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Iowa State University Capstones, Theses and Retrospective Theses and Dissertations Dissertations 1974 Histopathology of Peronospora manshurica on soybean cultivars of varying degrees of resistance and on a nonhost John Howard Riggle Iowa State University Follow this and additional works at: https://lib.dr.iastate.edu/rtd Part of the Agriculture Commons, and the Plant Pathology Commons Recommended Citation Riggle, John Howard, "Histopathology of Peronospora manshurica on soybean cultivars of varying degrees of resistance and on a nonhost " (1974). Retrospective Theses and Dissertations. 6362. https://lib.dr.iastate.edu/rtd/6362 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. It has been accepted for inclusion in Retrospective Theses and Dissertations by an authorized administrator of Iowa State University Digital Repository. 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Silver prints of "photographs" may be ordered at additional charge by writing the Order Department, giving the catzlog number, title, author and specific pages you wish reproduced. 5. PLEASE NOTE: Some pages may have indistinct print. Filmed as received. Xerox University Microfilms 300 North Zeeb Road Ann Arbor, Michigan 48106 75-3326 RIGGLE, John Ifoward, 1943- HISTOPATHOLOGY OF PERC3N0SPŒA MANSHURICA ON SOYBEAN CULTIVARS OF VARYING DEGREES OF RESISTANCE AND Œ A MMJOST. Iowa State Ibiiversity, Ph.D., 1974 Agriculture, plant pathology Xerox University Microfilms, Ann Arbor, Michigan 48io5 THIS DISSERTATION HAS BEEN MICROFILMED EXACTLY AS RECEIVED. Histopathology of Peronospora manshurica on soybean cultivars of varying degrees of resistance and on a nonhost by John Howard Riggle A Dissertation Submitted to the Graduate Faculty in Partial Fulfillment of The Requirements for the Degree of DOCTOR OF PHILOSOPHY Depar tment: Botany and Plant Pathology Major: Plant Pathology Approved; Signature was redacted for privacy. In Charge of Major Work Signature was redacted for privacy. For the Maior Department Signature was redacted for privacy. For (/he ffixIfaixSLte College Iowa State University Ames, Iowa 1974 ii TABLE OF CœTEENTS Page ABSTBAGT iv INTRODUCTION 1 LITERATURE REVIEW 2 Organism and Etiology 2 Importance of Downy Mildew 4 Histopathology of Fungal Infection 4 Histopathology of Other Peronosporales 5 Histopathology of Peronospora laanshurica 6 MATERIALS AND METHODS 7 Plant Material 7 Pathogen and Its Maintenance 9 Experimental Inoculations 11 Histological Methods 11 Observations 12 Electron Microscopy 13 Scanning Electron Microscopy 13 RESULTS 14 Infection Phenomena Common to All Hosts 14 Germination of conidia 14 Growth of germ tubes 14 Depth of hyphal penetration 17 Lateral hyphae growth 17 Formation of haustoria 18 Histopathology on the Susceptible Host 18 iii fage Histopathology on the Intermediate Hosts 19 Cultivar Acme 19 Cultivar Wayne at different ages 20 Heterozygous plants 23 Histopathology on the Resistant Host 25 Histopathology on a Nonhost 26 Ultrastructure of Host and Pathogen 27 DISCUSSION 101 LITERATURE CITED 109 ACKNOWLEDGMENTS 112 iv ABSTRACT Soybean leaves were inoculated with the downy mildew fungus Peronospora manshurica. Portions of the leaves were fixed, cleared, and stained for microscopic observation at intervals from 12 to 168 hr following inoculation. A susceptible cultivar inoculated at different ages, a resistant isogenic line, a lins heterozygous for resistance, a moderately resistant cultivar, and a nonhost were studied. Germination of conidia, germ tube growth, appressorium formation, development of penetration pegs, and hyphal growth for 36 hr were about the same on all categories of plants. After 36 hr hyphae grew within young leaves of the susceptible cultivar at the rate of about 59 pm/hr and formed about 57 haustoria/mm of hyphae. On mature leaves of the susceptible cultivar the fungus grew rapidly from 36 to 120 hr and then grew slowly until 144 hr when the final small lesion size was attained. On plants heterozygous for resist­ ance and on the moderately resistant cultivar, growth of the fungus hyphae was somewhat slower than on the susceptible cultivar. The number of haustoria/mm of hyphae was low on plants heterozygous for resistance, but high on the moderately resistant cultivar. On the resistant isogenic line growth was very slow after 36 hr and few haustoria were formed. On the nonhost, the common bean cultivar Tendercrop, growth of the fungus was very slow for 120 hr, but then, in a few infection sites, increased rapidly, before ceasing as lesions were delimited by veins; haustoria were numerous where lesions formed. Resistance generally appeared to V be related to or expressed as a slow hyphal growth rate between 36 and 168 hr after inoculation. The success of individual infections apparently was related to the ability of their hyphae to form haustoria. 1 INTRODUCTION The mechanism of plant disease resistance has been the subject of numerous investigations. While many workers have attempted to prove diverse hypotheses proposing chemical explanations for plant disease re­ sistance, only a few have concentrated on structural and ultrastructural aspects. Relatively few studies have been made, and few data are available on the histological aspects of infection of resistant plants by fungi. No data are available on development of lower obligate fungal parasites in resistant plants. Downy mildew of soybean is suitable to such a study. Several pathogenic races are available. Varieties of soybean with varying degrees of resistance are available for comparison. Recently, near-isogenic lines of resistant and susceptible soybeans be­ came available. Use of these lines can materially aid in eliminating spurious differences between varieties which might otherwise be suspected of accounting for disease resistance. The general procedures for handling the fungus had been worked out, and the necessary facilties were avail­ able. Techniques for study of ultrastructure by electron microscopy on a susceptible variety had been developed. Thus I was presented with an excellent opportunity to add to our knowledge of resistance to obligately parasitic lower fungi. Such information should lead to a more complete understanding of this most widespread of soybean diseases. A knowledge of any factor, structure, or activity that can be shown to be associated with disease resistance would be of material benefit to programs committed to breeding downy mildew resistant soybeans. 2 LITERATUBE REVIEW Organism and Etiology Peronospora manshurica (Naum.) Syd. ex Gaum, causes downy mildew disease of soybean. Glycine max L. (Merr.). Oogonia are fertilized by antheridia, which leads to the formation of oospores. Asexual reproduc­ tion is by meaus of conidia borne on dichotomously branched conidiophores 300-500 }m long. Conidia germinate only by germ tubes, never by zoo­ spores. The fungus is morphologically similar to Plasmopara viticola (Berk. & Curt.) Berl. & DeT., 1898, cause of downy mildew of grape, and Peronospora tabacina Adams, cause of downy mildew of tobacco. The disease cycle may begin in the spring with the infection of young seedlings arising from oospore coated seeds. Alternatively oospores contained in decaying leaves near the germinating seed may germinate at the same time as the seed and initiate primary infections. In either case infections may become systemic and result in lesions along the mid- vein of the first unifoliate leaves. Under humid conditions these lesions produce conidiophores and conidia on the leaf undersurface. Conidia are released as the humidity decreases and are carried by wind to other soybean plants (28). Conidia germinate directly by a germ tube and form an appressorium from which a penetration peg penetrates the epidermis of the young leaf.

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