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Kevin E. McPhee, Abebe Tullu, John M. Kraft, and Fred J. Muehlbauer

Plant breeders must be aware of sources of resistance to pathogens that affect their crops. Fusarium wilt caused by Fusarium oxysporum Schl. f. sp. pisi Snyd. & Hans. is a fungal disease that affects peas and is important worldwide. Resistance to the different races of the pathogen has been identified in adapted germplasm and from specific accessions in the United States World Collection of peas (Pisum sativum L.). The goal of this study was to evaluate the resistance to fusarium wilt race 2 in the Pisum core collection. Of the 452 accessions screened, 62 (14%) were resistant. The resistant accessions included accessions from P.s. ssp. elatius that were collected from 24 different countries. The wide distribution of resistance around the world precludes the identification of any single country or region as a source of resistance. Of the 62 accessions resistant to race 2, 39 are also resistant to race 1 based on data obtained from GRIN. One of the wild progenitors, PI 344012, possessed resistance to races 1 and 2.

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R. Provvidenti and R.O. Hampton

Resistance to white lupin mosaic virus (WLMV), a recently characterized member of the potyvirus group, was found in pea (Pisum sativum L.) plant introductions from Ethiopia (PI 193835) and India (PI 347485). In cross and backcross populations between plants of resistant PI 193835 with those of susceptible `Bonneville' and PP-492-5, this resistance was demonstrated to be governed by a single recessive gene. This gene was distinct from other genes previously found in PI 193835 and PP-492-5 (from PI 347492, India) conferring resistance to clover yellow vein virus (CYVV) and three strains of pea seedborne mosaic virus (PSbMV). Indirect evidence suggests that this newly recognized viral resistance gene, wlv, is a member of a cluster of closely linked genes located on chromosome 6. This gene cluster includes sbm-1, sbm-3, and sbm-4, which govern resistance to three PSbMV pathotypes, and cyv-2, which governs resistance to CYVV.

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Rebecca J. McGee and James R. Baggett

There was no difference in percentage in vitro germination of pollen from stringless pea (Pisum sativum L.) cv. Sugar Daddy and stringy `Oregon Sugarpod II' (OSP) and `OSU 705' (705). However, pollen tubes of `Sugar Daddy' grew more slowly in vitro than those of OSP or 705. Differences in pollen tube growth rate were demonstrated in vivo following time-course pollinations involving reciprocal crosses of `Sugar Daddy' with OSP and 705, along with the selfed parents. After 8 hours, pollen tubes from stringless peas (“stringless” pollen) had entered 13% of the ovules compared with 51% for those from stringy peas (“stringy” pollen). Stringless pollen tubes entered 29% and stringy pollen tubes 66% of the ovules after 10 hours. The slower growth of stringless compared with stringy pollen tubes is a plausible explanation for previously observed deficiencies of stringless plants in segregating populations.

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Gaétan Bourgeois, Sylvie Jenni, Hélène Laurence, and Nicolas Tremblay

The heat-unit system, involving the sum of daily mean temperatures above a given base temperature, is used with processing pea (Pisum sativum L.) to predict relative maturity during the growing season and to schedule planting dates based on average temperature data. The Quebec pea processing industry uses a base temperature of 5 °C to compute growing-degree days (GDD) between sowing and maturity. This study was initiated to verify if the current model, which uses a base temperature of 5 °C, can be improved to predict maturity in Quebec. Four pea cultivars, `Bolero', `Rally', `Flair', and `Kriter', were grown between 1985 and 1997 on an experimental farm in Quebec. For all cultivars, when using a limited number of years, a base temperature between 0.0 and 0.8 °C reduced the coefficient of variation (cv) as compared with 5.0 °C, indicating that the base temperature used commercially is probably not the most appropriate for Quebec climatic conditions. The division of the developmental period into different stages (sowing until emergence, emergence until flowering, and flowering until maturity) was also investigated for some years. Use of base temperatures specific for each crop phase did not improve the prediction of maturity when compared with the use of an overall base temperature. All years for a given cultivar were then used to determine the base temperature with the lowest cv for predicting the time from sowing to maturity. A base temperature from 0 to 5 °C was generally adequate for all cultivars, and a common base temperature of 3.0 °C was selected for all cultivars. For the years and cultivars used in this study, the computation of GDD with a base temperature of 3 °C gave an overall prediction of maturity of 2.0, 2.4, 2.2, and 2.5 days based on the average of the absolute values of the differences for the cultivars Bolero, Rally, Flair, and Kriter, respectively.

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Melissa T. McClendon, Debra A. Inglis, Kevin E. McPhee, and Clarice J. Coyne

Dry pea (Pisum sativum L.) production in many areas of the world may be severely diminished by soil inhabiting pathogens such as Fusarium oxysporum f. sp. pisi race 1, the causal organism of fusarium wilt race 1. Our objective was to identify closely linked marker(s) to the fusarium wilt race 1 resistance gene (Fw) that could be used for marker assisted selection in applied pea breeding programs. Eighty recombinant inbred lines (RILs) from the cross of Green Arrow (resistant) and PI 179449 (susceptible) were developed through single-seed descent, and screened for disease reaction in race 1 infested field soil and the greenhouse using single-isolate inoculum. The RILs segregated 38 resistant and 42 susceptible fitting the expected 1:1 segregation ratio for a single dominant gene (χ2 = 0.200). Bulk segregant analysis (BSA) was used to screen 64 amplified fragment length polymorphism (AFLP) primer pairs and previously mapped random amplified polymorphic DNA (RAPD) primers to identify candidate markers. Eight AFLP primer pairs and 15 RAPD primers were used to screen the RIL mapping population and generate a linkage map. One AFLP marker, ACG:CAT_222, was within 1.4 cM of the Fw gene. Two other markers, AFLP marker ACC:CTG_159 at 2.6 cM linked to the susceptible allele, and RAPD marker Y15_1050 at 4.6 cM linked to the resistant allele, were also identified. The probability of correctly identifying resistant lines to fusarium wilt race 1, with DNA marker ACG:CAT_222, is 96% percent. These markers will be useful for marker assisted breeding in applied pea breeding programs.

