Field studies were conducted to determine the relationships between soybean yield and foliar disease index (FDX) of sudden death syndrome (SDS) as well as root colonization by Fusarium solani f. sp. glycines, the causal agent. Single-row plots in a soybean field with relatively uniform SDS incidence and severity were identified at growth stage R6 on cultivar Pioneer 9492 in DeSoto, IL, in 1997 and 1998. For each plot, foliar disease index (FDX), yield, and yield components were determined. In 1997, linear relationships between yield (Y, grams per meter of row) and FDX were obtained from the wide-row (Y = 207.84 - 1.09 FDX) and narrow-row (Y = 126.66 - 0.745 FDX) plots, respectively. A linear relationship (Y = 124.23 - 1.11 FDX) also was observed in 1998. Increase in each FDX unit caused yield loss from 18 to 29 kg/ha (0.7 – 1.1 g/m of row). FDX was negatively correlated with seed weight (grams per plant) in both years and with seed size (grams per 100 seeds) in 1997. A no-tilled field at Southern Illinois University planted to soybean cultivar Asgrow 5403 was divided into 25 plots in 1997 and 40 plots in 1998. Root samples were taken from each plot at five or six sampling times during the seasons. Roots were used to isolate and enumerate F. solani f. sp. glycines on a selective medium to obtain the CFU. FDX was assessed and soybean yield was obtained from each plot. Soybean yield correlated negatively with FDX in both years. Both yield and FDX correlated significantly with CFU from slightly before growth stage R1 to R2 in both years, and with area under the pathogen population curve (AUPC) in 1997. An increase in one unit of AUPC or CFU per gram of root at R6 was associated with yield loss of 0.19 or 0.014%, respectively.

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