Ea. Adee et al., INOCULUM DENSITY OF PHIALOPHORA-GREGATA RELATED TO SEVERITY OF BROWN STEM ROT AND YIELD OF SOYBEAN IN MICROPLOT STUDIES, Plant disease, 79(1), 1995, pp. 68-73
Field microplots were used to determine the relationships among inocul
um density of Phialophora gregata, severity of brown stem rot and yiel
d of soybean. In the fall, microplots were infested with P. gregata by
introducing naturally colonized soybean residue either confined in ny
lon mesh bags or unconfined in corresponding microplots. Different sou
rces of residue were placed on the soil surface or buried in an attemp
t to establish a range of inoculum densities of the fungus. In the spr
ing, residue confined in nylon mesh bags was weighed and assayed for P
. gregata on a selective medium. Inoculum density of P. gregata was ex
pressed as colony-forming units (cfu) per square meter. Severity of br
own stem rot and yield data were recorded from a paired microplot by a
bioassay of unconfined residue using the cv. Corsoy 79. Inoculum dens
ity of P. gregata (cfu/m(2)) in confined residue was positively correl
ated 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 s
tem or leaf symptoms of brown stem rot was low when the inoculum densi
ty was less than 1.0 X 10(7) cfu/m(2). The severity of stem or leaf sy
mptoms of brown stem rot was negatively correlated with yield (P < 0.0
01, r = -0.741 to -0.853) in 3 of 4 yr of the study. There was a negat
ive 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 sur
face, did not affect the relationship between inoculum density and sev
erity of stem or leaf symptoms of brown stem rot.