Theses and Dissertations from UMD
Permanent URI for this communityhttp://hdl.handle.net/1903/2
New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a give thesis/dissertation in DRUM
More information is available at Theses and Dissertations at University of Maryland Libraries.
Browse
4 results
Search Results
Item SEASONAL DEVELOPMENT OF DOLLAR SPOT EPIDEMICS IN MARYLAND AND NITROGEN EFFECTS ON FUNGICIDE PERFORMANCE IN CREEPING BENTGRASS(2011) Ryan, Chris Patrick; Dernoeden, Peter H; Plant Science and Landscape Architecture (PSLA); Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Dollar spot (Sclerotinia homoeocarpa) is a common and destructive disease of creeping bentgrass (Agrostis stolonifera). The frequency and severity of dollar spot epidemics has not been quantified and there are no effective predictive models. High rates of nitrogen (N) reduce dollar spot injury, but low N rates applied in summer have not been assessed for disease suppression. Field studies were conducted from 2008 to 2010 with the following objectives: a) to describe the relationship among season, environmental factors and the severity of dollar spot epidemics in six creeping bentgrass cultivars; b) to evaluate six water soluble N sources applied at a low rate (7.3 kg N ha-1) in summer for their impact on dollar spot severity; and c) to assess the performance of low fungicide rates tank-mixed with N on dollar spot severity. Two epidemics were observed each year between spring and mid- autumn, with the second being most severe. A third, late autumn epidemic also was observed in each year. The first epidemic in May was effectively predicated using a degree day model having a biofix date of 1 April and a 15°C base temperature. Ammonium sulfate was most consistently effective in reducing dollar spot injury, but caused foliar injury. Tank-mixing a low chlorothalonil rate with N generally reduced fungicide efficacy.Item Corn Residual Nitrate and its Implications for Fall Nitrogen Management in Winter Wheat(2011) Forrestal, Patrick Joseph; Kratochvil, Robert J; Plant Science and Landscape Architecture (PSLA); Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Corn (Zea mays, L.) production typically requires supplemental nitrogen (N) to optimize yields. In dryland corn production systems, where N is applied during the early to mid-vegetative growth stages, inappropriate N applications or limited moisture during the growing season can result in large disparities between optimum and applied N rates. This leads to variable post-harvest residual nitrate (NO3-N) accumulation, which is susceptible to loss. However, this NO3-N could provide the starter N requirement of the subsequent winter wheat (Triticum aestivum, L.) crop. Accounting for residual NO3-N present at wheat planting is important to avoid compounding N loss potential due to corn residual NO3-N accumulation. The objectives of this study were to 1) examine plant based tools for assessing soil NO3-N; 2) to examine post-harvest residual NO3-N accumulation patterns following corn production; 3) to determine optimum fall starter N rates for winter wheat production; and 4) to identify a soil NO3-N level above which starter N could be forgone without negative agronomic effect. This study found that plant canopy measurements are useful tools for assessing corn N management and for identifying drought sites, which had the greatest NO3-N accumulations. The corn stalk nitrate test was significantly (p<0.001) and positively correlated with soil residual NO3-N (r2=0.41). Greatest soil residual NO3-N accumulation occurred where drought conditions reduced production. The agronomic optimum fall starter N rate for winter wheat in Maryland is 17 to 34 kg N ha-1 where soil NO3-N concentration to 15 cm depth is less than 15 mg kg-1. However, the fall starter N response was highly variable and declined significantly (p<0.01) as fall precipitation after planting increased. The results of this study indicate that residual NO3-N levels at planting should be considered before applying fall starter N to winter wheat.Item Scab Resistance QTLs are Associated with Quality and Agronomic Traits of Soft Red Winter Wheat(2011) Cardwell, Lydia Ann; Costa, Jose; Plant Science and Landscape Architecture (PSLA); Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Fusarium head blight (FHB) is a devastating fungal disease affecting Triticum aestivum crops worldwide. While many quantitative trait loci (QTL) responsible for FHB resistance have been reported, some widely used sources are from exotic cultivars that may carry undesirable alleles linked with resistance. Ning_7840, a Chinese hard red spring wheat, contains a major FHB QTL on the 3BS chromosome, along with two minor QTL on the 5A and 2DL chromosomes. Ning_7840 was crossed with Pioneer_2643, a soft red winter wheat, to create 86 recombinant inbred lines. The effect of the Ning_7840 alleles on agronomic traits and milling and baking quality traits was examined over three growing seasons in Maryland. While the 3BS QTL was not associated negatively with other traits, the 2DL and 5A QTL were. This suggests the introduction of FHB resistance QTL on 5A and 2DL into soft red winter wheat may negatively affect agronomic and quality traits.Item Comparing Source of Agricultural Contact Water and the Presence of Fecal Indicator Organisms on the Surface of 'Juliet' Grape Tomatoes(2010) Pahl, Donna Marie; Walsh, Christopher S; Plant Science and Landscape Architecture (PSLA); Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Microbial water quality standards exist to prevent food safety outbreaks due to the use of agricultural water, although little is known about how the levels the fecal indicator organisms in water relate to the counts on the tomato fruit surface. This study used fecal indicator organisms commonly used in microbial water quality standards (Enterobacteriaceae, total coliforms, fecal coliforms, and E. coli) to monitor the water quality of surface ponds, a groundwater source, and the phyllosphere of treated grape tomatoes over two growing seasons. Water source and date caused a significant difference in the counts of fecal indicator organisms. Variability in bacterial counts was found in the surface water sources over the course of the season, partially explained by environmental variables such as water temperature, pH, precipitation, and air temperature. The microbial counts on the surfaces of the tomato fruit did not reflect the water treatments applied to the plants.