John Hartman
Kentucky Pest News
August 14, 2000

Walnut, especially black walnut (Juglans nigra) and butternut (Juglans cinerea), is associated with an often difficult-to-diagnose wilt of plants in the garden and landscape. Affected plants growing near walnuts are exposed to juglone, a toxin which can cause plants to yellow, suddenly wilt, and die. Wilting plants often show a staining of the vascular system similar to that caused by other wilt diseases. Observed in the plant disease diagnostic laboratory are tomatoes, azaleas and rhododendrons, the most common victims of wilt caused by this walnut-produced toxin. Juglone, and its precursor, hydrojuglone can be found in all parts of black walnut including leaves, stems, fruit hulls, inner bark, and roots. Other walnuts and related trees such as hickories and pecans also produce juglone, but in much lower amounts.

Juglone can affect other plants in the garden and landscape through roots via contact, leakage, and decay; from rainwater leaching from leaves and branches; and from falling and decaying leaves. Plants located beneath the canopy of walnut trees are at greatest risk because juglone from roots and fallen leaves accumulates there - be aware that walnut roots extend well beyond the drip line of the tree and may affect plants at a distance equal to the height of the tree. Juglone can be toxic in very low doses and plant roots can encounter the juglone from a walnut root or, in some cases from a decomposing root of a walnut cut down several years before, by growing within a half-inch of the walnut root.

Walnut-associated wilt affects other trees such as apple, birch, cherry (sour), linden, pear, and pine. Shrubs such as azalea, mountain laurel, and rhododendron are sensitive as are flowers and herbs such as chrysanthemum, crocus (autumn), forget-me-not, lily-of-the valley, peony, and thyme. Fruits and vegetables such as blackberry, blueberry, cabbage, grape, pea, potato, and tomato will also turn yellow and wilt due to juglone exposure. In agricultural settings, alfalfa is also sensitive to the walnut toxin.

The toxic effects of juglone can be reduced by regularly raking up all fallen leaves and fruit from around black walnut trees, especially keeping debris away from sensitive landscape plants. Walnut debris may be detoxified by composting - walnut debris should not be used for mulch unless it has been composted. Maintain high soil organic matter levels so that active soil microbial populations can metabolize the toxin. Use tolerant plants in the landscape. Kentucky bluegrass and tall fescue are not affected by juglone. Except for the sensitive species listed above, most trees, shrubs, vines, ground covers, fruits, vegetables, and flowers are also tolerant.

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