Weidenbenner, Nicholas Hayes2012-12-132012-12-132012-08https://hdl.handle.net/11299/140886University of Minnesota M.S. thesis. August 2012. Major: Applied plant sciences. Advisor: Seth Naeve. 1 computer file (PDF); vi, 98 pages, appendix p. 77-91.United States soybean [Glycine max (L.) Merr.] yields have increased at an annual rate of 23.4 kg ha-1 yr-1. These gains have come from a variety of sources including genetic, agronomic, and environmental changes. The relative contribution of each source in yield gain and in protecting yield potential is difficult to estimate. The objectives of the study were 1: to compare yield across year of release attributed to greater foliar disease control and to understand the effects of fungicide applications on soybean yield, seed protein, seed oil, seed mass, seed number, lodging, and plant stands at establishment and harvest and 2: to understand the genetic response of soybean across year of release to seedling and foliar diseases including brown spot (Septoria glycines Hemmi), frogeye leaf spot (Cercospora sojina, Hara), cercospora blight (Cercospora kikuchii, T. Matsu. & Tomoyasu), and brown stem rot (Phialophora gregata Allington & D.W. Chamb. f. sp. sojae; BSR) over 85 years of cultivar releases in maturity groups (MG) II and III. One hundred and sixteen cultivars in MG II and III received a seed treatment and foliar fungicides at R1, R3, and R5, and were compared to untreated controls. These cultivars (released 1923-2008) were selected to represent a distribution of historically significant cultivars. The annual rate of yield gain was 1.1% and was unaffected by the fungicide treatment that could not control yield-limiting diseases. Seed protein decreased 0.06% yr-1, while oil increased 0.06% yr-1. Seed mass increased 0.12% yr-1 for MG III and had no change over time for MG II. Seed number increased 1.0% over time. The rate of lodging was suppressed by -0.026 unit yr-1 for treated and -0.022 unit yr-1 for untreated cultivars. The treatment increased plant stands at establishment but no treatment difference was observed at harvest. We attempted to evaluate the change in disease resistance over time but our methods limited our inference to the subset of naturally-occurring diseases. The area under the disease progress curve (AUDPC) for the foliar fungal diseases had a three-way interaction between treatment, maturity group, and year of release. Foliar diseases were reduced by both genetic sources and fungicides, even below economic injury levels. Stem browning decreased over the 85 years without significant contribution of BSR resistance genes.en-USApplied plant sciencesThesis or Dissertation