated and evaluated for disease severity. Hawaii 7981 was significantly more resistant than the 12 most resistant F3 selections even though all expressed hypersensitivity. A hypersensitive F3 with intermediate field resistance was crossed to Hawaii 7981 and subsequently, F2 and backcross generations were obtained. These generations were field inoculated with the race T3 pathogen and evaluated for disease severity. Hawaii 7981 was significantly more resistant than the F3 parent as in the previous year. The data did not fit an additive-dominance model and epistatic interactions were significant. Thus, it appears that field resistance to race T3 of bacterial spot found in Hawaii 7981 is conferred quantitatively by Xv3 and other resistance genes. Breeding implications are discussed.

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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.

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J.W. Scott, J.B. Jones, G.C. Somodi, and R.E. Stall

Tomato (Lycopersicon esculentum Mill.) accessions were tested for hypersensitivity and rated for resistance following field inoculation with tomato race 3 (T3) of the bacterial spot pathogen Xanthomonas campestris pv. vesicatoria (Doidge) Dye (Xcv) in 1992 and 1993. Hawaii 7981, PI 126932, PI 128216, and selections of the latter two expressed hypersensitivity. Hawaii 7981, only tested in the field in 1993, was nearly symptomless and developed significantly less disease than any other accession. PI 128216 had a level of disease similar to susceptible `Solar Set' when tested in 1993. However, a selection from it (PI 126218-S) was significantly more resistant than `Solar Set' in both years. Although PI 126932 had a level of disease similar to `Solar Set' in both years, a selection from it (PI 126932-1-2) was significantly more resistant than `Solar Set' in 1993. Other accessions without hypersensitive responses but more resistant than `Solar Set' for two seasons were PI 114490, PI 126428, PI 340905-S, and PI 155372. Hawaii 7975 was significantly more resistant than `Solar Set' in the one season it was tested.

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J.W. Scott, J.B. Jones, G.C. Somodi, D.O. Chellemi, and S.M. Olson

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J.W. Scott, D.M. Francis, S.A. Miller, G.C. Somodi, and J.B. Jones

Crosses were made between tomato (Lycopersicon esculentum Mill.) inbreds susceptible to races T2 and T3 of bacterial spot (Xanthomonas vesicatoria and Xanthomonas campestris pv. vesicatoria, respectively) and accession PI 114490 with resistance to races T1, T2, and T3. Resistance to race T2 was analyzed using the parents, F1, and F2 generations from one of the crosses. The F1 was intermediate between the parents for disease severity suggesting additive gene action. The segregation of F2 progeny fit a two-locus model (χ2 = 0.96, P = 0.9-0.5) where four resistance alleles are required for a high resistance level, two or three resistance alleles provide intermediate resistance, and zero or one resistance allele results in susceptibility. The narrow sense heritability of resistance to T2 strains was estimated to be 0.37 ± 0.1 based on F2 to F3 parent-offspring regression. A second cross was developed into an inbred backcross (IBC) population to facilitate multilocation replicated testing with multiple races. Segregation for T2 resistance in the inbred backcross population also suggested control was by two loci, lending support to the two-locus model hypothesized based on the F2 segregation. To determine if the same loci conferred resistance to the other races, selections for race T2 resistance were made in the F2 and F3 generations and for race T3 resistance in the F2 through F4 generations. Six T3 selections (F5), 13 T2 selections (F4's that diverged from seven F2 selections), and control lines were then evaluated for disease severity to races T1, T2, and T3 over two seasons. Linear correlations were used to estimate the efficiency of selecting for resistance to multiple races based on a disease nursery inoculated with a single race. Race T1 and race T2 disease severities were correlated (r ≥ 0.80, P< 0.001) within and between years while neither was correlated to race T3 either year. These results suggest that selecting for race T2 resistance in progeny derived from crosses to PI 114490 would be an effective strategy to obtain resistance to both race T1 and T2 in the populations tested. In contrast, selection for race T3 or T2 will be less likely to result in lines with resistance to the other race. PI 114490 had less resistance to T3 than to T2 or T1. Independent segregation of T2 and T3 resistance from the IBC population derived from PI 114490 suggests that T3 resistance is not controlled by the same genes as T2 resistance, supporting the linear correlation data.

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J.W. Scott, J.B. Jones, G. Cameron Somodi, and R.E. Stall

Resistant Hawaii 7981 (P1) was crossed with susceptible Fla. 7060 (P2), and F1, BCP1, BCP2, and F2 generations were obtained. Hypersensitive reactions (Hr) were measured 24 and 48 hours after inoculation in growth chambers at 24 and 30C. At 30C, there was no Hr. At 24C and 24 hours, 100% of Hawaii 7981 plants, 54.2% of BCP1 plants, and 21.7% of F2 plants had Hr. At 24C and 48 hours, 100% of Hawaii 7981, the F1, and BCP1 plants; 50% of BCP2 plants; and 73.3% of F2 plants had Hr. Other plants were inoculated and rated for race T3 in the field. Disease for each generation was significantly different (P < 0.05) and their order from most to least resistant was P1, BCP1, F1, F2, BCP2, and P2. The F1s were distributed between the parents with slight overlaps. BC plants had bimodal peaks similar to the F1 and their respective parents. The F2 had three peaks corresponding to P1, F1, and P2. The data suggest Hr and field resistance are controlled by the same incompletely dominant gene.

