Epidemiology of Soybean Stem Canker in the Southeastern United States: Relationship Between Time of Exposure to Inoculum and Disease Severity. E. F. Smith, Research Associate, Department of Plant Pathology, Alabama Agricultural Experiment Station, Auburn University, Auburn 36849. P. A. Backman, Professor, Department of Plant Pathology, Alabama Agricultural Experiment Station, Auburn University, Auburn 36849. Plant Dis. 73:464-468. Accepted for publication 21 November 1988. Copyright 1989 The American Phytopathological Society. DOI: 10.1094/PD-73-0464.

Experiments to determine the influence of the timing of exposure to inoculum and the influence of planting date to the development of soybean stem canker in the southeastern United States indicated that disease severity was primarily dependent upon plant growth stage at the time of exposure and was independent of planting date. The relationship between disease severity and the time of inoculum application was best described by Cauchy distribution models. These models predict exponentially increasing disease incidence in plants exposed to inoculum from emergence to a maximum for those exposed to inoculum at 22 and 24 days (V3 stage) postplanting. Plants exposed from the V3 stage through the V10 stage develop progressively less disease. Even a highly susceptible cultivar would not develop disease when exposed after the vegetative stages.