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      Contributions of Fusarium virguliforme and Heterodera glycines to the Disease Complex of Sudden Death Syndrome of Soybean

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          Abstract

          Background

          Sudden death syndrome (SDS) of soybean caused by Fusarium virguliforme spreads and reduces soybean yields through the North Central region of the U.S. The fungal pathogen and Heterodera glycines are difficult to manage.

          Methodology/Principal Findings

          The objective was to determine the contributions of H. glycines and F. virguliforme to SDS severity and effects on soybean yield. To quantify DNA of F. virguliforme in soybean roots and soil, a specific real time qPCR assay was developed. The assay was used on materials from soybean field microplots that contained in a four-factor factorial-design: (i) untreated or methyl bromide-fumigated; (ii) non-infested or infested with F. virguliforme; (iii) non-infested or infested with H. glycines; (iv) natural precipitation or additional weekly watering. In years 2 and 3 of the trial, soil and watering treatments were maintained. Roots of soybean ‘Williams 82’ were collected for necrosis ratings at the full seed growth stage R6. Foliar symptoms of SDS (area under the disease progress curve, AUDPC), root necrosis, and seed yield parameters were related to population densities of H. glycines and the relative DNA concentrations of F. virguliforme in the roots and soil. The specific and sensitive real time qPCR was used. Data from microplots were introduced into models of AUDPC, root necrosis, and seed yield parameters with the frequency of H. glycines and F. virguliforme, and among each other. The models confirmed the close interrelationship of H. glycines with the development of SDS, and allowed for predictions of disease risk based on populations of these two pathogens in soil.

          Conclusions/Significance

          The results modeled the synergistic interaction between H. glycines and F. virguliforme quantitatively in previously infested field plots and explained previous findings of their interaction. Under these conditions, F. virguliforme was mildly aggressive and depended on infection of H. glycines to cause highly severe SDS.

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          Most cited references 17

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          Sudden-death syndrome of soybean is caused by two morphologically and phylogenetically distinct species within the Fusarium solani species complex--F. virguliforme in North America and F. tucumaniae in South America.

          Soybean sudden-death syndrome has become a serious constraint to commercial production of this crop in North and South America during the past decade. To assess whether the primary etiological agent is panmictic in both hemispheres, morphological and molecular phylogenetic analyses were conducted on strains selected to represent the known pathogenic and genetic diversity of this pathogen. Maximum-parsimony analysis of DNA sequences from the nuclear ribosomal intergenic spacer region and the single copy nuclear gene translation elongation factor 1-α, together with detailed morphological comparisons of conidial features, indicate that SDS of soybean in North and South America is caused by two phylogenetically and morphologically distinct species. Fusarium virguliforme sp. nov., formally known as F. solani f. sp. glycines, is described and illustrated for the SDS pathogen in North America, and F. tucumaniae sp. nov. is proposed for the South American pathogen. The molecular phylogenetic results challenge the forma specialis naming system because pathogenicity to soybean might have evolved convergently in F. tucumaniae and F. virguliforme. Phylogenetic evidence indicates the two SDS pathogens do not share a most recent common ancestor, since F. tucumaniae was resolved as a sister to a pathogen of Phaseolus vulgaris, F. phaseoli comb. nov. All three pathogens appear to have evolutionary origins in the southern hemisphere since they are deeply nested within a South American clade of the F. solani species complex.
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            Survival and inoculum potential of conidia and chlamydospores of Fusarium oxysporum f.sp. lini in soil.

            The kinetics of survival and inoculum potential of Fusarium oxysporum f.sp. lini were studied in soil. Two types of inoculum were compared: microconidia freshly harvested from a laboratory-grown culture and microchlamydospores produced in sterilized soil. Introduced at the same inoculum densities into a natural soil, the two types of inoculum showed similar behavior; the inoculum densities changed little with time, at least during 100 days. However, the two types of inoculum did differ in disease potential. A higher percentage of microchlamydospores than microconidia germinated in the rhizosphere of flax seedlings, and the heterotrophic fluorescein diacetate hydrolysing activity of the microchlamydospores was 100 times higher than that of microconidia. Moreover, the microchlamydospores produced more disease on flax than the microconidia even at a much lower inoculum density.
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              Soybean sudden death syndrome species diversity within north and South america revealed by multilocus genotyping.