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J.W. Scott, S.A. Miller, R.E. Stall, J.B. Jones, G.C. Somodi, and V. Barbosa

Thirty-three tomato (Lycopersicon esculentum Mill.) or L. pimpinellifolium (L.) Mill. accessions were inoculated with race T2 of Xanthomonas campestris pv. vesicatoria (Xcv) in a field experiment at Wooster, Ohio, in Summer 1995. These included accessions selected for race T2 resistance in greenhouse tests in Florida, and accessions from Hawaii, Brazil, and Bulgaria. One L. esculentum (PI 114490-1-1) and three L. pimpinellifolium (PI 340905-S1, PI 128216-T2, and LA 442-1-BK) accessions had no Xcv symptoms. This is the first report of resistance to Xcv race T2. Partial resistance was found in PI 271385, PI 79532-S1, PI 155372-S1, PI 195002, and PI 126428. Most of the 33 genotypes were tested for race T1 resistance in Presidente Prudente, Sao Paulo, Brazil in summer 1993. Hawaii 7983, PI 155372-S1, PI 114490, PI 114490-S1, and PI 262173 had greater resistance to T1 than the susceptible control `Solar Set'. Comparisons with earlier experiments in which accessions were inoculated with race T1 or T3 indicated that the most consistent source of resistance to all three races was PI 114490 or selections from it.

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Susan L. Steinberg, Gerard J. Kluitenberg, Scott B. Jones, Nihad E. Daidzic, Lakshmi N. Reddi, Ming Xiao, Markus Tuller, Rebecca M. Newman, Dani Or, and J. Iwan D. Alexander

Baked ceramic aggregates (fritted clay, arcillite) have been used for plant research both on the ground and in microgravity. Optimal control of water and air within the root zone in any gravity environment depends on physical and hydraulic properties of the aggregate, which were evaluated for 0.25-1-mm and 1-2-mm particle size distributions. The maximum bulk densities obtained by any packing technique were 0.68 and 0.64 g·cm-3 for 0.25-1-mm and 1-2-mm particles, respectively. Wettable porosity obtained by infiltration with water was ≈65%, substantially lower than total porosity of ≈74%. Aggregate of both particle sizes exhibited a bimodal pore size distribution consisting of inter-aggregate macropores and intra-aggregate micropores, with the transition from macro- to microporosity beginning at volumetric water content of ≈36% to 39%. For inter-aggregate water contents that support optimal plant growth there is 45% change in water content that occurs over a relatively small matric suction range of 0-20 cm H2O for 0.25-1-mm and 0 to -10 cm H2O for 1-2-mm aggregate. Hysteresis is substantial between draining and wetting aggregate, which results in as much as a ≈10% to 20% difference in volumetric water content for a given matric potential. Hydraulic conductivity was approximately an order of magnitude higher for 1-2-mm than for 0.25-1-mm aggregate until significant drainage of the inter-aggregate pore space occurred. The large change in water content for a relatively small change in matric potential suggests that significant differences in water retention may be observed in microgravity as compared to earth.

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J.W. Scott, S.A. Miller, R.E. Stall, J.B. Jones, G.C. Somodi, V. Barbosa, D.L. Francis, and F. Sahin

Thirty-two tomato (Lycopersicon esculentum Mill.) or L. pimpinellifolium (L.) Mill. accessions were inoculated with race T2 of Xanthomonas campestris pv. vesicatoria (Xcv) in a field experiment at Wooster, Ohio, in 1995. Plants from accessions which segregated for race T2 resistance in greenhouse tests were selected and these are designated by hyphenated extensions below. The eight most resistant accessions from 1995 and PI 262173 were retested in 1996. Lycopersicon esculentum accession PI 114490-1-1 had virtually no Xcv symptoms either year. Lycopersicon pimpinellifolium accessions LA 442-1-Bk and PI 128216-T2 expressed a high level of resistance in 1995, but only partial resistance in 1996. Accessions with partial resistance for both seasons were PI 79532-S1, PI 155372-S1, PI 126428, PI 271385, PI 195002, PI 262173, Hawaii 7998, and Hawaii 7983. PI 79532-S1 is a L. pimpinellifolium accession and the remaining seven are L. esculentum. Twenty accessions tested in 1995 for T2 plus 10 other accessions were also tested for race T1 resistance in Presidente Prudente, Sao Paulo, Brazil, in 1993. Hawaii 7983, PI 155372-S1, PI 114490, PI 114490-S1, and PI 262173 had greater resistance to T1 than the susceptible control, `Solar Set'. Comparisons with earlier experiments, in which accessions were inoculated with race T1 or T3, indicated that the most consistent source of resistance to all three races was PI 114490 or selections derived from it.