Iowa State University Capstones, Theses and Retrospective Theses and Dissertations Dissertations 1987 Strategies for genetic transfer of an allele for resistance to Phytophthora megasperma f. sp. glycinea Kuan and Erwin in soybean [Glycine max (L.) Merr.] Verni Kitzmann Wehrmann Iowa State University Follow this and additional works at: https://lib.dr.iastate.edu/rtd Part of the Agricultural Science Commons, Agriculture Commons, and the Agronomy and Crop Sciences Commons Recommended Citation Wehrmann, Verni Kitzmann, "Strategies for genetic transfer of an allele for resistance to Phytophthora megasperma f. sp. glycinea Kuan and Erwin in soybean [Glycine max (L.) Merr.] " (1987). Retrospective Theses and Dissertations. 11658. https://lib.dr.iastate.edu/rtd/11658 This Dissertation is brought to you for free and open access by the Iowa State University Capstones, Theses and Dissertations at Iowa State University Digital Repository. 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Merr.] by Verni Kitzmann Hehrmann A Dissertation Submitted to the Graduate Faculty in Partial Fulfillment of the Requirements for the Degree of DOCTOR OF PHILOSOPHY Department: Agronomy Major: Plant Breeding and Cytogenetics Approved Signature was redacted for privacy. In Charge of Major Work Signature was redacted for privacy. ^r the Major Department Signature was redacted for privacy. For the Graduate College Iowa State University Ames, Iowa 1987 ii TABLE OF CONTENTS page INTRODUCTION 1 LITERATURE REVIEW 3 Physiologic Races of Phytophthora megasperma 4 Inheritance of Resistance to Phytophthora megasperma 4 Sources of Alleles for Resistance to Phytophthora megasperma in Soybean 9 Use of the Backcross Method in Plant Breeding 10 Systems of Designation of Backcross Derivatives 11 Number of Backcrosses Required to Recover the Phenotype of the Recurrent Parent 12 Use of the Backcross Method for Transferring a Specific Gene for Resistance to Phytophthora megasperma in Soybean 14 Association Between Resistance to Phytophthora and Other Agronomic Traits in Soybean 15 MATERIAL AND METHODS 19 Evaluation of Individual Lines 29 Evaluation of Bulks of Lines 29 Character Measurements 30 Statistical Analyses 31 RESULTS 41 DISCUSSION 81 SUMMARY AND CONCLUSIONS 90 APPENDIX A; THEORY OF BACKCROSSING 92 APPENDIX B: MEAN VALUES FOR FOUR TRAITS OF LINES FROM THE A78-123018 AND CUMBERLAND POPULATIONS AT INDIVIDUAL ENVIRONMENTS AND COMBINED ACROSS ENVIRONMENTS 97 lii page APPENDIX C; SET MEANS FOR FOUR TRAITS OF LINES FROM THE A78-123018 AND CUMBERLAND POPULATIONS AT INDIVIDUAL ENVIRONMENTS AND COMBINED ACROSS ENVIRONMENTS 99 APPENDIX D: MEAN VALUES FOR FOUR TRAITS OF EACH BACKCROSS GENERATION FROM THE INDIVIDUAL LINES OF THE A78-123018 AND CUMBERLAND POPULATIONS AT INDIVIDUAL ENVIRONMENTS 103. APPENDIX E: ANALYSES OF VARIANCE FOR FOUR TRAITS OF INDIVIDUAL LINES OF THE A78-123018 AND CUMBERLAND POPULATIONS AT INDIVIDUAL ENVIRONMENTS 110 APPENDIX F: MEANS FOR FOUR TRAITS OF BULKS OF LINES FROM THE A78-123018 AND CUMBERLAND POPULATIONS AT INDIVIDUAL ENVIRONMENTS AND COMBINED ACROSS ENVIRONMENTS 117 APPENDIX G: MEANS FOR FOUR TRAITS OF A BULK OF THE LINES FROM EACH BACKCROSS GENERATION OF THE A78-123018 AND CUMBERLAND POPULATIONS AT INDIVIDUAL ENVIRONMENTS 120 APPENDIX H; ANALYSES OF VARIANCE FOR FOUR TRAITS OF THE BULKS FROM LINES OF EACH BACKCROSS GENERATION OF THE A78-123018 AND CUMBERLAND POPULATIONS AT INDIVIDUAL ENVIRONMENTS 127 APPENDIX I: MEAN VALUES FOR FOUR TRAITS OF EACH INDIVIDUAL LINE OF THE A78-123018 AND CUMBERLAND POPULATIONS COMBINED ACROSS ENVIRONMENTS 132 REFERENCES 153 ACKNOWLEDGEMENTS 162 1 INTRODUCTION Phytophthora stem and root rot, caused by Phytophthora megasperma Drechs. f. sp. glyoinea Kuan and Erwin, is one of the most destructive diseases of soybeans [Glycine max (L.) Merr.J (Athow, 1984). It causes pre- and post-emergence damping-off of seedlings and a root and stem rot that results in wilting and death of plants (Kaufmann and Gerdemann, 1958). The disease may cause yield losses of more than 50% in susceptible soybean cultivars (Sinclair, 1982). Schmitthenner (1985) reported: "approximately 16 million acres are now infested with Phytophthora in the United States and Canada". Shortly after Phytophthora was recognized as a soybean disease, sources of resistance were identified (Suhovecky and Schmitthenner, 1955), and the genetic control of resistance was regarded as monogenic (Bernard et al., 1957). Twenty-five races of Phytophthora and several major genes for resistance have been reported. Backcrossing has been used to transfer major genes for resistance into susceptible cultivars. Two strategies of backcross have been used in developing soybean cultivars with resistance to Phytophthora. One strategy involves developing homozygous resistant lines and selecting the best line for release as a new cultivar, based on yield tests. The other strategy involves developing homozygous resistant lines, discarding those lines that do not conform 2 with the phenotype of the recurrent parent, without yield testing. Seeds of uniform lines are composited for release as a new cultivar. Pugsley (1949) emphasized the need for more precise information on the number of backcrosses necessary to recover the phenotype of the recurrent parent when breeding for disease resistance: "Californian workers have used four to six backcrosses, but experience in South Australia indicates that little is gained by making more than four backcrosses". Wilcox et al. (1971) investigated the number of backcrosses required to transfer a gene for resistance to Phytophthora into susceptible soybean cultivars. They concluded that seven backcrosses are necessary when no selection for agronomic traits is made during backcrossing. However, in recent years most soybean breeders have used less than seven backcrosses for development of Phytophthora-resistant cultivars. No studies have been conducted to verify the results of Wilcox et al. (1971), or to explore the possibility of recovering the yield of the recurrent parent at earlier generations of backcrossing. The objectives of this study were 1) to determine the number of backcrosses required to transfer a major gene for resistance to Phytophthora into a susceptible cultivar and obtain individual resistant lines with the yield potential of the recurrent parent, and 2) to determine the backcross generation in which a composite of phenotypicaly similar lines will provide the same yield as that of the recurrent parent. 3 LITERATURE REVIEW Phytophthora was first detected in soybean in northeastern Indiana in 1948 and in northwestern Ohio in 1951 (Hartwig, 1975). Since that time, the disease has been recognized in several soybean growing