Date of Award

5-1-2017

Degree Name

Doctor of Philosophy

Department

Agricultural Sciences

First Advisor

Fakhoury, Ahmad

Abstract

Seedling diseases are both common and destructive on soybean, Glycine max (L.) Merr. They are caused by a variety of plant pathogens including fungi, bacteria, and oomycetes. The goal of this research was to identify fungal pathogens causing seedling diseases of soybean through a multi-state effort. Two different approaches were used for genus and species determination, one relying on morphological features using microscopy and the other through DNA sequencing of specific genetic loci using up to three different barcodes. The internal transcribed spacer (ITS), the elongation factor EF-1α, and the intergenic spacer (IGS) regions were sequenced for the various isolates to confirm speciation. Data produced from the study includes identification of fungal isolates from eight states in the Unites States, each one contributing approximately 150 isolates. Data also involves correlating species and genus distributions to factors including geographical distribution, cultural practices, soil type, and climatic conditions. More than 3,000 fungal isolates were processed for identification to help determine the predominant causal agents of seedling diseases on soybean in the production fields where the isolates were collected. Our findings showed that Fusaria were the most predominant of the genera identified followed by Trichoderma species. Of sixty-one different species identified over the course of two years of the study, F. oxysporum was the most abundantly isolated. Evidence was shown that different cultural practices and environmental factors do have effects on the distribution of the various species seen in soybean fields with seedling diseases. Through the two-year efforts of seedling diseases causing fungal identification studies, further research studies were developed and pursued. In addition to the fungi believed to be contributing to seedling diseases, we also identified fungal isolates with potential to serve as biological control agents (BCAs). We found fungi, e.g. Trichoderma species, known for their control potential and proceeded with in vitro and in vivo studies to further understand their efficacy and mechanisms on a variety of pathogens with a focus on the sudden death syndrome (SDS) pathogen, Fusarium virguliforme. Species of the Trichoderma genus more significantly inhibited pathogen growth than other genera tested and T. harzianum was the most efficient of Trichoderma species.

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