              Sudden death syndrome (SDS) of soybean has become a serious constraint to the production of this crop in North and South America. Phenotypic and multilocus molecular phylogenetic analyses, as well as pathogenicity experiments, have demonstrated that four morphologically and phylogenetically distinct fusaria can induce soybean SDS. Published molecular diagnostic assays for the detection and identification of these pathogens have reported these pathogens as F. solani, F. solani f. sp. glycines, or F. solani f. sp. phaseoli, primarily because the species limits of these four pathogens were only recently resolved. In light of the recent discovery that soybean SDS and Phaseolus and mung bean root rot (BRR) are caused by four and two distinct species, respectively, multilocus DNA sequence analyses were conducted to assess whether any of the published molecular diagnostic assays were species-specific. Comparative DNA sequence analyses of the soybean SDS and BRR pathogens revealed that highly conserved regions of three loci were used in the design of these assays, and therefore none were species-specific based on our current understanding of species limits within the SDS-BRR clade. Prompted by this finding, we developed a high-throughput multilocus genotyping (MLGT) assay which accurately differentiated the soybean SDS and two closely related Phaseolus and mung BRR pathogens based on nucleotide polymorphism within the nuclear ribosomal intergenic spacer region rDNA and two anonymous intergenic regions designated locus 51 and 96. The single-well diagnostic assay, employing flow cytometry and a novel fluorescent microsphere array, was validated by independent multilocus molecular phylogenetic analysis of a 65 isolate design panel. The MLGT assay was used to reproducibly type a total of 262 soybean SDS and 9 BRR pathogens. The validated MLGT array provides a unique molecular diagnostic for the accurate identification and molecular surveillance of these economically important plant pathogens.
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                Author and article information

                Contributors
                Role: Editor
                Journal
                PLoS One
                PLoS ONE
                plos
                plosone
                PLoS ONE
                Public Library of Science (San Francisco, USA )
                1932-6203
                2014
                16 June 2014
                : 9
                : 6
                Affiliations
                [1 ]Julius Kühn-Institut, Institute for Plant Protection in Field Crops and Grassland, Braunschweig, Germany
                [2 ]Department of Botany and Plant Pathology, Purdue University, West Lafayette, Indiana, United States of America
                [3 ]State Key Laboratory of Environmental Aquatic Chemistry, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing, China
                [4 ]Marrone Bio Innovations Inc., Davis, California, United States of America
                [5 ]South Australian Research and Development Institute, Plant Research Centre, Urrbrae, South Australia, Australia
                [6 ]Department of Plant Pathology, University of Minnesota, St. Paul, Minnesota, United States of America
                Agriculture and Agri-Food Canada, Canada
                Author notes

                Competing Interests: Lijuan Xing worked at Marrone Bio Innovations at the time of submission of this manuscript. This does not alter the authors' adherence to PLOS ONE policies on sharing data and materials.

                Conceived and designed the experiments: AW CL LX AM DM. Performed the experiments: AW CL LX AM DM. Analyzed the data: AW CL DM. Contributed reagents/materials/analysis tools: AW AM DM. Wrote the paper: AW DM. Contributed to writing sections of the manuscript and approved of submission of the overall manuscript: CL LX AM.

                Article
                PONE-D-14-05882
                10.1371/journal.pone.0099529
                4059700
                24932970

                This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

                Page count
                Pages: 13
                Funding
                We appreciate the support of the College of Agriculture, Purdue University where the microplot experiments were carried out during the employment of the first three authors, and the College of Food, Agriculture, and Natural Resources, University of Minnesota where the qPCR analyses were conducted. The grant support from the United Soybean Board, the North Central Soybean Research Program, and the Indiana Soybean Alliance is greatly appreciated. The funders had no role in the study design, data collection and analysis, decision to publish, or preparation of the manuscript.
                Categories
                Research Article
                Biology and Life Sciences
                Agriculture
                Crops
                Crop Diseases
                Pest Control
                Integrated Control
                Microbiology
                Plant Microbiology
                Plant Science
                Plant Pathology
                Plant Pathogens
                Plant Pests
                Zoology
                Nematology
                Ecology and Environmental Sciences
                Soil Science
                Soil Ecology
                Medicine and Health Sciences
                Pathology and Laboratory Medicine
                Pathogenesis
                Host-Pathogen Interactions

                Uncategorized

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