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Soybean plants can be attacked by the soilborne pathogen Phytophthora sojae at all growth stages and may result in damping-off of seedlings, root and stem rots; all of which cause significant losses in soybean harvestable yield. To gain a better understanding of the biology and ecology of Phytophthos sojae, and for the purpose of developing novel disease management practices, this Phd dissertation investigated two major themes: (1) The biological role of elicitins as sterol carrier proteins in P. sojae: and (2) Development of a cellular lipid profiling method as a basis for determining infestations of P. sojae in soil. The first theme investigated extracellular proteins belonging to class-I elicitins in Phytophthora. It is presumed elicitins may be involved in sterol acquisition, but very little is known about the relationship between sterols and elicitin gene expression. The objective was to determine the pattern of class-I elicitin gene expression in P. sojae, when its growth medium contains different types of sterol (fungal, plant or animal). It was discovered that the growth of P. sojae was stimulated by nanomolar concentrations of all the sterols tested. This also resulted in a differential regulation of class-I elicitin gene expression compared to controls when monitored over-time using real time Reverse Transcription Polymerase Chain Reaction (RT-PCR). Generally, class-I elicitin genes became down-regulated over time which also coincided with a reduction in elicitin biosynthesis when any of the sterols was present in the growth medium. However, kinetics of down-regulation varied as a function of sterol structure, which may be related to binding efficiencies for sterols with elicitins. Also, using Elemental Analysis-Isotopic Ratio Mass Spectrometry (EA-IRMS) I discovered that P. sojae rapidly assimilated 15N-labeled extracellular proteins (which predominantly constitute class-I elicitins) into its mycelium. This happened when stigmasterol was added to the growth medium, suggesting that elicitins are involved in sterol sequestration by this organism. This study is the first to show that sterols regulate the expression of class-I elicitin genes in Phytophthora which provide strong evidence for the involvement of elicitins in sterol acquisition. The second theme of this dissertation was on the development of a biochemical assay to detect P. sojae infestation in soil. The approach was based on profiling total cellular Fatty Acid Methyl Esters (FAME) of P. sojae in pure culture. A total of 12 fatty acids (14:0, 16:0, 18:0, 16:1 ω7, 18:1 ω9, 18:2 ω6, 18:3 ω6, 20:1 ω9, 20:3 ω6, 20:4 ω6, 22:1 ω9 and 24:1 ω9) were identified in the FAME profiles of P. sojae pure cultures. The predominant fatty acids in the FAME profiles are the unsaturated 18C fatty acids (18:1ω9 and 18:2 ω6) followed by the saturated and unsaturated 16C fatty acids (16:0 and 16:1 ω7). Zoospores of P. sojae additionally contained the long-chain saturated fatty acids (20:0 and 22:0), which were not detected in the mycelium of this organism. The potential of using FAME profiles of P. sojae for detecting the pathogen in soil was evaluated by adding a known number of zoospores of P. sojae to soil. The results showed that fatty acids such as 18:1w9, 18:2w6, 20:1w9, 20:4w6 and 22:1w9 could be detected and quantified against the background levels of fatty acids present in soil. This outcome is significant because it offers the potential for a simple and rapid method for determining P. sojae infestations in soil.