A statewide soybean disease survey was carried out during the 2005 to 2007 growing seasons in Iowa to determine the relative risks for soybean diseases in Iowa. Soybean plants were collected from almost 1,000 fields in each growing season. A systematic design was used to collect 30 plants from 3-5 fields in each county at four growth stages, V2-V3, R1-R2, R4-R5, and R6-R7. The survey provided a unique opportunity to study the spatial and temporal prevalence and incidence of several soybean diseases in Iowa. This thesis will focus on soybean mosaic and stem canker.;Soybean mosaic virus (SMV) is seedborne and can be transmitted by 32 species of migratory aphids. Soybean yield losses ranging from 8 to 35% have been reported. The soybean aphid (Aphis glycines) is considered the only colonizing aphid species of soybean in North America, and this aphid species is also a vector of SMV. This aphid was first found in Iowa in 2000 and has since been reported in every county; however, little information is available concerning its impact on the prevalence and incidence of SMV. The middle leaflet of the topmost fully-developed leaf from each plant sampled from a soybean field was removed, and the 30 leaflets from each field were divided into five, 6-leaflet subsamples. Following sap extraction, samples were tested for the presence of SMV by ELISA. The prevalence and incidence of SMV at the county and field scales were calculated and mapped using ArcGIS. In 2005, 43 of 921 soybean fields (4.7%) tested positive for SMV, compared with 37 of 1058 soybean fields (3.5%) in 2006, and 310 of 1073 soybean fields (28.9%) in 2007. The highest incidence of SMV among counties was 23.1%, 13.3% and 46.0% in 2005, 2006, and 2007, respectively. The seasonal pathogen progress (cumulative SMV prevalence versus day of year and SMV incidence versus day of year) were similar in 2005 and 2006, but very different for 2007, which had a substantially higher rate of pathogen increase versus time. Counties with similar incidence of SMV showed a weak clustered spatial pattern in each of the three years using Moran's Index. Based on this three-year survey, there was weak (2005) or no association (2006, 2007) between the absence/presence of SMV and the absence/presence of soybean aphid in the same soybean fields.;Members of Diaporthe/Phomopsis complex cause several late season diseases of soybean, including northern stem canker ( Diaporthe phaseolorum var. caulivora, DPC) and southern stem canker (Diaporthe phaseolorum var. < meridionalis , DPM). Southern stem canker has not been reported in Iowa, however the disease was found in neighboring Wisconsin (2003). Since agronomists in Iowa have reported an increase in the prevalence of stem canker over the past few years, one objective of this study was to determine if DPM is present in Iowa soybean fields. As part of the Iowa Soybean Disease Survey, soybean plants with stem canker symptoms were collected and isolates of Diaporthe/Phomopsis characteristics were obtained from these samples. During the 2005 growing season, 62 isolates were isolated from plant samples that exhibited stem canker symptoms. Amplification of the fungal internal transcribed spacer (ITS) region followed by digestion with the restriction enzyme Alu I was used to identify isolates to variety, and 14 of the 62 isolates were identified as DPC, and the remaining 48 isolates were Phomopsis . No DPM isolates were identified from the 62 isolates. To quantify and compare isolate aggressiveness, nine of the isolates representing four different geographic areas in Iowa were arbitrarily selected for components analysis. Soybean cultivar "S35" was inoculated with each isolate at growth stage V2-V3 by inserting a single DPC-infested toothpick below the first trifoliate node. Each replication consisted of three 10-cm pots, each containing four plants. The entire experiment was conducted twice using the same controlled environmental conditions. Components of aggressiveness (incubation period, rate of lesion expansion, final lesion length, and time to death) for each isolate and for isolates from the same geographic area were analyzed using analysis of variance (ANOVA), and mean separations were performed using the Waller-Duncan K-ratio test (P=0.05). There were significant differences among the nine isolates evaluated and among the isolates grouped from different geographic areas for each of the aggressiveness components. Stem canker disease was not observed in the 2006 and 2007 growing season of this survey (approximately 1,000 soybean fields sampled per season).