INOCULUM DENSITY OF PHIALOPHORA-GREGATA RELATED TO SEVERITY OF BROWN STEM ROT AND YIELD OF SOYBEAN IN MICROPLOT STUDIES

Field microplots were used to determine the relationships among inoculum density of Phialophora gregata, severity of brown stem rot and yield of soybean. In the fall, microplots were infested with P. gregata by introducing naturally colonized soybean residue either confined in nylon mesh bags or unconfined in corresponding microplots. Different sources of residue were placed on the soil surface or buried in an attempt to establish a range of inoculum densities of the fungus. In the spring, residue confined in nylon mesh bags was weighed and assayed for P. gregata on a selective medium. Inoculum density of P. gregata was expressed as colony-forming units (cfu) per square meter. Severity of brown stem rot and yield data were recorded from a paired microplot by a bioassay of unconfined residue using the cv. Corsoy 79. Inoculum density of P. gregata (cfu/m(2)) in confined residue was positively correlated with severity of either foliar or stem symptoms of brown stem rot in all experiments (P < 0.005, r = 0.596 to 0.646). The severity of stem or leaf symptoms of brown stem rot was low when the inoculum density was less than 1.0 X 10(7) cfu/m(2). The severity of stem or leaf symptoms of brown stem rot was negatively correlated with yield (P < 0.001, r = -0.741 to -0.853) in 3 of 4 yr of the study. There was a negative correlation between inoculum density and yield (P < 0.001, r = -0.692) in an experiment with buried residue, as well as in an experiment with residue exposed on the soil surface and buried (P = 0.036, r = -0.512). Placement of soybean residue, either buried or on the soil surface, did not affect the relationship between inoculum density and severity of stem or leaf symptoms of brown stem rot.