Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)


Plant Pathology and Crop Physiology

First Advisor

Edward C. McGawley


Calonectria crotalariae enhanced root penetration of Lee 74 (susceptible) and Centennial (resistant) soybeans by juveniles of race 3 of Heterodera glycines. The presence of the fungus resulted in greater numbers of females and both soil and root cysts on Lee 74 during the first 30 days. Greater numbers of soil cysts were recovered from Lee 74 at 80 and 120 days and from Centennial at 120 days after inoculation with both pathogens. Root colonization by the fungus increased from 19% to 57% at 40 days on Lee 74 and from 58% to 86% on Centennial at 120 days in the presence of the nematode. Root injury ratings increased significantly when both organisms were present suggesting a synergistic nematode x fungus interaction on Lee 74 at 40 days. An inoculation timing study in which Lee 74 was infested with the nematode and fungus individually, sequentially, and in combination at days 0 and 35 indicated that enhanced nematode reproduction was related more to early plant-fungus interactions than early plant fungus-nematode interactions. Inoculum density studies with eggs of the nematode revealed augmented nematode reproduction in coinoculated plants from a 10,000 egg/pot inoculum level but not from either 100 or 1,000 egg/pot levels. Enhanced nematode reproduction paralleled increases in microsclerotia densities from 500 to 5,000 per pot, but not at a 50,000 microsclerotia per pot density. The effects of H. glycines and C. crotalariae on 60 day root injury were antagonistic; the sum of the root weight reductions caused by coinoculation with both pathogens was less than the sum of that caused by each alone.