Walnut wilt is a noninfectious disease which causes the sudden wilt and death of many plant species. The problem most often occurs in the beds of sensitive plants growing under or near walnut trees: Juglans negiaand J. nigra. The cause of the wilt is an active compound known as juglone produced by these trees and to a lesser extent under or near Butternut: J. cinerea and J. sieboldiana; hickories: Carya ovata, C. alba, and C. olivaeformis; and Pterocarya causcasica. Althouhg these species also produce some juglone, the amount is greatly reduced and toxicity is less often noticed in conjunction with these plants.
Juglone has been isolated from the roots, leaves, bark, and husks of walnut and related species. The compound can be found in the soil surrounding the living roots, and the highest concentrations occur in the roots, the hulls of the nuts, and the buds. It is found in the leaves and stems in lesser amounts. As the juglone is produced by living tissue and slowly leached out of dead tissue and into the surrounding soil, poisoning may occur when the roots of sensitive plants grow into the toxic zone. Juglone is not very water soluble compound, so even after the plant material dies, it may take several months for all of the juglone to be leached from the tissue and additional time for it to be leached from the ground. Poor drainage may play a role even when roots do not actually touch as the susceptible plant remains exposed to the juglone for a longer period before it is leached from the soil.
Different plants may show varying degrees of susceptibility to Juglone. Some may survive near the edge of the crown or just beyond the root zone, but may not survive beneath the canopy. Highly sensitive plants may not even be able to grow at the edge of the root zone.
Where walnuts are located near your property, it may be good to keep in mind that quirrels may gather and store these nuts in areas relatively far away from the trees. It is not uncommon to find whole walnuts burried in flower beds. Often these are buried before the hulls have fallen off. As the hulls contain a high concentration of juglone that may be released over several months, where this occurs, you may observe the deterioration or death of some bedding plants. If found, remove the nuts as soon as possible.
Symptoms may appear at any time during the growing season. Affected plants wilt suddenly and often show a browning of the vascular tissues just under the outer layers of the main stem. These symptoms may be easily confused with those of the vascular wilt diseases such as Fusarium and Verticillium. Some plants with mild tolerance may show stunting, lack of flower development, and/or lack of vigor.
Toxicity does not occur every year. Juglone does not persist in the soil over winter. It is produced only by living walnut, butternut, and hickory juglone producing roots when they are active during the growing season, or it may be leached from recently killed tissues. Apparently, the production and release of juglone is influenced by the growing conditions of a particular year and in some years the toxin is not produced in sufficient concentrations to become toxic to sensitive plant species. Walnut wilt can be avoided by planting vegetables or ornamentals outside of the toxic root zone. Plants will usually be safe if they are planted at a distance from the tree equal to the height of the tree. In cases where a juglone producing tree is removed, sensitive plant species may be safely planted on the site one year after the death of the tree. Juglone will not persist in dead roots or in the soil surrounding the dead roots for more than a few months following the death of the original tree. Recommendations: Don't plant the juglone producing trees in urban areas. If they are there, plant tolerant species under them. Keep in mind that the roots can spread 50 to 60 ft. from the trunk. This may be well beyond the drip line. Plants particularly sensitive to juglone include: tomato, potato, pea, cabbage, pear, apple, sour cherry, red pine and rhododendron. See the table below for a list of plants unaffected by juglone.
Table 1: PLANTS SEEMINGLY TOLERANT TO JUGLONE