Walnut tree produces a chemical called juglone which is toxic for most of the plants. Juglone is produced from all parts of tree. Its production varies between walnut species; black walnut and butter nut produce more juglone than English or Persian walnut. The main purpose of this toxin production is to ensure survival of walnut.

Juglone induces wilting in sensitive plants by inhibiting plant respiration, depriving needed energy and cell division as well as water and nutrient uptake. In spite of this some plants overcome this toxin by metabolizing it (like black cherry, sugar maple etc.,)

Among vegetables onion, corn, beans, carrot, beets, squash and melons, parsnips are tolerant to juglone. And in fruit trees cherry, nectarine, plum, peach and Quince are tolerant to this toxin.

For more information please follow our Penn State Extension link
http://extension.psu.edu/plants/garde...