Browsing by Subject "Soybean cyst nematode"

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    Fungal communities of soybean cyst nematode-infested fields under corn and soybean monoculture and crop rotation
    (2019-12) Strom, Noah
    Understanding how continuous monoculture impacts fungal communities of corn and soybean is essential for developing agricultural practices that minimize yield losses and protect plants from disease. In continuous crop monoculture, negative plant-soil feedbacks involving a build-up of detrimental fungi are thought to be involved in yield declines. However, positive plant-soil feedbacks involving a build-up of microbial antagonists to specific plant pathogens may also occur over long-term monoculture. Changes in soil properties and depletion of key nutrients may also negatively impact yields under crop monoculture. In my dissertation, I investigate soil and root-associated fungal communities under long-term corn-soybean monoculture and crop rotation. I focus on relationships between the mycobiota in different agroecosystem compartments (bulk soil, rhizosphere, rhizoplane, and root endosphere) and on shifts in fungal communities in relation to soil properties, pathogen density, and yield. In chapters 1 and 2, I explore fungal communities in bulk soils, rhizosphere soils, and root endospheres of corn and soybean using a high throughput amplicon sequencing approach and ask whether variation in communities is related to continuous monoculture, soil properties, or the density of a major soybean pathogen, the soybean cyst nematode (Heterodera glycines, SCN). Long-term monoculture of both crops resulted in dramatically different soil and root-associated mycobiota compared to annual crop rotation. Nematophagous fungi increased in abundance and diversity over continuous soybean monoculture, while arbuscular mycorrhizal fungi (AMF) increased in abundance and diversity over continuous corn monoculture. In chapter 1, positive relationships between soil P and Mortierellales, an order containing phosphate-solubilizing fungi, and negative relationships between soil P and AMF suggest biological causes for the observed shifts in available soil P under continuous soybean and corn monoculture. However, structural equation modeling did not show that changes in soil P had a direct effect on yield in either crop and identified soil N and crop host-specific pathogens (fungal pathogens of corn and the SCN) as the most important factors in monoculture yield decline. In chapter 2, I found evidence of selective filtering and enrichment of nematophagous taxa in soybean root-associated compartments due to the "rhizosphere effect" and overlap between fungal communities in soybean roots and SCN cysts, suggesting that the soybean root promotes the growth of fungi that ultimately colonize the SCN cysts. In chapter 3, I used a culture-based approach to characterize the root endophytic mycobiota of corn and soybean and investigated the potential for soybean and corn root endophytes to produce substances that are toxic to the SCN. This project resulted in the first reported isolation of the nematode biocontrol fungus Hirsutella rhossiliensis from a soybean root and showed that the diversity of soybean fungal endophyte communities was correlated with the density of the SCN. Several corn and soybean endophytes produced nematicidal metabolites, in vitro. These projects open the door to future research investigating the utilization of fungi for improved P uptake by plants and the use of root endophytes in the biocontrol of the SCN.
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    Genome wide association mapping and genomic selection for agronomic and disease traits in soybean
    (2014-09) Bao, Yong
    Genome-wide association mapping and genomic selection are two emerging genomic approaches for investigating genetic architecture and improving breeding efficiency for complex traits in crop species. The objectives of our study were to: 1) dissect the genetic basis of resistance to soybean cyst nematode (SCN) and sudden death syndrome (SDS) through association mapping (AM) and 2) evaluate genomic selection (GS) as an improved marker-based selection tool for predicting agronomic and disease traits in a public soybean breeding program. For AM, we genotyped 282 common breeding parents from the University of Minnesota soybean breeding program using a genome-wide panel of 1,536 single nucleotide polymorphism (SNP) markers and evaluated plant responses to SCN and SDS in the greenhouse. AM rediscovered reported resistance genes (rhg1 and FGAM1 for SCN resistance; cqSDS001, cqRfs4, and SDS11-2 for SDS resistance) and also identified novel loci. For GS, average prediction accuracy through cross-validation studies was 0.67 for SCN resistance and 0.64 for root lesion severity associated with SDS resistance. We also empirically assessed the prediction accuracy and responses to GS for agronomic traits. Soybean lines in the AM panel were used as a training set and a validation set consisting of 273 breeding lines were selected from the ongoing breeding program. Existing historical trial data were used to train the GS model. GS was then conducted to select the top 20% individuals from the validation set based on a comprehensive consideration including genomic estimated breeding values. Our GS model predicted yield with a significant positive accuracy in only two MN x MN crosses, while the prediction accuracy was near to zero or negative for protein and oil, and for the rest of crosses. Moreover, one generation of GS didn't significantly change the population mean of yield, seed protein and oil content. Overall, our study suggested AM holds promise to be used as an alternative approach for mapping QTL in soybean breeding germplasm, and GS deserves further investigation prior to implementation in genetic improvement in existing soybean breeding programs.
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    Management strategies for control of soybean cyst nematode and their effect on the nematode community
    (2013-06) Grabau, Zane Joseph
    Soybean cyst nematode (SCN), Heterodera glycines, is the major yield-limiting pathogen on soybean and various plant-parasitic nematodes can damage corn. Additionally, the nematode community is a useful bioindicator for soil health. In chapter 1, relevant research is reviewed. Chapter 2 describes experiments testing ten organic soil amendments at various rates for SCN control in the greenhouse. Some amendments-particularly canola meal, pennycress seed powder and condensed distiller's solubles-effectively reduced SCN populations at 40 days after planting soybeans. By 70 days after planting, SCN control by amendments was diminished. Additionally, phytotoxicity was a concern, particularly at 40 days after planting. Based on these experiments, organic soil amendments have value for SCN management, but more work is needed to optimize amendment efficacy particularly at the field scale. Chapter 3 describes the impact of tillage, granular nematicide (aldicarb or terbufos), synthetic fertilizers (NPKS combinations), and organic fertilizer (swine manure) on plant-parasitic nematodes, the nematode community, and plant yield as assessed in a corn-soybean cropping system. H. glycines, Helicotylenchus spp, Xiphinema spp, and Pratylenchus spp were the major plant-parasitic nematodes present at the sites. Tillage had only minor impacts on populations of major plant-parasitic nematode genera. While aldicarb reduced H. glycines and Helicotylenchus populations, albeit inconsistently, terbufos did not affect major plant-parasitic nematode populations. Nematicides increased soybean and corn yields under some conditions suggesting plant-parasitic nematodes impacted corn and soybean, although this impact was inconsistent. Tillage, fertilizer, and nematicide impacts on the nematode community were often site- and season-specific. Manure application compellingly shifted the nematode community to one of increased enrichment and decreased community structure. The inorganic fertilizers had minimal impact on the nematode community. Conventional tillage decreased nematode community structure based on some measures, but increased bacterivore and fungivore population densities. In contrast, aldicarb nematicide decreased bacterivore and fungivore population densities. Effects of terbufos nematicide on nematode populations and community composition were inconsistent.