Browsing by Author "Todd Wehner, Committee Chair"

Filter results by typing the first few letters
Now showing 1 - 3 of 3
  • No Thumbnail Available
    Inheritance of Fruit Yield and other Horticulturally important Traits in Watermelon [Citrullus lanatus (Thunb.) Matsum. & Nakai]
    (2009-07-09) Kumar, Rakesh; Todd Wehner, Committee Chair
    Watermelon is a diverse crop in terms of qualitative genetic traits, with many different fruit types among the cultivars. We were interested in determining the optimum method for improving yield, and measuring the heritability and rate of natural outcrossing within elite populations. The rate of natural outcrossing is dependent upon the distance between plants, as well as other factors such as location and cultivar. The objective of this study was to 1) estimate narrow-sense heritability for yield using parent-offspring regression in two watermelon populations (NCHYWI and NCHYW2); 2) determine the inheritance of fruit yield, rind pattern, and fruit shape in six generations (PaS1, PaS2, F1, F2, BC1Pa and BC1Pb) of three families; and 3) determine the rate of natural outcrossing effected by in-row spacing and cultivars. Field trials were conducted at two locations in North Carolina (Clinton and Kinston) to determine the narrow-sense heritability for yield in NCHYW1 and NCHYW2 watermelon populations using parent-offspring regression. Low estimates of narrow-sense heritability were recorded for total fruit weight (0.04-0.12), marketable fruit weight (0.06-0.15), total fruit number (0.04-0.16), fruit size (0.18-0.19), and percent culls (0.02-0.09) for NCHYW1 and NCHYW2 populations, respectively. Only low gain in yield can be made due to single-plant selection, based on the populations used. Strong positive genotypic correlations were observed between total fruit weight and marketable fruit weight; total fruit weight and marketable fruit weight with fruit size, and negative correlation was recorded between total fruit number and fruit size, and total fruit weight and marketable fruit weight with percent culls. In the second experiment consisting of three families and six generations, a low to intermediate level of heritability was reported for total fruit weight, total fruit number, and fruit size. Recurrent selection is recommended to improve populations for these traits to accumulate favorable genes. The family ‘Mountain Hoosier’ x ‘Calsweet’ did not fit the expected segregation ratios for the single dominant gene for solid dark green rind pattern (G) (solid dark green vs. wide stripe), or for the single incompletely dominant gene for elongate fruit shape (O) (elongate vs. round) in the F2 and backcross, different from previous reports. Deviation from expected ratios was also observed for the single dominant gene controlling solid dark green rind (G) and light green rind (g) in the families ‘Mountain Hoosier’ x ‘Minilee’ and ‘Early Arizona’ x ‘Minilee’. However, segregation ratios showed that light green rind was controlled by two recessive genes, g-1 and g-2, by duplicate dominant gene action when crossed to a line having solid dark green rind. The rate of natural outcrossing was determined by using split plot in randomized complete block design in two locations: Kinston and Clinton; eight in-row spacing as whole plots: 0.6, 1.2, 1.8, 2.4, 3.0, 3.7, 4.3, and 4.9 m; and two cultivars as sub plots: ‘Allsweet’ and ‘Mickylee’. ‘Moon and Stars’ was used as the pollen donor of marker gene to track the rate of natural outcrossing. Location and cultivar, and all interactions were not significant. Closer in-row spacing (0.6 m and 1.2 m) had a significantly higher rate of natural outcrossing (11.0% and 16.9%, respectively) than wide in-row spacing (> 4.3 m), which had a low rate of outcrossing (1.8 %). In conclusion, watermelon appears to act more like a self-pollinated crops when spaced >5 m apart. The rate of natural outcrossing should also be taken in to account while estimating heritability and genetic variance in watermelon populations.
  • No Thumbnail Available
    Screening Cucumber (Cucumis sativus) for Resistance to Downy Mildew (Pseudoperonospora cubensis)
    (2008-07-22) Criswell, Adam; Todd Wehner, Committee Chair
    Downy mildew, a foliar disease caused by the oomycete Pseudoperonospora cubensis (Berk. and Curt.) Rostow. is one of the most destructive pathogens of cucurbits. Resistant cultivars are available but nevertheless yield losses are high in North Carolina and Poland if fungicides are not used. The first objective of this experiment was to test all available plant introduction accessions from the U.S. National Plant Germplasm System of cucumber for downy mildew resistance under field conditions. The 1289 cultigens were tested at Clinton NC, USA, and Skierniewice, Poland during 2005-2007 under natural field epidemics of the disease. Averaged over locations, eighty-one cultigens were classified as highly resistant, 130 as moderately resistant, 406 as intermediate, 408 as moderately susceptible, and 271 as highly susceptible. The 40 most resistant and 10 most susceptible cultigens from these field trials, were further evaluated in replicated field and greenhouse experiments in North Carolina, along with 22 check cultivars. Results from the retest study in North Carolina confirmed the results of the initial screening study, although the range of downy mildew ratings in the North Carolina field retest were much narrower compared with the screening results obtained in the larger study. The most resistant and most susceptible lines in the screening study were also the most resistant and most susceptible lines in the field retest. The most resistant 10 cultigens averaged over both locations were Ames 2353, Ames 2354, PI 197085, PI 197088, PI 234517, PI 321008, PI 330628, PI 432878, PI 605996 and PI 618931. These cultigens originated from India, the United States, Pakistan, P.R. China and Taiwan. Despite the identification of resistant plant introduction accessions, they were not significantly better than the most resistant cultivars currently used in either North Carolina or Poland. The most positive aspect of the screening effort was that resistant plant introductions originated from diverse geographic regions. Because geographic diversity is often associated with genetic diversity in germplasm collections, the newly identified resistant typed may carry unique alleles as compared to commercial material. If so, then the potential exists to develop recombinant types from crosses