Toxicité - juglone
Most members of the Walnut family (Juglandaceae) produce a chemical called "juglone" (5 hydroxy-alphanapthoquinone) which occurs naturally in all parts of these plants. Black walnut, pecan, hickory and others members of the family including Carya, Engelhardtia, Juglans, Platycarya and Pterocarya can produce juglone.
Landscaping and Gardening Around Walnuts and Other Juglone Producing Plants
The greatest concentration of juglone and hydroxyjuglone (a nontoxic, colorless precursor that is converted into the toxic form juglone by sensitive plants and through oxidation) is found in the vegetative buds, leaves, stems, nut hulls, and roots of the plants.