Article: The Effects of Mycorrhizal Fungi Inoculum on Potato Yield

The Effects of Mycorrhizal Fungi Inoculum on Potato Yield Written by: Janet Didur, Master Gardener

Another good example from the folks at the Agriculture and Natural Resources, University of California web site on the effects of adding mycorrhizae to your soil. 

In this experiment the results aren't as obvious as far as size and yield, but you can tell that the quality, look and overall development of underground crops was effected.