Population Dynamics of Phialophora gregata in Soybean Residue

Brown stem rot frequently is more severe in no-till cropping systems. Population density of Phialophora gregata was monitored in soybean residue positioned below and on the soil surface. Current season residue was confined in nylon mesh bags that were placed in the field in the fall. Population density of the pathogen was measured monthly through the winter, or seasonally for 30 months, and expressed as CFU per gram of soybean residue. Population dynamics of P. gregata differed in surface residue, compared with buried residue. Population density of P. gregata was not significantly different in buried and surface residue from November/December to April. Beginning in late April or May, population density of P. gregata was significantly greater in surface residue than in buried residue. The population density of P. gregata in surface residue remained above 10⁵ CFU per gram of residue after 30 months in the field. In contrast, the fungus was not detected in buried residue after 11 to 17 months. From November to May, the population destiny of P. gregata in surface residue increased more than 14 times the initial density. In contrast, the density of P. gregata in buried residue increased sixfold by April, then decreased to densities not significantly different from the initial population by May and June. Mass of buried residue decreased more rapidly than that of surface residue during April, May, and June. There was a positive correlation between residue weight and the population density of P. gregata in residue. P. gregata survived longer and at higher population densities in residue positioned on the soil surface (simulated no-till) than in buried residue (simulated conventional tillage). Although inoculum density of P. gregata remained high in surface residue, total inoculum declined because of loss of residue biomass.