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Mohamed F. Mohamed, Dermot P. Coyne, and Paul E. Read

The leaf reaction of the Phaseolus vulgaris L. germplasm—UNECA (M6 mutant derived from the cultivar Chimbolito, Costa Rica), `Chimbolito', BAC-6 (Brazil), XAN-159 (Centro Internacional de Agricultura Tropical, Cali, Colombia), and `PC-50' (Domican Republic)—to Xanthomonas campestris pv. phaseoli strain V4S1 (Dominican Republic) were determined in two replicated trials conducted in a greenhouse in Lincoln, Neb. (Feb.–Mar. and July–Aug. 1993). `PC-50' and `Chimbolito' were susceptible to Xcp strain V4S1 in both tests. UNECA, BAC-6, and XAN-159 had similar levels of resistance to Xcp in the July to August trial. However, in the February to March trial, the resistance of UNECA was greater than that of BAC-6 but less than that of XAN-159.

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Haytham Z. Zaiter, Dermot P. Coyne, and James R. Steadman

Sixteen Alubia lines (15 with long, straight hairs and one with short, hooked hairs on trifoliolate leaves) derived from single-plant selections made in an Alubia landrace (Argentine) were used to evaluate the relation of abaxial leaf pubescence to reaction to rust in a greenhouse experiment. The pinto cultivar UI-114 (short, hooked hairs) was used as a susceptible check. One plant per pot, replicated six times, in a randomized complete-block design was used. The primary leaves and the sixth trifoliolates of all plants from 12- and 50-day-old plants, respectively, were inoculated with a water suspension of urediniospores (105 cells/ml) of rust isolate US-NP85-10-1. Pustule size and rust intensity were assessed 14 days later. No rust pustules were observed on the sixth trifoliolate leaves of the pubescent (long, straight hairs) Alubia lines, but large pustules were observed on the primary leaves (short, hooked hairs) of all Alubia lines and pinto `UI-114'. as well as on the sixth trifoliolate leaf of A-07-2 and pinto `UI-144' (the latter two with short, hooked hairs).

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Mohamed F. Mohamed, Paul E. Read, and Dermot P. Coyne

A new in vitro protocol was developed for multiple bud induction and plant regeneration from embryonic axis explants of four common bean (Phaseolus vulgaris L.) and two tepary bean (P. acutifolius A. Gray) lines. The explants were prepared from two embryo sizes, 3 to 4 mm and 5 to 7 mm, corresponding to pods collected after 15 and 25 days from flowering, respectively. The embryonic axis was cultured on Gamborg's B5 basal medium with 0, 5, 10, or 20 μm BA in combinations with 0, 1, or 2 μm NAA. The cultures were maintained at 24 to 25C under continuous light or incubated in darkness for 2 weeks followed by continuous light before transfer to the secondary B5 medium (0 or 2 μm BA or 2 μm BA plus 4 μm GA3). Adventitious roots or a single shoot with roots formed on the explants cultured on media without plant growth regulators. Multiple buds were induced on all BA media, but more were produced with 5 or 10 μm for most lines. Dark incubation greatly enhanced multiple bud initiation. Shoot buds were not produced on media containing NAA alone or in combinations with BA. On the secondary medium, six to eight shoots per explant for common bean and up to 20 shoots per explant from tepary bean were observed after 3 weeks. Mature, fertile plants were produced from these shoots. Chemical names used: benzyladenine (BA); 1-naphthaleneacetic acid (NAA); gibberellic acid (GA3).

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Soon O. Park, Dermot P. Coyne, and James R. Steadman

Bean rust, caused by Uromyces appendiculatus, is an important disease of common bean (Phaseolus vulgaris L.). The objective was to identify RAPD markers linked to the gene (Ur-6) for specific resistance to rust race 51 using bulked segregant analysis in an F2 segregating population from the common bean cross pinto `Olathe' (resistant to rust) × great northern Nebraska #1 selection 27 (susceptible to rust). A single dominant gene controlling specific resistance to race 51 was hypothesized based on F2 segregation, and then was confirmed in the F3 generation. A good fit to a 3:1 ratio for band presence to band absence for each of three markers was observed in 100 F2 plants. Three RAPD markers were detected in a coupling phase linkage with the Ur-6 gene. Coupling-phase RAPD marker OAB14.600 was the most closely linked to the Ur-6 gene at a distance of 3.5 cM among these markers. No RAPD markers were identified in a repulsion phase linkage with the Ur-6 gene. The RAPD markers linked to the gene for specific rust resistance of Middle American origin detected here, along with other independent rust resistance genes from other germplasm, could be utilized to pyramid multiple genes into a bean cultivar for more durable rust resistance.

