Theses and Dissertations from UMD
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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
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Item Investigating Cultural Practices to Improve the Efficacy and Reliability of Biological Control Organisms in Turfgrass Systems(2021) Doherty, Joseph; Murphy, Angus; Plant Science and Landscape Architecture (PSLA); Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Biological control, or suppressing plant pathogens through natural predators or competitors, has been an area of scientific intrigue for many decades. However, inconsistent efficacy remains the chief reason for a lack of widespread adoption by growers. This dissertation was developed to address the inconsistencies of biological control through three practices. First, reducing competition from resident foliar microbial communities, or niche clearing, was explored for brown patch suppression and biological control organism establishment. Second, biological control organisms were applied to the rhizosphere and evaluated for suppression of root infecting pathogens. Finally, combinations of monthly biochar topdressing and biweekly or weekly biological control organism applications were evaluated for foliar pathogen suppression and biological control organism establishment. In each study, biological control organisms were observed to suppress pathogens and reduce disease severity. However, neither niche clearing nor biochar topdressing increased disease control compared to the biological control organism applied alone. While biochar applications did not improve the efficacy of biological control organisms, they did reduce the severity of disease unaffected by biological control organisms and increased turfgrass quality. Furthermore, biological control organisms were most successful under low to moderate levels of disease pressure, as control was lost when disease pressure peaked each year. Once again, neither niche clearing nor biochar topdressing increased populations of biological control organisms compared to the organisms applied alone. Weekly applications of biological control organisms did result in higher population levels compared to biweekly applications, and rhizosphere targeted applications appeared to have resulted in an establishment of the biological control organism into the rhizosphere community. Findings from these research projects suggest that biological control organisms should be applied weekly to increase odds of successful disease reduction. Given the lack of control under high disease pressure in all studies, successful implementation of biological controls should be targeted to the shoulders of the season when disease pressure is lower, or biological control organisms should be implemented into a season long program to supplement a traditional fungicide program.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 INTEGRATED PEST MANAGEMENT OF FUSARIUM HEAD BLIGHT IN SOFT RED WINTER WHEAT(2011) Reed, Elizabeth; Grybauskas, Arvydas; Plant Science and Landscape Architecture (PSLA); Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Fusarium head blight (FHB), caused by Fusarium graminearum, is a destructive disease of the soft red winter wheat grown in the Mid-Atlantic region. Management of FHB focuses primarily on foliar fungicides or cultivar resistance. The purpose of this research was to examine how type II resistance (resistance to spread of the pathogen) is affected by multiple infections along the spike. The combination of type II resistance and fungicide as a way to manage FHB was evaluated in both the greenhouse and field settings. Finally, the role of increased foliage density in an integrated pest management program that included fungicide and cultivar resistance was also evaluated. Multiple infections occurring along a single wheat spike can overwhelm the type II resistance present in some cultivars. The combination of type II resistance and fungicide was the best management practice for FHB than either alone. Foliage density did not improve FHB disease ratings.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.