of commercial by exotic materials which may be more resistant. The second objective of this study was to measure the correlation of four response traits. A low correlation among the four response traits on a diverse array of cucumber cultigens would suggest that the traits are controlled by different genes. Field studies were conducted to measure the response traits of plant stunting, leaf necrosis, chlorosis and sporulation caused by downy mildew infection. Each of the four traits were measured on 67 diverse cucumber cultigens in North Carolina and India. All cucumber cultigens were tested in four replications and two locations under natural field epidemics of the disease. A significant genotype by location interaction was found by analysis of variance and data from the two locations were analyzed separately. In North Carolina, necrosis and chlorosis were highly correlated (r=0.90) while sporulation was moderately correlated with necrosis and chlorosis(r=0.71 and r=0.70, respectively) and not significantly correlated with stunting. Stunting was moderately correlated with necrosis and chlorosis (r=0.43 and r=0.34, respectively). In India, chlorosis and sporulation were highly correlated (r=0.97) while necrosis was moderately correlated with chlorosis, sporulation and stunting (r=0.67 and r=0.0.65 and r=0.76, respectively). Stunting was moderately correlated with chlorosis and sporulation (r=0.55 and r=0.57, respectively). Sporulation or necrosis may be controlled by a different gene(s) but another year of testing is required. Stunting may also be controlled by a different gene(s) but difficulties in differentiating between stunting resulting from genotype and stunting resulting from disease must be resolved. Different degrees of correlation among chlorosis, necrosis and sporulation in North Carolina and India may be due to the presence of different races in the two locations. These differences may also be explained by the variable number and timing of ratings between the two locations. Availability of only one set of data for sporulation in North Carolina may have reduced the correlation between it and necrosis and chlorosis. Sporulation ratings need to be taken on a weekly basis rather than once during the last rating. Therefore, the possibility exists that chlorosis, necrosis and sporulation are response traits controlled by the same genes.
  • No Thumbnail Available
    Studies on Resistance to Downy Mildew in Cucumber (Cucumis sativus L.) Caused by Pseudoperonospora cubensis
    (2010-04-30) Call, Adam Dean; Peter Ojiambo, Committee Member; Paul Murphy, Committee Member; Todd Wehner, Committee Chair
    CALL, ADAM DEAN. Studies on Resistance to Downy Mildew in Cucumber (Cucumis sativus L.) Caused by Pseudoperonospora cubensis. (Under the direction of Todd C. Wehner, M.S.) Downy mildew, caused by the oomycete pathogen Pseudoperonospora cubensis (Berk. And Curt) Rostov, is a major foliar disease of cucumber (Cucumis sativus L.) Currently, high yield and quality in the presence of downy mildew is achieved with multiple fungicide applications. Most of the currently grown cultivars have some resistance to downy mildew. Prior to a 2004 outbreak in the United States, host resistance was sufficient to control the disease, and downy mildew was only a minor problem on cucumber. There are currently no cultivars that show resistance at a level equal to that seen prior to 2004. However, differences in resistance among cultivars exist, ranging from moderately resistant to highly susceptible. Both host resistance and fungicides contribute to control of downy mildew for growers. Experiments were conducted to identify resistant and high yielding cultigens and and to determine the contribution of resistance and fungicides to overall control of downy mildew. There were three major experiments. All experiments rated disease using a 0 to 9 scale (0=none, 1-2=trace, 3-4=slight, 5-6=moderate, 7-8=severe, 9=dead). The objective of experiment 1 was to identify new sources of resistance to downy mildew among plant introduction accessions from the U.S. National Plant Germplasm System, elite cultivars, and breeding lines of cucumber. The 1300 cultigens were tested at Clinton NC, USA, and Skierniewice, Poland during 2005-2007 under natural field epidemics of the disease in unreplicated trials. Mean ratings for downy mildew leaf damage in the germplasm screening ranged from 1.0 to 7.3 in North Carolina and from 0.3 to 9.0 in Poland. The 40 most resistant and 10 most susceptible cultigens, along with 22 check cultivars were further evaluated in replicated field and greenhouse experiments in North Carolina and India in 2007 to 2009 for a total of eight environments (year by location). A fungicide component was added in 2008 and 2009 in that one environment each year was a field treated weekly with applications of Previcur Flex and Mancozeb alternating with Bravo and Tanos. Results from the retest study in NC confirmed the results of the initial screening study. The most resistant and most susceptible cultigens in the screening study were also the most resistant and most susceptible cultigens in the field retest. Cultigens were found that significantly outperform checks. High yielding and tolerant cultigens were also been identified, which could be used in developing improved cultivars. The objective of experiment 2 was to identify cultivars having high yield and resistance to the new downy mildew. The experiment had 86 cultigens, three locations (Clinton and Castle Hayne, NC, and Bath, MI), three years (2007 to 2009) and 4 replications per location. None of the cultigens tested in this study showed a high level of resistance, although differences in resistance and yield among cultigens do exist. The objective of experiment 3 was to evaluate different fungicide treatments, chosen to represent different levels of efficacy, combined with different levels of resistance (resistant - M 21, moderate - 'Sumter', susceptible - 'Wisconsin SMR-18') among cultigens for the effect on disease severity and yield. There were six and twelve replications in 2008 and 2009, respectively. Cultigen had a large effect in both years. As expected, using a resistant cultivar significantly improved resistance and most yield traits compared to a susceptible cultivar. Fungicide has a smaller effect on resistance traits and larger effect on total yield and percent marketable yield. To achieve maximum yield both a resistant cultivar and fungicide spray program should be used.