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Assessment of Resistance in Soybean to Pythium Ultimum and Sensitivity of Ohio’S

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Assessment of Resistance in Soybean to Pythium Ultimum and Sensitivity of Ohio’S Assessment of Resistance in Soybean to Pythium ultimum and Sensitivity of Ohio’s Diverse Pythium species towards Metalaxyl THESIS Presented in Partial Fulfillment of the Requirements for the Degree Master of Science in the Graduate School of The Ohio State University By Christine Susan Balk Graduate Program in Plant Pathology The Ohio State University 2014 Master's Examination Committee: Dr. Anne E. Dorrance, Advisor Dr. Pierce Paul Dr. Francesca Peduto Hand Copyrighted by Christine Susan Balk 2014 Abstract In Ohio, seedling blight caused by oomycetes is an annual problem in crop production. More than 25 different species of Pythium have been identified that contribute to seed and seedling loss in Ohio; which is economically detrimental to soybean producers. Several factors have been proposed that may contribute to the high incidence of seedling blights caused by Pythium. These include long term no-till production, changes in seed treatments, and environmental conditions that favor infection, which occurs shortly after planting. Host resistance and seed treatments are two management strategies that could be deployed for these pathogens. Pythium ultimum var. ultimum and Pythium ultimum var. sporangiiferum are aggressive Pythium species that are abundant in Ohio soils. There has not been an in-depth evaluation of resistance in soybeans towards Pythium ultimum. The first objective of this study was to 1a) identify sources of resistance to these two varieties of Pythium ultimum. Much of the germplasm that was evaluated were previously identified as sources of resistance Phytophthora sojae and P. irregulare. Therefore objective 1b) was to determine if genotypes resistant to the varieties of Pythium ultimum are the same or different to each other as well as other oomycete pathogens Phytophthora sojae and P. irregulare. Multiple isolates of Pythium ultimum var. ultimum and Pythium ultimum var. sporangiiferum were used to evaluate germplasm to identify resistant soybean genotypes. ii Metalaxyl and pyraclostrobin are fungicides currently used as seed treatments to manage Pythium species. However, some species are insensitive to one or both of the active ingredients. A second objective was to determine which proportion of isolates within a species, were sensitive to metalaxyl. A collection of isolates of Pythium species was evaluated on amended agar (127 isolates) or amended broth assays (292). The results from these studies may identify new sources of resistance to P. ultimum v. ultimum, and P. ultimum v. sporangiiferum and if metalaxyl is an effective seed treatment to improve current management and limit further losses. Resistance to both varieties of P. ultimum was expressed as a reduction in root rot, higher stands, and larger root mass in these cup assays. A high level of resistance was found in soybean genotypes Dennison, Hutchinson, PI 424354, OSU038, OSU015, OSU028, PI 408225A, OSU049, OSU027 and OhioFG1 to P. ultimum var. sporangiiferum. While Williams, Kottman, Streeter, and Wyandot; had high levels of resistance to P. ultimum v. ultimum. Among the 298 genotypes that were evaluated, there was one that had high levels of resistance to both species, which was Dennison. There was growth on metalaxyl amended broth (100 ppm) for 96 of the 252 isolates evaluated in this study from years previous to 2014. The sensitivity to metalaxyl was variable both among and within isolates for Pythium irregulare, Pythium sylvaticum, Pythium torulosum, Pythium aphanidermatum, and Phytophthora sojae. There was complete sensitivity to pyraclostrobin for all 127 Pythium or Phytophthora isolates tested. iii Dedication I would like to dedicate this thesis to my family and friends. My parents, Daniel and Barbara Balk have been with me every step of the way; not only for this degree, but in life. They are such supportive and encouraging role models. My brother Tommy has been an inspiration to me to be myself, no matter who’s watching. My sister Melissa has taught me that if you want something, go get it. The sky is the limit. Thank you all! iv Acknowledgments I would like to acknowledge Dr. Anne E. Dorrance in her constant leadership, support, and drive throughout this intense process. She has been an inspirational advisor. I’d also like to acknowledge Dr. Aswini Pai for pushing me throughout my undergraduate years and having faith in me at St. Lawrence University. I never would have started this path had she not put me on it. I’d like to acknowledge St. Lawrence University for a phenomenal liberal arts degree that I will forever be grateful for. Lastly, I’d like to acknowledge The Ohio State University for making this degree a reality in my life. v Vita July 25, 1988 ..................................................Born- Buffalo, New York June 2006 .......................................................Amherst Central High School 2010................................................................B.S. Biology, St. Lawrence University 2014................................................................M.S. Plant Pathology, The Ohio State University Fields of Study Major Field: Plant Pathology vi Table of Contents Abstract ............................................................................................................................... ii Acknowledgments............................................................................................................... v Vita ..................................................................................................................................... vi List of Tables ...................................................................................................................... x List of Figures .................................................................................................................. xiii Chapter 1: Assessment of Resistance in Soybean to Pythium ultimum and Effective Rates of Metalaxyl Towards Ohio’s Diverse Pythium species ...................................... 1 Literature Review ............................................................................................................ 1 Chapter 2: Quantitative Resistance in Elite Soybean Germplasm to Pythium ultimum var. ultimum and Pythium ultimum var. sporangiiferum .............................. 12 Introduction ....................................................................................................................... 12 Materials & Methods:.................................................................................................... 15 Pythium Isolates. ....................................................................................................... 15 Inoculum for greenhouse assays. .................................................................................. 15 Greenhouse Studies. .................................................................................................. 16 vii Soybean Genotypes. .................................................................................................. 16 Data collection. .......................................................................................................... 17 Comparison between Pythium species. ..................................................................... 18 Data analyses. ............................................................................................................ 18 Results. .......................................................................................................................... 18 Checks ....................................................................................................................... 19 There were 10 genotypes used as controls across experiments. Conrad was excluded from the final analysis of checks due to problems with the seed. The Virginia Tech experiments were also excluded due to less disease in those 2 experiments. Once analyzed, there was not an isolate x genotype interaction (Root score P-value= 0.9152, Root weight P-value= 0.2190, Appendix A). ............................................... 19 Nested Association Mapping ..................................................................................... 19 University of Missouri ............................................................................................... 20 Virginia Tech Genotypes ........................................................................................... 20 Genetic Gain .............................................................................................................. 21 Ohio State University Genotypes .............................................................................. 22 Comparison of isolates and genotypes ...................................................................... 22 viii Discussion ..................................................................................................................... 24 Chapter 3: Efficacy of Metalaxyl and Pyraclostrobin to Pythium species affecting soybean and corn in Ohio ............................................................................................... 73 Introduction ................................................................................................................... 73 Materials & Methods ..................................................................................................... 75 Phytophthora and Pythium isolates ........................................................................... 75 Sensitivity to metalaxyl ............................................................................................
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