Oomycetes are a diverse group of organisms with worldwide distribution. The phylum contains a large number of pathogenic species which cause immense damage to both animal and plant host species in cultured, managed and wild ecosystems. As such, the majority of research is focused on host-pathogen interactions and the environmental study of oomycetes is greatly neglected. However, due to their devastating economic and ecological effects it is becoming increasingly clear that oomycete ecology and diversity studies are vital to the monitoring and management of diseases caused by oomycete species. By studying oomycete diversity, we further expand our knowledge of species ecological adaptation, distribution and host ranges, allowing for development of improved biosecurity systems. The diversity of oomycete species within the biogeographically important Falkland Island archipelago was assessed. The Islands contained relatively low oomycete species diversity, with a total of twenty-five species identified, in nine genera; Pythium s. str., Phytopythium, Saprolegnia, Leptolegnia, Newbya, Achlya, Elongisporangium, Globisporangium and Phytophthora. Thirteen of the identified species from the Falkland Islands were putative novel species, within the genera; Saprolegnia, Leptolegnia, Newbya, Pythium s. str., Phytopythium and Elongisporangium. Four of these putative novel species were confirmed as novel taxa through the morphological and molecular analyses presented herein. A fifth species requires further analyses to confirm the taxonomic position. One of the presented species was mildly pathogenic towards Atlantic salmon (Salmo salar) eggs, while another was pathogenic towards ryegrass (Lolium sp.) seedlings. These results expand our knowledge of oomycete taxonomy and provide further knowledge of pathogenic oomycete species.

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Oomycetes, like fungi, are filamentous heterotrophs, but unlike true fungi are diploid and share a photosynthetic ancestor. Many of these organisms are plant and animal pathogens, and members of the genus Phytophthora cause devastating disease on a diverse array of agricultural plant hosts. Several diverse topics in oomycete biology are investigated in this dissertation. Chapter 2 is a report on loss of heterozygosity in Phytophthora capsici in response to chemical mutagenesis.The research presented in Chapters 3 and 4 are centered on documenting biodiversity and genetic diversity of populations of Phytophthora species obtained from infected plant hosts. The final chapter (Chapter 5) involves determining genetic diversity, ecology, and enzymatic activities of Pythiaceous oomycetes from marsh wetlands of the southeastern US.

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Many oomycetes, such as the notorious potato late blight pathogen Phytophthora infestans, have devastating effects on crops. Recent findings have implied that eukaryotic plant pathogens deliver effector proteins inside host cells to facilitate colonization by modulating plant defences. Although the translocation mechanisms remain unknown, in oomycetes this process depends on a short conserved amino acid sequence located near the signal peptide of many secreted proteins. This sequence, termed the RxLR motif, is strikingly similar to the core RxLxE/D/Q host cell targeting-signal that is found in virulence proteins from the malaria parasite Plasmodium falciparum. Common infection strategies have been described for these divergent pathogens, albeit one is a plant pathogen while the other infects human cells. In this thesis, stable transformation of P. infestans, combined with a validated assay based on the intracellular recognition of the RxLR effector, A vr3a, was used to study the specificity of effector translocation in oomycetes and to demonstrate the functional similarity between translocation motifs from Plasmodium and two distantly related oomycetes. While accumulating evidence shows that RxLR effectors are delivered into the host cell, the subcellular targeting of these proteins is still unclear. Throughout this PhD project, the difficulty of visualizing translocated effectors during infection was tackled by seeking alternative approaches allowing the detection of fluorescently- tagged effectors once delivered into the plant cell. A further key research question addressed in this thesis was whether the mechanism of translocation required pathogen-encoded proteins or a pathogen- induced environment. Purified fluorescent protein fusions of A vr3a were used to demonstrate that this plant pathogen effector may hold the intrinsic ability to traverse the plasma membrane of animal cells.

Thesis (Ph.D.)--Aberdeen University, 2008. / Title from web page (viewed on Mar., 24, 2009). Includes bibliographical references.

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Soil-borne oomycetes include plant pathogens that cause substantial losses in the agricultural sector. To better manage this important group of pathogens, it is critical to understand how they respond to common agricultural practices, such as tillage and crop rotation. Here, a long-term field experiment with a split-plot design with tillage as the main plot factor (conventional tillage [CT] vs. no till [NT], 2 levels) and rotation as the subplot factor (monocultures of soybean, corn, or wheat, and corn-soybean-wheat rotation, 4 levels) was sampled. Post-harvest oomycete communities were characterized over three consecutive years (2016-2018) by metabarcoding the Internal Transcribed Spacer 1 (ITS1) region of soil DNA extracts. The community contained 292 Amplicon Sequence Variants (ASVs) and was dominated by Globisporangium spp. (85.1% in abundance, 203 ASV) and Pythium spp. (10.4%, 51 ASV). NT decreased diversity and community compositional structure heterogeneity, while crop rotation only affected the community structure under CT. Soil and crop health represented by soybean seedling vitality was lowest in soils under CT cultivating soybean or corn, while grain yield of the three crops responded differently to tillage and crop rotation regimes. The interaction effects of tillage and rotation on most oomycetes species accentuated the complexity of managing these pathogens.

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Thesis (Ph. D.)--Ohio State University, 2007. / Full text release at OhioLINK's ETD Center delayed at author's request