View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Digital Repository @ Iowa State University Iowa State University Capstones, Theses and Retrospective Theses and Dissertations Dissertations 2007 Phytophthora root and stem rot of soybean in Iowa: minimizing losses through an improved understanding of population structure and implementation of novel management strategies Sarah Marie Cerra Iowa State University Follow this and additional works at: https://lib.dr.iastate.edu/rtd Part of the Agricultural Science Commons, Agronomy and Crop Sciences Commons, and the Plant Pathology Commons Recommended Citation Cerra, Sarah Marie, "Phytophthora root and stem rot of soybean in Iowa: minimizing losses through an improved understanding of population structure and implementation of novel management strategies" (2007). Retrospective Theses and Dissertations. 14837. https://lib.dr.iastate.edu/rtd/14837 This Thesis 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. For more information, please contact [email protected]. Phytophthora root and stem rot of soybean in Iowa: minimizing losses through an improved understanding of population structure and implementation of novel management strategies by Sarah Marie Cerra A thesis submitted to the graduate faculty in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE Major: Plant Pathology Program of Study Committee: Alison Robertson, Major Professor Silvia Cianzio Edward Braun Iowa State University Ames, Iowa 2007 Copyright © Sarah Marie Cerra, 2007. All rights reserved. UMI Number: 1443152 UMI Microform 1443152 Copyright 2007 by ProQuest Information and Learning Company. All rights reserved. This microform edition is protected against unauthorized copying under Title 17, United States Code. ProQuest Information and Learning Company 300 North Zeeb Road P.O. Box 1346 Ann Arbor, MI 48106-1346 ii Table of Contents Abstract iii Chapter 1. Introduction and literature review of Phytophthora sojae 1 Introduction 1 Phylum Oomycota: the Oomycetes 1 Phytophthora versus Pythium 2 Phytophthora sojae: classification and biology 3 Phytophthora Root and Stem Rot disease cycle 6 Managing Phytophthora Root and Stem Rot 8 My Project 13 Justification for research 13 Literature 14 Chapter 2. Within field diversity of Phytophthora sojae populations in production soybean fields in Iowa 19 Abstract 19 Introduction 20 Materials and Methods 23 Results 28 Discussion 37 Appendix 2.1 41 Literature 44 Chapter 3. Search for new sources of resistance to Phytophthora sojae in soybeans 47 Abstract 47 Introduction 47 Materials and Methods 49 Results 52 Discussion 59 Literature 60 Chapter 4. Use of mefenoxam in combination with soybean genotype as a Phytophthora disease management tool in Iowa 62 Abstract 62 Introduction 62 Materials and Methods 65 Results 67 Discussion 73 Appendix 4.1 76 Literature 82 iii Abstract Phytophthora root and stem rot (PRR), caused by the oomycete Phytophthora sojae, is an economically important soybean disease in the North Central Region of the United States, including Iowa. Management of PRR includes the use of specific resistance (Rps genes), partial resistance, and seed treatment fungicides. We had three objectives specific to PRR in Iowa; i) to determine the within field population diversity of P. sojae in fields with a history of PRR, ii) to screen 32 PI lines identified with unique resistance to P. sojae with isolates from Iowa and determine their value for future breeding efforts and iii) to determine if seed treatment fungicides in combination with partial resistant cultivars of soybean would be a useful management tool against PRR. Previous surveys in Iowa recovered 12 races of the pathogen between 1966 and 2004. These surveys collected a single isolate from each site. We proposed multiple races could be identified from a single site, based on evidence from Ohio, Arkansas and Australia surveys. Three fields in Iowa with a history of PRR were extensively sampled. Soil samples were baited using leaf disc and seedling baiting methods. A total of 36 isolates of P. sojae were collected. Of those, 32 were identified to race or pathotype. We found multiple races and pathotypes in two fields sampled. Eleven races and 13 pathotypes are reported new to Iowa. Rps genes generally are defeated by P. sojae populations between 8 and 15 years after deployment. Due to the pathogen’s ability to defeat Rps genes, new sources of resistance need to be discovered and incorporated into commercial soybean lines. As part of a collaborative study with the North Central Soybean Research program, we screened 32 previously identified plant