Juglone does not move much in the soil. Greatest quantities are found in the area immediately under the walnut tree, where roots are concentrated and decaying nut hulls and leaves accumulate. Problems occur for sensitive plants primarily when these plants are growing in the root zone area of the black walnut. This is known as allelopathy, or one plant producing a substance that affects growth of another. Affected plants may vary in symptoms from the toxicity.
Symptoms range from stunting, yellowing, partial to total wilting, to complete death.
Apparently the problem occurs when roots of susceptible species contact the tissue of the Juglans species, rather than the juglone being released into the soil. Even if the tree is removed, as long as the roots remain in the soil problems could continue for sensitive plants. Tomatoes and potatoes are two of the popular plants sensitive to juglone. Other species known to be affected include alfalfa, some apple varieties, rhododendron, white pine, white birch, eggplant, pepper, lilac, cotoneaster, and privet, among others. Some plants are known not be to affected by juglone.