Brown Spot Severity and Yield Reduction in Soybean

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Abstract
The effect of brown spot, which is caused by Septoria glycines, on yield and 300-seed weight of 2 soybean cultivars, ''Wells'' and ''Williams'', and the relationship between brown spot severity and yield reduction were studied in 1977 and 1978. Different levels of brown spot epidemics were established by inoculating soybean plants at various growth stages. Disease severity and apparent infection rate in 1977 were not significantly different between the 2 cultivars when they were inoculated and evaluated at corresponding growth stages. Infection rates were higher in both cultivars when they were inoculated at late growth stages (R4-R5). ''Williams'' was more severely diseased than was ''Wells'' at corresponding stages in 1978, but there were no significant differences in apparent infection rates between cultivars or between plants inoculated at the different growth stages. Yield and 300-seed weight differed significantly between inoculated, control (protected with benomyl spray) and naturally infected plots. Yield reduction in both ''Wells'' and ''Williams'' ranged 12-34% during the 2 yr. The regression of yield reduction on the area under the brown spot progress curve (AUBC) gave a good fit with the data. Regression of yield reduction on disease severity rated at the R6 stage also satisfactorily explained the relationship between brown spot severity and yield.

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