Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)


Plant Pathology and Crop Physiology

First Advisor

John S. Russin

Second Advisor

Raymond W. Schneider


Red crown rot of soybean caused by Calonectria ilicicola is a serious disease in Louisiana. The pathogen infects soybean roots and the above ground symptoms and signs appear during reproductive stages of the plant. Little information is available on root infection as well as effects of environmental factors or varietal resistance on this process. Field studies were conducted in 1994, 1995, and 1996 to determine the effects of planting date, cultivar susceptibility, and soil pathogen population on soybean root colonization by C. ilicicola and subsequent disease development. Early season colonization of tap as well as lateral roots was important for red crown rot symptom development. Symptom development in more susceptible Sharkey was reduced following delayed planting but remained low in less susceptible Cajun regardless of planting date. Root colonization correlated positively with soil pathogen levels. A weak positive correlation was detected between taproot colonization and pathogen population level during all 3 growing seasons. In the case of lateral root colonization, a strong positive correlation was detected in 1994, the only year that foliar disease symptoms were detected. Pathogen population changes in the experimental field were not consistent during 3 growing seasons. Considerable decrease in the pathogen population in soil in 1995, along with reduced soybean root colonization could be attributed to high soil temperature experienced during that summer. High temperature effects on C. ilicicola microsclerotia in heavy alluvial soil were examined. Microsclerotia survived at soil temperatures between 20--35°C. Optimal infectivity of microsclerotia was detected, when microsclerotia were incubated in soil between 25--30°C. Results supported the role for high soil temperature in controlling field pathogen population in 1995. Temperature effects on soybean root colonization by C. ilicicola were examined in growth chambers. The optimal soil temperature range for root colonization was between 20--30°C. The effect of plant age on soybean root colonization by C. ilicicola was examined by exposing the pathogen to plants at different ages. Soybean plants were most susceptible to C. ilicicola during the first week after seedling emergence. Susceptibility was then reduced to nearly half and fluctuated at that level till the end of 8 weeks.