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Steven J. Guldan, Charles A. Martin, and Constance L. Falk

`Sugar Snap' snap peas (Pisum sativum L.) were interseeded into a stand of `Española Improved' chile pepper (Capsicum annuum L.) in July or Aug. in 1995, 1996, and 1997. Peas were interseeded as one or two rows per bed, giving planting rates of about 92 or 184 kg·ha-1, respectively. Our objectives were to determine: 1) if intercropped pea would reduce chile yield and vice versa; 2) the effects of pea planting rates and dates on pea yield. Intercropped peas reduced chile yield by about 22% in 1995, but had no significant effects in other years. Pea plants from the August intercrops reached the flowering stage but did not produce pods in 1995 or 1996; some small pods were produced from August intercrops in 1997. Final plant densities were lower and less uniform in 1996 than in 1995 or 1997. Intercropped peas yielded less than monocropped peas in all years. Pea yields ranged from 1370 to 3960 kg·ha-1 when monocropped, 31 kg·ha-1 (1996 single-row) to 646 kg·ha-1 (1995 double-row) when intercropped. In 1995 only, the double-row intercrop yielded more peas than the single-row intercrop. Pod yield/plant was reduced 80%, 98%, and 96% in 1995, 1996, and 1997, respectively, by intercropping. Estimated gross revenues for the treatments indicate that, under the price assumptions used in the study, interseeding snap peas into stands of chile in north-central New Mexico is not economically advantageous compared with monocropped chile.

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S. Brauner, R.L. Murphy, J.G. Walling, J. Przyborowski, and N.F. Weeden

DNA primers for 37 genes have been developed in pea (Pisum sativum L.). Two-thirds of these primers also amplify orthologous sequences in lentil (Lens culinaris). The primers were designed to be complementary to highly conserved sequences in exons of known genes. In addition, most of the priming sequences were selected to be 1000 to 3000 bp distant on the genomic DNA and to amplify a fragment that contained at least one intron. Segregating sequence polymorphism in mapping populations of recombinant inbred lines (RILs) derived from wide crosses in Pisum was observed by restriction of the amplified fragment with endonucleases recognizing four-base restriction sites. Successful mapping of 36 of these genes in pea demonstrated the utility of these primers for mapping, and it appears likely that the primers should have general utility for comparative mapping in legumes.

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Orion P. Grimmer and John B. Masiunas

Winter-killed oats (Avena sativa) may have potential for use to suppress weeds in early seeded crops such as pea (Pisum sativum). Residue biomass and surface coverage are generally correlated with weed suppression. Oat residues also contain allelochemicals. Our objective was to determine if oat cultivars vary in residue production and allelopathy. Differences between oat cultivars were observed in residue production, and for effects on emergence of common lambsquarters (Chenopodium album) and shepherd's-purse (Capsella bursa-pastoris) in the greenhouse, and germination of pea and common lambsquarters in an infusion assay. Two of the oat cultivars producing the greatest biomass, `Blaze' (in the field) and `Classic' (in the greenhouse), interfered minimally with pea germination and were among the best cultivars in inhibiting common lambsquarters and shepherd's-purse. `Blaze' also greatly inhibited common lambsquarters germination in the infusion assay that measured allelopathy. Thus, `Blaze' and `Classic' possess suitable characteristics for use as a cover crop preceding peas.

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Michael P. Croster and John B. Masiunas

Studies established the critical period for eastern black nightshade (nightshade) (Solanum ptycanthum Dun.) competition in pea (Pisum sativum L.) and determined the effect of N fertility on pea and nightshade growth. In 1992, pea yields were most affected when nightshade was established at planting and remained for 4 or 6 weeks, while in 1993, competition for 6 weeks caused the greatest reduction in pea yields. In a sand culture study, pea biomass and N content were not affected by three N levels (2.1, 21, and 210 mg·L-1). Nightshade plants were five to six times larger in the highest N treatment than at lower N levels. Nitrogen content of nightshade was 0.76% at 2.1 ppm N and 3.22% at 210 ppm N. Choosing soils with low N levels or reducing the N rates used in pea may decrease nightshade interference and berry contamination of pea.

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Hideyuki Takahashi, Christopher S. Brown, Thomas W. Dreschel, and Tom K. Scott

Orientation of root growth on earth and under microgravity conditions can possibly be controlled by hydrotropism-growth toward a moisture source in the absence of or reduced gravitropism. A porous-tube water delivery system being used for plant growth studies is appropriate for testing this hypothesis since roots can be grown aeroponically in this system. When the roots of the agravitropic mutant pea ageotropum (Pisum sativum L.) were placed vertically in air of 91% relative humidity and 2 to 3 mm from the water-saturated porous tube placed horizontally, the roots responded hydrotropically and grew in a continuous arch along the circular surface of the tube. By contrast, normal gravitropic roots of `Alaska' pea initially showed a slight transient curvature toward the tube and then resumed vertical downward growth due to gravitropism. Thus, in microgravity, normal gravitropic roots could respond to a moisture gradient as strongly as the agravitropic roots used in this study. Hydrotropism should be considered a significant factor responsible for orientation of root growth in microgravity.