Dear Rich,

Thank you for contacting us about growing mushrooms in black walnut. Juglone is a chemical excreted by the roots of some trees, including walnut. There seems to be conflicting information about its effect on fungi.

There are some recent studies on the effect of organic matter and mycorrhizal fungi in the soil on the toxicity of juglone. However, I have not been able to locate any credible information about the success or safety of growing mushrooms in juglone-producing species of trees.

It may be possible that the juglone will inhibit the fungi, or vice versa. but without credible research, it is best not to use black walnut or other juglone-producing species. 

Good luck with your mushrooms. Please write again anytime you have gardening questions.