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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 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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J.E. Flaherty, B.K. Harbaugh, J.B. Jones, G.C. Somodi and L.E. Jackson

Bacteriophages specific to Xanthomonas campestris pv. pelargonii (Xcp), the causal agent of bacterial blight of geranium, Pelargonium ×hortorum L.H. Bailey, were isolated from soil and sludge samples from Florida, California, Minnesota, and Utah. Sixteen phages were evaluated for their potential to lyse 21 Xcp strains collected from around the world. The Xcp strains varied in their susceptibility to the phage isolates with 4 to 14 phages producing a lytic or highly virulent reaction. A mixture of five h-mutants was developed from phages that exhibited the broadest host-ranges and tested against the same Xcp strains. The h-mutant phage mixture lysed all 21 Xcp strains. Three experiments were designed to determine the efficacy of using a mixture of four h-mutant phages to control the spread of the bacterial blight pathogen on potted and seedling geraniums under greenhouse conditions. Plants surrounding diseased inoculated plants were treated with a phage mixture at 5 × 108 pfu/mL daily, biweekly, or triweekly, or treated with Phyton-27®, at 2.0 mL·L-1 every 10 or 14 days. In potted geraniums, daily foliar sprays of the phage mixture had reduced disease incidence and severity by 50% and 75%, respectively, relative to control plants after 6 weeks. In two plug experiments, the phage mixture applied daily also had reduced disease incidence and severity by 69% and 86%, and 85% and 92%, respectively, when compared with controls after 5 weeks. In all three experiments, disease incidence and severity were less for plants treated daily with phages than for those treated less frequently with phages or with Phyton-27®. Chemical name used: copper sulfate pentahydrate (Phyton-27®).

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J.E. Flaherty, G.C. Somodi, J.B. Jones, B.K. Harbaugh and L.E. Jackson

A mixture of host-range mutant (h-mutant) bacteriophages specific for tomato race 1 (T1) and race 3 (T3) of the bacterial spot pathogen, Xanthomonas campestris pv. vesicatoria (Doidge) Dye was evaluated for biological control of bacterial spot on `Sunbeam' tomato (Lycopersicon esculentum Mill.) transplants and field-grown plants for two seasons (Fall 1997 and Fall 1998). Foliar applications of bacteriophages were compared with similar applications of water (control) and of copper/mancozeb bactericides, the commonly used chemical control strategy for tomato seedling and field production. In 1997, the incidence of bacterial spot on greenhouse-grown seedlings was reduced from 40.5% (control) to 5.5% or 0.9% for bactericide- or bacteriophage-treated plants, respectively. In 1998, the incidence of bacterial spot was 17.4% on control plants vs. 5.5% and 2.7% for bactericide- and bacteriophage-treated plants, respectively, although these differences were not statistically significant at P ≤ 0.05. Applications of bacteriophages to field-grown tomatoes decreased disease severity as measured by the area under the disease progress curve (AUDPC) by 17.5% (1997) and 16.8% (1998) compared with untreated control plants. Preharvest plant vigor ratings, taken twice during each field season, were higher in the bacteriophage-treated plants than in either bactericide-treated plants or nontreated controls except for the early vigor rating in 1998. Use of bacteriophages increased total weight of extra-large fruit 14.9% (1997) and 24.2% (1998) relative to that of nontreated control plants, and 37.8% (1997) and 23.9% (1998) relative to that of plants treated with the chemical bactericides. Chemical names used: manganese, zinc, carboxyethylene bis dithiocarbamate (mancozeb).

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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. 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, 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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S.S. Snapp, R.B. Jones, E.M. Minja, J. Rusike and S.N. Silim

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D.W. Lickfeldt, N.E. Hofmann, J.D. Jones, A.M. Hamblin and T.B. Voigt

An efficient deoxyribonucleic acid (DNA) extraction procedure that yields large quantities of DNA would provide adequate DNA for a large number of different analytical procedures. This study was conducted to compare three DNA extraction procedures for cost, time efficiency, and DNA content while extracting DNA from Kentucky bluegrass (Poa pratensis L.). Three students at the Univ. of Illinois with varying levels of DNA extraction experience conducted DNA extractions using Plant DNeasy™ Mini Kits, Plant DNAzol® Reagent, and a PEX/CTAB buffer. Costs varied significantly with cost (US$) per DNA sample of $3.04 for the DNeasy™ method, $0.99 for the DNAzol® method, and $0.39 for the PEX/CTAB extraction. The DNAzol® method was the fastest; although extracting 2.8 ng less DNA than the DNeasy™ method, it did not require the use of hazardous organic solvents, and random amplified polymorphic DNA (RAPD) markers were satisfactory for DNA fingerprinting of Kentucky bluegrass cultivars. The PEX/CTAB method, which did not include a tissue homogenization step, did not have reproducible banding patterns due to miniscule and inconsistent quantities of DNA extracted, or possibly due to inadequate purification. The investigator with the least DNA extraction experience was the slowest, while extracting 75% more DNA. All three methods are easily adapted to laboratories having personnel with different levels of experience. The DNAzol® Reagent method should save time and money, with reproducible results when many individual plant samples need to be identified. Chemical names used: potassium ethyl xanthogenate (PEX); cetyltrimethyl ammonium bromide (CTAB)