Degree Type


Date of Award


Degree Name

Doctor of Philosophy


Plant Pathology and Microbiology


During the 1979 and 1980 growing seasons, field experiments were conducted near Ames, Iowa, to determine the relative importance of seed, soil, and crop residue as sources of inoculum for Phomopsis sp., the causal agent of pod and stem blight of soybeans (Glycine max). Soybean seedlots with Phomopsis seed-borne infection levels ranging from 0.0-10.0% and 0.0-77.0% were planted in 1979 and 1980, respectively, in fields with continuous soybean, corn-soybean, or continuous corn rotation histories. Progression of Phomopsis infection throughout both growing seasons showed that the severity of disease on stems, pod, and seeds was greater in the continuous soybean, less in the corn-soybean, and least in the continuous corn rotations. No relationship between seed-borne inoculum and disease severity was detected in either growing season, thus providing circumstantial evidence that soil-borne inoculum was the major source of inoculum for the disease. Pattern of infection on plants and relationships between lodging and pod infection further substantiated this conclusion. Differences in soil potassium levels between the fields did not influence seed infection by Phomopsis in either growing season;In a 1980 field experiment, in which Phomopsis-free and artificially inoculated viable infected and nonviable infested seed were planted in the continuous soybean and continuous corn fields, disease progression was similar for all seed treatments in the continuous soybean field, but more disease developed in the continuous corn field in plots containing viable infected seed. These results further emphasized the importance of soil-borne inoculum, but also suggested that viable infected seed may be a significant source of inoculum if there is concern about introducing the pathogen into an area previously free of the disease;Direct evidence that crop residue could act as a source of inoculum was obtained in a greenhouse experiment in which extensive seedling infection occurred in autoclaved soil infested with soybean crop residue, but not in noninfested soil. Results of a bio-assay to quantify soil-borne inoculum in both crop residue and soil fractions showed that soil-borne inoculum was primarily located in soybean crop residues, although some was found in soil particles. Sensitivity of the bio-assay was improved by including the severity of seedling infection from only soybean crop residues and the amount of crop residue in the sampled field into a bio-assay index.



Digital Repository @ Iowa State University,

Copyright Owner

Dennis Michael Garzonio



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86 pages