Volume 13 No. 4 May 19, 2006 Scouting Report | Feature Article |Things to Look For | Miscellany | Growing Degree Days | Walnut Allelopathy Compiled by W. A. Sinclair, Department of. Plant Pathology Cornell University, Ithaca, NY Abstract Eastern black walnut (Juglans nigra) roots exude chemicals that can suppress or kill many kinds of plants. Juglone (5-hydroxy-1,4-naphthoquinone), a nonspecific cytotoxin, is the best known and most potent of these chemicals. It suppresses or kills sensitive plants whose roots are exposed to it. Above-ground symptoms include retarded growth, stunted and distorted leaves, wilting, premature leaf senescence, and death. Although juglone is produced in all organs of a walnut tree, the roots are the main source of trouble for other plants. Juglone in soil and dead walnut materials is degraded to nontoxic products, but the supply in soil adjacent to living walnut roots is renewed annually. Walnut allelopathy can be mitigated by killing source trees (and removing large roots for quickest benefit) or placing root barriers between walnuts and sensitive plants. Many kinds of plants are relatively tolerant of juglone and can be grown successfully in the root zones of black walnuts. Symptoms of juglone poisoning in plants Affected plants may be stunted, and/or have yellow or brown, undersized or deformed leaves, or may wilt and die. Trees and shrubs undergo decline with external symptoms indistinguishable from those caused by other agents that kill roots. Brownish discoloration of vascular bundles has been noted in tomato. Symptoms may develop rapidly, or perennial plants that are somewhat tolerant may survive for years. Seed germination is less affected than root and shoot growth. Sensitive trees of considerable size (e.g. white pines up to 40 feet tall and 12 inches in diameter) may decline and die as their roots intermingle with walnut roots. Some sensitive plants and plant groups: * black alder, alfalfa, apple, asparagus, autumn-olive, azalea, barley, bean, birch, blueberry, brambles, buttercup, cabbage, sweet cherry, chokeberry,  chrysanthemum, crimson clover, columbine, corn, crabapple, garden cress, cucumber, eggplant, hackberry, Amur honeysuckle, hydrangea, Japanese larch, lespedeza, lettuce, lilac, lily, linden, lucerne, saucer magnolia, silver maple, mountain-laurel, narcissus, garden pea, peony, pepper, petunia, pine, potato, potentilla, privet, quince, rhododendron, rhubarb, rose, Norway spruce, strawberry, tomato, viburnum, watermelon, wheat, yew. * Sensitivity may vary among species in a group or cultivars within a species.      Many plants are tolerant. Lists are available in numerous on-line extension publications. The Morton Arboretum has a particularly nice one.         www.mortonarb.org/plantinfo/plantclinic/Selection_BlackWalnutToxicity.pdf Mode of action Juglone suppresses root oxygen uptake, water uptake and acid efflux. Little is known about juglone uptake by roots and movement within plants. Brown vascular tissue observed in tomato stems poisoned by juglone seems likely to be due to internal movement of the toxin, but could also be caused by mildly pathogenic microorganisms that enter xylem through dead roots. Side effects Some beneficial soil bacteria, including Frankia spp. and Bradyrhizobium japonicum, which induce N-fixing nodules on plant roots, are inhibited by juglone in lab assays. However, studies in Ontario indicated no inhibitory effect of juglone on ammonification or nitrification in soils where black walnuts were growing. Effects of juglone on mycorrhizae have not been reported. Site aspects Juglone concentrations in soil are highest adjacent to walnut roots. Walnut toxicity to sensitive plants is usually observed within 50 feet of a walnut trunk, but the hazard zone increases in relation to tree size (and thus in relation to size of rooting zone). Walnut roots can grow many feet beyond the zone overhung by walnut branches.      In theory, fallen walnut fruits and leaves may contribute to the juglone concentration in soil, but no effect of these materials on plant health under natural conditions has been documented.      Juglone has been reported to persist in soil for up to a year after removal of walnut roots, but persistence is conditioned by soil conditions. It persists longest in poorly drained soils and diminishes rapidly in well-drained soils, in which it is oxidized to nontoxic products. Various bacteria and fungi degrade juglone. Addition of 10-3 M juglone solution to a silt-loam soil in Ontario suppressed the growth of alder seedlings for only 22 days. In laboratory studies with a fine sandy loam soil, juglone and its inhibitory activity disappeared more rapidly under a ‘dry’ moisture regime than under a ‘wet’ regime. In mixed plantations of pines and black walnut growing in Ontario, pines seemed unaffected by the walnut on excessively drained sites, while walnut suppressed or killed the pines on imperfectly drained and poorly drained sites. Influence of walnut mulch and chips on plants Composting of walnut materials for a minimum of six months provides a safe mulch even for plants sensitive to juglone. In tests with wood chips, substances eluted from black walnut proved less inhibitory to growth of lettuce seedlings than did substances eluted from red maple, swamp chestnut oak, red-cedar, neem, and magnolia. Prevention or avoidance of damage Don’t grow eastern black walnut, or don’t grow sensitive plants near black walnuts, especially not on soils with impaired internal drainage. Mitigation of damage From practical experience, if all walnut roots extending into the root zones of sensitive plants are severed and the larger roots pulled from the soil, sensitive plants can be grown there in the next growing season. Walnut roots will recolonize the area, however. Detection and identification of large black walnut roots is easy because they and the soil around them have a pungent noxious odor.      Root barriers can prevent growth of walnut roots into the root zones of other plants. Polyethylene barriers in soil minimized juglone concentration beyond the barriers to trace levels in one study. In a garden in New York State where “walnut wilt” occurred in tomato and several white pines adjacent to black walnuts declined and died, damage was halted by a 4-step procedure: 1) Dig a trench 1 foot wide and 2 feet deep (deeper than the maximum root depth on the site) between the walnuts and plants to be protected. 2) Detect severed walnut roots in the trench wall, and starting there, pull or dig up all large walnut roots beyond the trench. 3) Line both sides of the trench with polyethylene (held in place with wooden stakes). 4) Refill the trench with soil, and seed it with grass. The rationale for the double barrier was that if a walnut root grew over, under, or through a hole in the first barrier, the root would then grow in the former trench, not beyond the second barrier. The rationale for root removal beyond the barrier was that it would have immediate beneficial effect and would favor the demise of smaller walnut roots left in the soil. How long the procedure may be effective is unknown.      (For a list of literature references for this article please contact Branching Out at ddo1@cornell.edu) Top of Page