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Mohamed F. Mohamed, Dermot P. Coyne, and Paul E. Read

Plant regeneration has been achieved in two common bean lines from pedicel-derived callus that was separated from the explant and maintained through successive subcultures. Callus was induced either on B5 or MS medium containing 2% sucrose and enriched with 0.5 or 1.0 mg thidiaznron/liter alone or plus various concentrations of indoleacetic acid. The presence of 0.07 or 0.14 g ascorbic acid/liter in the maintenance media prolonged the maintenance time. Up to 40 shoot primordia were observed in 4-week-old cultures obtained from 40 to 50 mg callus tissues on shoot-induction medium containing 1-mg benzyladenine/liter. These shoot primordia developed two to five excisable shoots (>0.5 cm) on medium with 0.1-mg BA/liter. A histological study confirmed the organogenic nature of regeneration from the callus tissues. The R2 line from a selected variant plant showed stable expression of increased plant height and earlier maturity. Chemical names used: ascorbic acid, N- (phenylmethyl)-1H-pnrin-6-amine [benzyl-adenine, BA], 1H-indole-3-acetic acid (IAA), N- phenyl-N'-1,2,3-thiadiazol-5-ylurea [thidiazuron, TDZ].

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Guenhwa Jung, Dale T. Lindmen, and Dermot P. Coyne

Eight species and 57 selections/cultivars of Penstemon were compared for genetic variability using Random Amplified Polymorphic DNAs (RAPDs). The RAPD technique was used to help understand the genetic relationships in species and cultivars in the genus Penstemon. Ten RAPD primers (from Operon) were screened to identify polymorphisms among these eight species and 57 selections. More than 100 RAPD polymorphic bands were obtained. A principle component analysis was used to study genetic relationships. Variation among species was greater than variation among selections/cultivars within species. RAPD markers distinguished differences between most cultivars tested. DNA fingerprints generated by RAPDs should be useful to distinguish cultivars of Penstemon, as well as to assist in determining genetic relationships between species.

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Soon O. Park, Dermot P. Coyne, Atilla Dursun, and Geunhwa Jung

Common bacterial blight (CBB), incited by Xanthomonas campestris pv. phaseoli (Xcp), is an important seed-transmitted disease of common bean (Phaseolus vulgaris L.). Tepary bean (Phaseolus acutifolius A. Gray) has high resistance to Xcp. The objective of this study was to identify RAPD markers linked to genes controlling resistance to three isolates of Xcp using bulked segregant analysis in an F2 population from the tepary bean cross CIAT-G40005 (resistant to Xcp) × Nebr.#4B (susceptible to Xcp). Twelve RAPD markers were mapped in a coupling-phase linkage with three genes for resistance to Xcp. The linkage group spanned a distance of 19.2 cM. A marker L7750 was linked to the genes for resistance to Xcp strains EK-11 and LB-2 at 8.4 cM and 2.4 cM, respectively. Markers U10400 and Y14600 were detected as flanking markers for the resistance gene to Xcp strain SC-4A at 2.4 cM and 7.2 cM, respectively. The symbols Xcp-1, Xcp-2, and Xcp-3 were assigned for the genes for resistance to Xcp strains EK-11, LB-2, and SC-4A, respectively. RAPD markers linked to the genes for resistance to Xcp could be used for transferring all of the resistance genes from P. acutifolius to a susceptible P. vulgaris cultivar.

