Degree Type


Date of Award


Degree Name

Doctor of Philosophy


Plant Pathology and Microbiology

First Advisor

Xiang B. Yang

Second Advisor

Charlie A. Martinson


In 1995-97, experiments were conducted in the field, greenhouse, laboratory, and growth chamber to identify methods that may be used to screen soybean cultivars for field resistance to S. sclerotiorum, and to determine pathogen dispersal patterns in the field. Differences in resistance to S. sclerotiorum were detected among cultivars in the field (p = 0.0001) and when mycelial inoculations were made on detached leaves, stems, or foliage (p < 0.05). Percentage of leaf area covered with lesions obtained from detached leaf assays had the strongest correlation with incidence of stem rot in the field (R = 0.63);Three methods not involving fungal inoculations were tested to determine if they may be useful in identifying soybean cultivars with field resistance to S. sclerotiorum. Levels of soluble pink pigments from soybean stems were different among cultivars (p < 0.05), were negatively correlated with disease incidence in the field (R = -0.55), and may be useful in identifying resistant cultivars. Lengths of discolored portions of stems that were severed near soil level and placed in 40 mM oxalic acid were different among cultivars (p < 0.05) and were correlated with disease incidence in the field (R = 0.66). Total concentrations of the isoflavones daidzein and genistein and their glucosyl conjugates in oven-dried leaves of non-inoculated and inoculated greenhouse-grown plants differed among cultivars (p < 0.05) and were negatively correlated with disease incidence in the field (R = -0.66). Genistein inhibited mycelial growth of S. sclerotiorum on water agar at 120 and 240 [mu]M when incubated at 18 and 26 C;Diseased plants were detected in an inoculum-free soybean field at least 50 m from an adjacent corn field infested with S. sclerotiorum. Disease gradients in the soybean field were best described by the Kiyosawa and Shiyomi model (R2 = 0.86). Within soybean fields, diseased plants were detected 20-45 m from area and strip sources of S. sclerotiorum inoculum. Disease incidence decreased with increasing distance from point sources of apothecia placed at the centers of soybean plots.



Digital Repository @ Iowa State University,

Copyright Owner

Stephen Ngakhala Wegulo



Proquest ID


File Format


File Size

130 pages