Cannot answer the question about the rootstock situation, but using Black Walnut rootstock is common. However, juglone (the allelopathic chemical in this family) is also produced by English Walnut (as well as most other members of the family) and isn't just a root exudate....it occurs in leaf litter, nut hulls, and even run-off from leaves and stems!

Juglone's effects can be minimized enormously by paying attention to the soil! Well-aerated soil with a high population of soil microorganisms can quickly nullify juglone, or the worst of it, anyway. Using a good mulch will be beneficial, too.

Your example is one of many...an exception to the rule. I'm not sure that I would change what I was doing, if I were you. Well, I'd probably try to remove most of the leaf litter in the fall, and let it compost. It could be safely used on anything later on.

The problems occur more severely with Black Walnut (Juglans nigra L.) and Butternut (Juglans cinerea L.) not English walnuts.

The roots of Black Walnut (Juglans nigra L.) and Butternut (Juglans cinerea L.) produce a substance known as juglone (5-hydroxy-alphanapthaquinone). Persian (English or Carpathian) walnut trees are sometimes grafted onto black walnut rootstocks. Many plants such as tomato, potato, blackberry, blueberry, azalea, mountain laurel, rhododendron, red pine and apple may be injured or killed within one to two months of growth within the root zone of these trees. The toxic zone from a mature tree occurs on average in a 50 to 60 foot radius from the trunk, but can be up to 80 feet. The area affected extends outward each year as a tree enlarges. Young trees two to eight feet high can have a root diameter twice the height of the top of the tree, with susceptible plants dead within the root zone and dying at the margins. The juglone toxin occurs in the leaves, bark and wood of walnut, but these contain lower concentrations than in the roots. Juglone is poorly soluble in water and does not move very far in the soil. [From Ohio State University Extension Fact Sheet HYG-1148-93 by Richard C. Funt and Jane Martin]

The Ohio State University Extension and the American Horticultural Society have reported that R. periclymenoides, formerly R. nudiflorum, Pinxterbloom Azalea, and Exbury Azaleas Gibraltar and Balzac will grow near Black Walnut and Butternut trees. They also list many other plants that will grow in the root zone of these trees.

Here is a link that might be useful: Cornell Extension Fact Sheet

Thanks for the link and information.

Dax