Disease Pressure on Soybean in Illinois. S. R. Eathington, Graduate Research Fellow, Department of Agronomy, University of Illinois, Urbana 61801. S. M. Lim, C. D. Nickell, and J. K. Pataky. Head, Department of Plant Pathology, University of Arkansas, Fayetteville 72701; and Professor, Department of Agronomy, Professor, Department of Plant Pathology, and Associate Agronomist, Department of Agronomy, University of Illinois, Urbana 61801. Plant Dis. 77:1136-1139. Accepted for publication 31 July 1993. Copyright 1993 The American Phytopathological Society. DOI: 10.1094/PD-77-1136.

Development of disease resistant cultivars is a primary emphasis of many soybean (Glycine max) breeding programs. By understanding the magnitude and frequency of economic loss caused by soybean pathogens, breeding programs can establish breeding priorities. Therefore, 13 yr of disease monitoring data comprising 89 trials and 52 cultivars were analyzed to determine disease pressure on soybeans in Illinois. Of the 12 diseases observed, brown spot (Septoria glycines) was the predominant foliar disease, while brown stem rot (Phialophora gregata) was the most frequent soilborne disease. Prior probabilities of disease occurrence were developed for brown spot, bacterial blight (Pseudomonas syringae pv. glycinea), and downy mildew (Peronospora manshurica) by partitioning (based on a z-score of ±0.43 from the sample mean) the distribution of disease severity ratings taken at the R6 growth stage. Based on this analysis, the probability of severe symptoms of brown spot is 23% in northern, 31% in central, and 15% in southern Illinois. Brown stem rot occurred at 64 and 52% of the locations in the northern and central regions, respectively, but was not observed in the southern region. Brown spot caused yield reduction throughout Illinois, while other diseases reduced yields at specific locations.