Thesis (Ph. D.)--University of Missouri-Columbia, 1998. / Typescript. Vita. Includes bibliographical references. Also available on the Internet.

Thesis (Ph. D.)--University of Missouri-Columbia, 1998. / Typescript. Vita. Includes bibliographical references. Also available on the Internet.

Greenhouse and microplot studies were conducted for understanding the effects of cover crop species/cultivars for hosts and population reduction of soybean cyst nematode (SCN; Heterodera glycines) from the fields of North Dakota. Moreover, early-maturing soybean [Glycine max (L.)] accessions from different countries of origin were screened for resistance against two common SCN populations for finding new sources of resistance. Thirty-eight cover crop species/cultivars were evaluated for their hosting ability of two SCN populations (SCN103 and SCN2W) from two fields of North Dakota in greenhouse experiments. The majority of the tested crops were non-hosts for both SCN populations. However, a few of them, such as Austrian winter pea (Pisum sativum L.), crimson clover (Trifolium incarnatum L. cv. Dixie), crambe (Crambe abyssinica, cv. BelAnn), field pea, cvs. Aragorn and Cooper, hairy vetch (Vicia villosa Roth), turnip (Brassica rapa L. cv. Purple top), and white lupine (Lupinus albus L.) were poor-hosts/hosts of both SCN populations. Furthermore, thirteen of them were tested for the SCN population reduction either or both in the greenhouse and microplot experiments. Out of 13, at least four crops, such as annual ryegrass (Lolium multiflorum L.), brown mustard (Brassica juncea L. cv. Kodiak), daikon radish (Raphanus sativus L.), and turnip cv. Pointer showed more than 50% population reduction compared with initial population densitiy, consistently in the greenhouse or microplot experiments. The resistance screening of 152 early-maturing soybean accessions showed that a majority of the accessions were susceptible/moderately susceptible to both SCN populations (SCN HG type 0 and 2.5.7), while a few (n=18) showed good resistance responses to both or either of the SCN populations. The cover crops, which were non-hosts/poor-hosts and have a greater ability for the SCN population reduction have great potential to be included in an integrated SCN management strategy. The novel resistant accessions identified in this study have the potential to be used in soybean breeding for developing SCN-resistant cultivars after confirming their resistance response and identifying the resistance genes/loci. The results obtained from this study helps in developing a sustainable SCN management strategy in the northern Great Plains.

Heterodera glycines, soybean cyst nematode (SCN) causes more than one billion dollars soyben production loss in the U.S. annually. SCN is an obligate parasite of specialized feeding cells within the host root known as syncytium. The SCN resistance genes and signaling pathways in soybean have not been fully characterized. Gene expression analysis in syncytium from compatible and incompatible interactions identified candidate genes that might involve conferring resistance to SCN. This dissertation aimed to investigate the biological functions of the candidate resistance genes to confirm the roles of these genes in resistance to SCN. The study demonstrated a role of syntaxin 31-like genes (Gm-SYP38) in resistance to SCN. Overexpression of Gm-SYP38 induced the transcriptional activity of the cytoplasmic receptor-like kinase BOTRYTIS INDUCED KINASE 1 (Gm-BIK1-6). Overexpression of Gm-BIK1-6 rescued the resistant phenotype. In contrast, Gm-BIK1-6 RNAi increased parasitism. In another experiment, the expression of a Glycine max homolog of LESION SIMULATING DISEASE1 (LSD1) resulted in the transcriptional activation of ENHANCED DISEASE SUSCEPTIBILITY1 (EDS1) and NONEXPRESSOR OF PR1 (NPR1), that function in salicylic acid (SA) signaling, implicating the involvement of the antiapoptotic, environmental response gene LESION SIMULATING DISEASE1 (LSD1) in defense that is demonstrated here. The study also investigated the role of SNARE components (genes functioning in membrane fusion) in resistance to SCN. Experiments showed that SNARE functions in concert with a beta-glucosidase having homology to PEN2 and an ATP binding cassette transporter having homology to PEN3. This study provides novel information for the genetic improvement of soybean for enhanced disease resistance.

