Degree Type


Date of Award


Degree Name

Doctor of Philosophy


Plant Pathology and Microbiology

First Advisor

Xiang B. Yang


Studies were conducted to investigate the direct effects of herbicides on Rhizoctonia solani, effects of herbicides on damping-off and root rots caused by R. solani on glyphosate-tolerant and glyphosate non-tolerant soybeans, and distribution of different anastomosis groups (AGs) of R. solani across the soybean growing regions of the USA.;Effects of three commonly applied soybeans herbicides (glyphosate, imazethapyr, and pendimethalin) were evaluated on the mycelial growth, sclerotial production, and viability of Rhizoctonia solani isolates (anastomosis groups AG-1, AG-2-2, and AG-4) under controlled conditions. Only pendimethalin significantly reduced mycelial growth of all three R. solani isolates investigated. Sclerotial production was affected differently by the three herbicides. Viability of sclerotia produced in the presence of herbicides was not significantly different from the no herbicide control.;In 1998 and 1999, studies were conducted to evaluate the interactive effects between glyphosate-tolerant soybean and herbicides (pendimethalin, imazethapyr, glyphosate, and lactofen) on damping-off and root rots caused by R. solani under greenhouse and field conditions. Generally, no significant interactions occurred between herbicides and plant growth variables (plant height, shoot and root dry weight, root rot severity index, and plant stand) in the presence of R. solani for both glyphosate-tolerant and non-tolerant cultivars. In a two year field study, plant stands decreased with both glyphosate-tolerant and non tolerant cultivars in the presence of R. solani and herbicides. Other variables measured such as plant height, shoot and root dry weights, and root rot severity ratings gave differential responses with the various treatments.;The last study was conducted to assess the frequency of different anastomosis groups (AGs) of R. solani from different soybean growing regions of the U.S. Multinucleate R. solani isolates were recovered from soybean fields at five latitude levels ranging from 33°N to 46°N. Out of 143 multinucleate isolates recovered, anastomosis grouping revealed that 51 isolates were AG-1 (35.6%), nine were AG-2-2 (6.2%), 40 were AG-4 (28%), and 15 were AG-5 (10.6%). Effects of temperature on growth, sclerotial production, and survival, respectively, were studied with isolates from each AG. Results indicate differential effects of temperature on growth rate, sclerotial production, and survival among various AGs.



Digital Repository @ Iowa State University,

Copyright Owner

Harikrishnan Ramasubramaniam



Proquest ID


File Format


File Size

121 pages