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Haytham Z. Zaiter, Dermot P. Coyne, Ralph B. Clark, and James R. Steadman

Nine bean cultivars/lines were grown in a Tripp sandy-clay loam (high pH), a Sharpsburg silty clay loam (neutral pH), and a potting mix (equal volume of sand, soil [Sharpsburg silty clay loam], vermiculite and moss pest) (low pH) in greenhouse (one experiment), growth chamber (two experiments), and field (two experiments) in Lincoln, NE, in order to evaluate the leaf reaction of the plants to a Nebraska rust (Uromyces appendiculatus var. appendiculatus) isolate US85-NP-10-1. A factorial arrangement of soil media and cultivars/lines in a randomized complete block design was used in the greenhouse and growth chamber experiments, while a split-plot design (soil media as main plots and cultivars/lines as sub-plots) was used in the field experiments. Significant differences were observed for rust pustule size of cultivars/lines grown on the three different soil media. Plants grown on potting mix medium showed significant Increases in rust pustule size compared with Tripp (high pH) or Sharpsburg silty clay loam soils (neutral pH). A significant interaction occurred between soil media and cultivars/lines for the rust reaction. A positive correlation (R= +0.5) was observed between the increased concentration of C1 and Mn,, and a negative correlation for lower K (R+ -0.44) and soil pH in the potting mix and larger rust pustule size of leaves. These results have implications for plant breeders and pathologists involved in evaluating bean progenies and lines for rust resistance.

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Haytham Z. Zaiter, Dermot P. Coyne, Ralph B. Clark, and James R. Steadman

Nine bean cultivars/lines were grown in a Tripp sandy-clay loam (high pH), a Sharpsburg silty clay loam (neutral pH), and a potting mix (equal volume of sand, soil [Sharpsburg silty clay loam], vermiculite and moss pest) (low pH) in greenhouse (one experiment), growth chamber (two experiments), and field (two experiments) in Lincoln, NE, in order to evaluate the leaf reaction of the plants to a Nebraska rust (Uromyces appendiculatus var. appendiculatus) isolate US85-NP-10-1. A factorial arrangement of soil media and cultivars/lines in a randomized complete block design was used in the greenhouse and growth chamber experiments, while a split-plot design (soil media as main plots and cultivars/lines as sub-plots) was used in the field experiments. Significant differences were observed for rust pustule size of cultivars/lines grown on the three different soil media. Plants grown on potting mix medium showed significant Increases in rust pustule size compared with Tripp (high pH) or Sharpsburg silty clay loam soils (neutral pH). A significant interaction occurred between soil media and cultivars/lines for the rust reaction. A positive correlation (R= +0.5) was observed between the increased concentration of C1 and Mn,, and a negative correlation for lower K (R+ -0.44) and soil pH in the potting mix and larger rust pustule size of leaves. These results have implications for plant breeders and pathologists involved in evaluating bean progenies and lines for rust resistance.

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Soon O. Park, Dermot P. Coyne, James R. Steadman, Paul W. Skroch, and Geunhwa Jung

The objective was to detect molecular markers associated with QTL for partial physiological resistance (PPR) to two white mold (WM) isolates, partial field resistance (PFR), plant architecture (PA), and plant height (PH) in a genetic linkage map constructed using recombinant inbred lines (RILs) from the cross `PC-50' (resistant to WM) × XAN-159 (susceptible to WM). Significant correlations (+0.39 and +0.47) were noted between the WM reactions in the greenhouse and field. A significant but negative correlation (–0.33) was observed between the WM reaction and PH in the field. Six QTL affecting PPR to isolate 152 were found on LGs 4, 5, 7, and 8. Six QTL affecting PPR to isolate 279 were found on LGs 2, 3, 4, 7, and 8. Five QTL for PFR were observed on LGs 2, 5, 7, 8, and 11. Two QTL affecting PA were detected on LGs 7 and 8. Two QTL affecting PH were identified on LGs 7 and 8. On one end of LG 8 marker H19.1250 was significant for PPR to both isolates. On the other end of LG 8 the region closely linked to the C locus was significantly associated with PPR to both isolates, PFR, PA and PH. Marker J09.950 on LG 7 was significantly associated with PPR to both isolates, PFR, PH and seed weight. Marker J01.2000 on LG 2 was the most significant locus for both PPR to the isolate 279 and PFR. QTL on LG 5 were found for PPR to the isolate 152 and PFR. Overall, four of the five QTL affecting PFR were also found for PPR to one or both isolates.