If you have one or more black walnut trees in your yard, you
may have noticed that some plants have difficulty surviving
near the trees.

The roots of black walnut trees produce a toxic substance
called juglone which adversely affects plants that are
sensitive to it. Many plants are highly sensitive to
juglone, but there are some that will tolerate it.

Plants that cannot tolerate juglone will show symptoms
such as yellowing, wilting foliage. Juglone acts as a
respiration inhibitor, sapping a plant's energy and leaving
it unable to breathe. Plants that cannot tolerate juglone
will eventually give up and die.

Although juglone is produced in the trees' roots, all
parts of a black walnut tree contain the toxin, with the
strongest concentration in the buds and nut hulls.
Black walnut trees have a habit of dropping leaves, nuts
and twigs from late summer through autumn and this debris
adds to the juglone levels in the soil beneath the trees.
Rain dripping from the leaves also adds juglone to the
soil, making the entire drip zone beneath the tree a
hazardous environment for juglone-sensitive plants.

Cutting down the offending tree won't solve the problem,
as the roots will continue to release juglone into the soil,
making the area toxic for several years after the tree is
gone. Debris from black walnut trees should not be added to
compost, nor should the wood or bark be used for mulch.

There are also other trees and plants that produce allelopathic substances that can harm companion plants. I googled 'allelopathic plant' and found this article that gives lots of relevant information.