What causes toxicity in walnut trees?
Many years ago, a chemical compound called juglone, which is a quinone, was isolated from walnut husks and was found to be highly toxic to alfalfa and tomatoes when injected into the plants. It was also noted that apple trees that even grew close to walnut trees were subsequently killed. It has since been discovered that walnut bark also produces juglone and this has a toxic effect on the soil due to root presence. The chemical causes wilting in plants that contact walnut roots and this usually becomes so severe that death is the result. It is interesting to note that the effect juglone has on the soil seems to deal only with root contact since walnut hulls and leaves used as mulch have not had the same effect. This is a significant observation due to the fact that walnut roots do not occupy the surface layers in most soils, thus allowing growth of many shallow rooted species that don’t come in contact with the roots under the tree. Jason W. Boggs Email: bjason@bgnet.bgsu.
