Apparently quince (Cydonia oblonga) can tolerate being in the proximity of a Walnut tree. My guess is that Medlar probably has some degree of tolerance but is not completely immune to the juglone. In other words, yes, there are issues, but Medlar is not going to be as vulnerable as an Apple tree.

Also Black Walnut roots exude much more juglone than other Walnut species.

Thank you for your reply! Due to construction, we had to transplant our medlar under our ......(uh oh)......black walnut. Hoping it makes it! I'm not as concerned about fruiting, just more about the tree thriving.

Ademink,

How did this turn out? I'm looking to plant 10 medlars where we just cut down 3 black walnuts.

Thanks!

Hey there! So far it's alive. It was a bit more puny but I'm positive that it was because it was such a big tree and the transplanting knocked it back. It actually did fruit though! I would call it a success!