Black walnut trees produce a nontoxic chemical that becomes highly toxic when it is exposed to air or soil. How does this chemical help black walnut trees compete with plants growing nearby?

Answer 1

From Google:

Walnuts and butternuts produce a compound called juglone that is toxic to many other plants. Tomatoes and other members of the nightshade family are particularly susceptible. They can wilt suddenly when in contact with juglone from the roots, husks or uncomposted leaves of the walnut tree.