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My research is focused on understanding the ecology, genetics, and population biology of soil fungi that cause plant disease. For several years we have been examining the phylogeny and taxonomy of fungi classified as Rhizoctonia based on DNA sequence analysis of β-tubulin and ribosomal RNA genes. Information from these experiments has contributed to our understanding of species concepts in Rhizoctonia and provided a conceptual framework for defining individuals and populations. From a population perspective, I am particularly interested in understanding the 1) relative contribution of asexual and sexual reproduction to the genetic diversity and structure of field populations, 2) genetic relatedness and distribution of fungal individuals in nature, 3) evolutionary processes such as gene flow and recombination that shape populations, and 4) genetic isolation or subdivision (differentiation) of populations associated with geographic location and/or host specialization. I am currently involved in a collaborative project with Stellos Tavantzis (University of Maine) to determine the occurrence of double strand RNA elements (mycoviruses) in natural populations of Rhizoctonia solani and the role they play in fungal fitness and pathogenesis. Information generated from population-based experiments will be utilized to develop novel approaches for managing plant diseases caused by R. solani.
1) Molecular systematics and taxonomy of Rhizoctonia fungi
Fungi classified as Rhizoctonia represent a diverse assemblage of microorganisms that are important pathogens of plants, saprophytes responsible for decomposition and recyclingof organic matter and beneficial symbionts associated with orchids and mosses. This project has been investigating the genetic diversity and taxonomy of fungi classified as Rhizoctonia by examining DNA sequences of nuclear genes that encode for the ribosomal RNA (rDNA) subunits and β-tubulin. Phylogenetic analysis of DNA sequence data has provided information on the evolutionary relationships of the asexual and sexual stages of Rhizoctonia fungi represented in different taxonomic classes, orders and families. This database is currently being employed to better characterize species and provide a conceptual framework for defining individuals and populations of Rhizoctonia.
Collaborators: Rytas Vilgalys (Department of Biology, Duke University, Durham) and Dolores Gonzalez (Departamento de Sistemática Vegetal, Instituto de Ecologia, Xalapa, Vera Cruz, Mexico)
2) Population biology and genetics of Rhizoctonia solani anastomosis group 3
The soil fungus Rhizoctonia solani anastomosis group 3 (AG-3) is an important pathogen of potato and commonly occurs in most production regions throughout the world. The fungus can infect all parts of the potato plant when environmental conditions favor disease development. Historically, Rhizoctonia disease of potato has been difficult to manage with traditional approaches such a crop rotation and breeding for host plant resistance. In order to develop more effective management of Rhizoctonia disease of potato, we have been investigating the genetic diversity and structure of pathogen populations from potato seed tubers and soil in North Carolina using DNA-based (AFLP and PCR-RFLP) and somatic incompatibility criteria. Results from this research suggest that populations of R. solani AG-3 from potato are genetically diverse and support a model of population structure that includes both clonality and recombination. Our results also suggest that movement (migration) of R. solani AG-3 on potato seed tubers from source populations is an important component for long distance dispersal of the pathogen and provides an efficient mechanism to introduce new genes into recipient soil populations in North Carolina.
Collaborators: Rytas Vilgalys and Tim James (Department of Biology, Duke University, Durham); Liane Rosewich-Gale (Department of Plant Pathology, USDA/ARS, University of Minnesota, St. Paul) and David Shew (Department of Plant Pathology, North Carolina State University, Raleigh)
Personnel involved: Jeremy Benson. Paulo Ceresini, Bryan Cody, and Amir Morgan
3) Role of double stranded RNA (dsRNA) elements in populations of Rhizoctonia solani anastomosis group 3
Like many fungi, the soil fungus Rhizoctonia solani anastomosis group 3 (AG-3) can harbor double stranded RNA (dsRNA) viruses (called mycoviruses) in their cells. We are currently using direct isolation and reverse transcription, polymerase chain reaction (RT-PCR) based methods to examine field population of R. solani AG-3 from potato and soil for the presence of dsRNA mycoviruses. Preliminary results suggest that at least five unique dsRNA mycoviruses are present in approximately 30% of the isolates sampled. Research is currently in progress to determine the association of specific dsRNA mycoviruses with their ability to cause disease on potato in growth chamber and field experiments. The long-term objective of this research project is to better understand how the genetic diversity and presence of dsRNA mycoviruses influence pathogen fitness and ecology.
Collaborator: Stellos Tavantzis and Dilip Lakshman (University of Maine, Orono)
Personnel involved: Nikki Charlton and Marike Boerema
4) Population biology and genetics of Sclerotinia minor and Sclerotinia sclerotiorum
The soil fungi Sclerotinia sclerotiorum and S. minor are important pathogens on more than 400 and 100 species of plants, respectively. For the past several years we have utilized DNA fingerprinting and mycelial compatibility criteria to examine the genetic diversity and structure of field populations of S. sclerotiorum and S. minor from cabbage and peanut, respectively. Results from these studies suggest that populations of these pathogens are predominantly clonal (asexual) in North Carolina, but there was also evidence for recombination. Research is currently in progress to understand the role that species of annual winter weed hosts and sexual reproduction play on the population dynamics of S. minor on peanut.
Collaborators: Barbara Shew (Department of Plant Pathology, North Carolina State University, Raleigh) and Linda Kohn (University of Toronto)
Personnel involved: Bryan Cody, Joyce Hollowell, Joseph Hudyncia, Diana Sermons, and Jason Smith
5) Role of mycoparasite secondary metabolites for managing rust diseases of cereals
Fungi that cause rust diseases of cereals are important on a worldwide basis and result in billions of dollars in annual yield losses. The complex life cycle of rust fungi and selection pressure associated with the deployment of resistant varieties and fungicides contribute significantly to our ability to effectively manage rust diseases. Although rust diseases have been studied for many years, they remain a major problem and can spread very rapidly due to effective short and long distance dispersal of airborne asexual spores called urediospores. The urediospores represent an important stage of development of rust fungi because they are associated with subsequent secondary infection (repeating stage of the disease) of cereals. As plants begin to senesce at the end of the growing season, urediospores are transformed into non-infective teliospores and provide a mechanism for the fungus to survive from season to season in soil. This research is focused on the isolation and characterization of secondary metabolites produced by fungal parasites (mycoparasites) of rust urediospores and their ability to reduce rust disease of cereals. The ability to manipulate the transition from infective (urediospore) to non-infective (teliospore) stage of rust fungi would provide a novel method for managing rust diseases, allow us to delve more deeply into the complex life cycle rust fungi and provide an opportunity to understand the mechanism(s) of how urediospores are converted into teliospores.
Collaborators: Ven Subbiah (PhytoMyco Research Co) and David Marshall (Department of Plant Pathology, USDA/ARS North Carolina State University, Raleigh)
Personnel involved: Bill Brown
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