Vertical and temporal distribution of Fusarium solani and Heterodera glycines in fields with sudden death syndrome of soybean

Sudden death syndrome (SDS) is an important soilborne disease of soybean caused by the fungus Fusarium solani. This disease is also associated with the soybean cyst nematode (SCN), Heterodera glycines. To quantify the vertical and temporal distribution of these pathogens in the soil profile, a study was conducted at the University of Arkansas Pine Tree (PTS) and Cotton Branch Stations (CBS) in 1991. Soil samples were taken at six depths (0 to 7, 7 to 15, 15 to 30, 30 to 50, 50 to 75 and 75 to 100 cm) at four growth stages (planting, R2, R6 and harvest) and numbers of F. solani, SCN and total fungi were determined. Soil physical and chemical characteristics were measured at planting and soybean root densities were measured at R2 and R6. Disease development was followed weekly in August and September. Soil numbers of F. solani were greatest in the top 15 cm of soil throughout the season. SCN egg numbers also were greatest in this zone until harvest. At harvest, numbers significantly increased at depths of at least 50 cm. The greatest root densities were in the 0- to 15-cm zone. At both locations, the 0- to 15-cm zone was over a plow pan under which the pH decreased from 6.4 to 7.0 above the pan to 3.8 to 4.5 below the pan. Total fungal numbers were greatest in the upper soil layers at planting but declined until there were no significant differences in numbers of total fungi with depth from R2 to harvest. SDS developed sooner and was more severe at PTS than at CBS. PTS had greater numbers of F. solani but supported lower numbers of SCN at the end of the season than CBS.