Plant-parasitic nematodes represent a substantial constraint on global food security by reducing the yield potential of all major crops, including soybean (Glycine max L.). The soybean cyst nematode (SCN) (Heterodera glycines I.) is widely distributed across all soybean production areas of the US, and is the major yield-limiting factor, especially in the Midwestern US. Double cropping (DC) is defined as producing more than one crop on the same parcel of land in a single growing season. Compared to conventional single annual crops, DC provides many advantages, including improving soil health, enhanced nutrient provisioning to plants, improvement of soil physical properties, control of erosion, decrease in tillage requirements, and enhanced profitability. In some double-cropping systems, soybean is planted following winter wheat (Triticum aestivum L.), and several reports suggest the potential of wheat to suppress SCN populations. Field trials were conducted from 2017 to 2018 to investigate the effect of wheat on SCN populations in double-cropping soybean. Nine fields with three levels of initial SCN populations (low, moderate, and high) were selected in Illinois. Wheat was planted in strips alternating with strips-maintained weed-free and under fallow over winter and early spring. Soybean was planted in all strips after the wheat harvest. Soybean cyst nematode egg densities were acquired at four time points: wheat establishment, post-wheat/pre-soybean, mid-soybean (R1 growth stage or beginning of flowering), and post-soybean harvest. Wheat strips reduced SCN egg densities compared with fallow strips at the R1 stage (−31.8%) and after soybean harvest (−32.7%). Field locations with noted SCN suppression were selected for a metagenomics study. The structure of fungal communities differed significantly between DC and fallow plots at soybean planting and after harvest (P<0.001). Fungal populations were affected by location at all sampling times and by treatments before planting and after soybean harvest. Several enriched fungal and bacterial taxa in wheat plots, including Mortierella, Exophiala, Conocybe, Rhizobacter spp., and others, were previously reported to parasitize SCN and other plant-parasitic nematodes. Knowing that phytocompounds with potential nematicidal activity are released via the root system of plants, we implemented a gas chromatography–mass spectrometry (GC-MS) pipeline to investigate the profile of phytochemicals in soil samples collected from these fields and reviewed the potential nematicidal activity of compounds with higher concentration in double cropping fields. A total of 51 compounds were detected with the GSMS analysis, 8 with unknown identification. Several compounds, including multiple fatty acids, had greater relative peak areas in double-cropping samples compared to fallow. This multi-approach study provides a better understanding of the mechanisms governing wheat's effect on SCN populations. Rather than a single mechanism, the suppression of SCN in soybean fields double-cropped with winter wheat is potentially linked to enriched microbial communities, increased populations of beneficial organisms, and higher concentrations of phytochemicals with nematicidal activity. This is the first study implementing metagenomics and GCMS to characterize soil microbial and chemical profiles in soybean fields double-cropped with winter wheat. Finally, a set of studies were conducted to evaluate the effect of two seed-applied succinate dehydrogenase inhibitors (SDHI) compounds, fluopyram and pydiflumetofen, on SCN population densities, plant injury, and plant growth. Next-generation sequencing was later employed to identify transcriptomic shifts in gene expression profiles of soybean seedlings treated with fluopyram and pydiflumetofen. Fluopyram reduced both SCN egg and cyst counts in comparison to pydiflumetofen and the untreated control. RNA expression patterns of seed treatments clustered by sampling time (5 DAP vs 10 DAP). Multiple DEGs identified in soybean seedlings treated with fluopyram are reported to be linked to systemic resistance, suggesting a role of systemic resistance on the suppression of SCN by fluopyram. The non-target inhibition of soybean succinate dehydrogenase genes by fluopyram may be the origin of the phytotoxicity symptoms observed and potentially the cause of the systemic resistance activation reported in the current study. To our knowledge, this is the first report of systemic resistance being activated by fluopyram in addition to the suppression of soybean succinate dehydrogenase (SDH) and ubiquinone oxidoreductase genes. This work helps to elucidate the mechanisms of suppression of SCN by fluopyram

