Because Asian Soybean Rust drastically reduces leaf area across a short time period, defoliation studies are an appropriate method for predicting disease effects. Soybean becomes progressively more tolerant to defoliation-induced yield loss as seed filling progresses. Thus, farmers are interested in knowing when fungicide application is warranted, should the disease threaten. Studies were conducted at Baton Rouge, LA and Lexington, KY in which soybeans were defoliated (0, 33, 66, and 100%) at weekly intervals during seed filling. Greatest yield losses were for 100% defoliation administered in early seed filling, which resulted in 80% yield loss. The 66% defoliation also created significant yield losses during the first 3 weeks of seed filling (about 40-50% yield declines), whereas 33% defoliation had little yield effect. As soybean advanced through the seed filling period, yield losses from defoliation became less. This trend was especially pronounced for the 100% defoliation. By week 4 (shortly past mid seed filling), all defoliation treatments affected yield similarly, resulting in about a 30% yield loss. During the remainder of seed filling, no significant yield losses were created by defoliation, although numerical yield loss at week 5 was still sizable (about 30%). Because total defoliation even at 70% through seed filling resulted in a 30% yield loss, occurrence of a rust threat at any time during the first half of seed filling would justify protective fungicide application. This recommendation is based on a typical yield of 50 Bu/A, a selling price of $6.50/Bu, and spraying cost of $25/A. Because defoliation has little effect on yield during the last quarter of seed filling, and the minimal time from rust invasion to defoliation is about three weeks, fungicide application may not be warranted when rust threatens a field after mid seed filling.