introduction (PI) lines against isolates of P. sojae collected iv in Iowa to identify potential resistance beneficial to Iowa soybean growers. We screened the PIs in four fields as well as in the greenhouse using four layer tests with multiple combinations of isolates. We found six PI lines (PI 398693, PI 399079, PI 408029, PI 424487B, PI 567374, and PI 407974B) with possible new sources of resistance to be incorporated into Iowa soybeans. Partial resistance does not become active until the VC growth stage of the soybean, causing the seed and seedling to be susceptible to disease at planting and early stages of growth. Application of fungicide seed treatments, including the active ingredients metalaxyl and mefenoxam, are suggested in areas of strong disease pressure and favorable environmental conditions for disease development. We evaluated the use of fungicide seed treatments in combination with partial resistance as a potential effective management tool, as part of a collaborative study of the North Central Soybean Research Program. Three fields with a history of PRR were identified and field trials were conducted over two years in a split plot design, with the main plot including cultivar of soybean (2 susceptible; Sloan and Williams, 2 reportedly partial resistant; Stine 2402 and S27-T7, and 2 with Rps genes; Archer with Rps1k and 6 and 2834RR with Rps1k). The subplot included three seed treatments; a control untreated, Apron XL® and Apron XL® plus Apron Maxx®. Evaluations included initial stand counts, disease incidence taken twice in the growing season, final stand counts and yields. Disease pressure was low due to unfavorable environmental conditions for P. sojae development. As results of these conditions, we are unable to make any conclusions to the effectiveness of using fungicide seed treatments in combination with partial resistance as an effective management tool. 1 Chapter 1: Introduction and literature review of Phytophthora sojae Introduction Phytophthora root and stem rot (PRR) of soybean, Glycine max (L.) Merrill, is caused by the oomycete, Phytophthora sojae Kaufmann & Gerdemann (26). The disease is the second most important disease of soybean in the United States after soybean cyst nematode (24, 41). Phytophthora sojae causes disease in every soybean growing state in the United States as well as Canada, China, Argentina, Australia, Hungary, Russia and Italy (29, 40). The disease was first described in 1948 in Indiana, followed by identification in Ohio and North Carolina in the early 1950’s (11, 19). PRR was first identified in Iowa in 1966 (35). In 1998, PRR caused 1113.6 x 103 metric tons of yield loss in the north central region of the United States (41). In Iowa, PRR of soybean is a destructive seedling disease present in over 63% of the soybean fields (30, 49). During 1998 - 2002, the estimated annual loss in Iowa averaged 92,480 metric tons of soybeans with an approximate value of $6 million (41). Greater yield losses can occur during years with favorable environmental conditions for disease development. Phylum Oomycota: the Oomycetes Phytophthora sojae is a water mold in the phylum Oomycota. Molecular phylogenetic analysis has shown that the oomycetes are more closely related to the diatoms, brown and golden brown alga in the kingdom Chromista (Stremenophila) than to the true fungi kingdom Eumycota (1, 25). Oomycetes have diploid nuclei in their hyphae and the only haploid state occurs in the gametes (1, 3, 25). Hyphal cell walls contain cellulose-like glucans instead of chitin (1, 3, 25). The hyphae are aseptate (1). Oomycetes produce zoospores with heterokont flagella that point in opposite directions 2 of the spore, one tinsel flagellum pointing forward and one whiplash flagellum pointing to the rear (3, 25). Oogamous sexual reproduction occurs in the oomycetes. Fertilization between distinct oogonia and antheridia results in the formation of a thick walled oospore (1, 3, 25). There are five orders in the phylum Oomycota but only two are economically significant, Saprolegniales (water molds) and Peronosporales, which include many plant pathogens such as Pythium spp., Phytophthora spp., Peronospora spp. (downy mildews) and Albugo spp. (white rusts) (3, 25). Phytophthora versus Pythium Phytophthora and Pythium belong to the family Pythiaceae. Both genera are closely related and often times it is difficult to distinguish between the two. Both have coenocytic mycelium, produce zoosporangia and produce a single oospore per oogonium (12). The major difference between Phytophthora and Pythium is zoospore formation in relation to the sporangium.