Freed, Gretchen Marie2014-04-242014-04-242014-04https://hdl.handle.net/11299/163056University of Minnesota M.S. thesis. April 2012. Major: Plant Pathology. Advisor: Dr. Dean Malvick. 1 computer file (PDF); viii, 76 pages, appendix p. 59-76.Symptoms of sudden death syndrome of soybean (SDS), caused by Fusarium virguliforme, include root rot and leaf scorch symptoms The goal of this study was to understand the roles of inoculum rate, crop residues, and seed exudates on growth of F. virguliforme and the development of SDS. The first study was a greenhouse experiment to investigate the influence of inoculum rate and crop substrate on disease development using moderately resistant and susceptible soybean cultivars to SDS. Soybean seeds were planted in soil mix containing an inoculum at four rates (0, 101, 102, or 103 conidia/cc) and with one of seven crop residue substrate treatments (none, soybean seed, corn seed, sorghum seed, corn stalk, corn root, or soybean stem) incorporated into the soil mix. Root rot severity was assessed 15 and 50 days after inoculation (dai) and foliar disease severity and fresh biomass were assessed 50 dai. Root rot and foliar symptom severity were positively associated with the increase of inoculum rate, especially in cultivar MN1410 Plants grown with no added substrate exhibited very low to no disease severity. Disease severity was greater in the treatments with the soybean, corn, and sorghum seed substrates compared to the other treatments examined. Early root rot severity (15 dai) corresponded with the foliar disease severity for all treatments examined. In the second study, the influence of seed exudates on the growth of F. virguliforme was investigated. Seed exudates from four soybean cultivars, two moderately resistant and two susceptible to SDS, and one corn hybrid, which is an asymptomatic host, were collected at different time points during seed germination. These seed exudates were transferred to the wells of a 96-well plate along with macroconidia of one of four F. virguliforme isolates or one Fusarium solani isolate. Optical density values, used as an indication of fungal growth, were recorded after five days of incubation. Soybean seed exudates of the SDS moderately resistant cultivar MN1606, collected just prior to radicle emergence, triggered significantly (p<0.001) more fungal growth compared to all other exudates studied. Exudates from soybean cultivars susceptible to SDS did not promote greater growth of F. virguliforme than the moderately resistant cultivars tested. The effect of corn exudates on fungal growth was similar to the moderately resistant and susceptible soybean cultivars tested. Overall, these findings indicate that organic substrates from soybean and corn promote the growth of F. virguliforme, seedling root infection and the development of SDS in soybean.en-USCrop residueEarly root infectionFoliar resistance ratingFusarium virguliformeInoculum doseSudden death syndromeThesis or Dissertation