VIEW ARTICLE

Disease Detection and Crop Losses

Brown Spot Severity and Yield Reduction in Soybean. S. M. Lim, Research plant pathologist and associate professor, Agricultural Research, Science and Education Administration, U. S. Department of Agriculture, and Department of Plant Pathology, University of Illinois, Urbana 61801; Phytopathology 70:974-977. Accepted for publication 31 March 1980. Copyright 1980 The American Phytopathological Society. DOI: 10.1094/Phyto-70-974.

The effect of brown spot, which is caused by Septoria glycines, on yield and 300-seed weight of two 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 two 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.