http://www.utextension.utk.edu/fieldCrops/soybean/diseases/disease_images/2005Soybean_Disease_Ratings_Yields.pdf

Yield information from UT about disease ratings and trials with sprayed vs. unsprayed. Starting on page 32 you can see the data from some standard trials that show significant yield increases with a fungicide application. UT has been showing this kind of increase for several years now.