Master of Science / Department of Agronomy / William T. Schapaugh Jr / Fusarium virguliforme, the soilborne fungus which causes sudden death syndrome (SDS) of soybean, and Heterodera glycines Ichinohe, soybean cyst nematode (SCN), are two economically important pathogens in the Midwest. The pathogens are often found together in soybean (Glycine max (L.) Merr.) fields. This study was conducted to determine the effect of soybean genotype, F. virguliforme populations, and H. glycines populations have upon yield and to examine the interaction between the two pathogens. In 2008 and 2009, four genotypes with different levels of resistance to SDS and H. glycines were planted at seven environments. F. virguliforme and H. glycines soil populations were quantified at planting, midseason, and harvest. At the end of the growing season, area under the disease progress curves of SDS, F. virguliforme root populations, and H. glycines reproductive indices were determined and plots harvested for seed yield. Soil populations of F. virguliforme and H. glycines at planting, midseason, and harvest varied across environments. Within environments, generally, they were not significantly different. Seed yield varied within and across environments. As disease pressure increased, the performance of resistant genotypes increased compared to susceptible genotypes. Genotypes resistant to SDS yielded higher than susceptible genotypes. There were negative correlations between yield and disease rating and F. virguliforme root populations. F. virguliforme soil populations and H. glycines populations at planting were positively correlated. It is important to manage both SDS and H. glycines in fields with a history of the two diseases. This can be achieved through genetic resistance. Information in this study will improve decisions regarding genotype selection to minimize losses to SDS and H. glycines.

Master of Science / Department of Statistics / Perla Reyes / Heterodera glycines, or the soybean cyst nematode (SCN), is a parasite that targets and damages the roots of soybean plants. It is the most yield-limiting pathogen of soybean in the U.S. and the reliable detection and accurate estimation of population densities is crucial to research and management of this pathogen. A study was performed to understand the effects of crop rotation on the prevalence of SCN. Standard sampling procedures in the plant pathology community dictate taking soil samples from potentially infected fields, processing them and counting the number of eggs in one 1 mL subsample via microscope. Suspecting the traditional procedure may lead to invalid results, false negatives in particular, the researcher created and implemented a sampling procedure based on his knowledge of sampling methods and constraints of sampling in the field. Using the data collected, we will discuss the strengths and limitations of the procedure in estimating the population density of SCN in the field. In addition, a simulation study informed by the data will be conducted to determine a sampling strategy that will yield accurate results while still considering the conditions in the field. Knowledge on how the different stages of the sampling procedure for SCN affect the accurate detection of the pathogen would extend to experimental designs and sampling methodologies for other soil dwelling organisms.

Thesis (Ph. D.)--University of Missouri-Columbia, 1996. / Typescript. Vita. Includes bibliographical references (leaves 160-168). Also available on the Internet.

Thesis (Ph. D.)--University of Missouri-Columbia, 1996. / Typescript. Vita. Includes bibliographical references (leaves 160-168). Also available on the Internet.

Soybean cyst nematode (SCN), Heterodera glycines Ichinohe, is the most devastating pathogen of soybeans, Glycine max (L.) Merr., causing over $1 billion in yield losses annually in the United States alone. Currently, planting of genetically resistant cultivars is the most commonly employed management strategy. Due to an overuse of genetic resistance derived from the soybean variety ‘PI 88788’, many populations of soybean cyst nematodes are becoming virulent on previously resistant cultivars, urging the understanding and discovery of alternative mechanisms of SCN resistance. In this study, we will delve into the history and epidemiology of Heterodera glycines, learn about the molecular etiology underlying SCN pathogenesis, begin to understand the mechanism of resistance by Peking-type soybeans, and look to discover a novel mechanism of resistance by establishment of a mutagenized population of the soybean variety ‘PI 567516C’.