This is from Mike McGroarty of freeplants.com. He posted it in his newsletter.

How Black Walnut Trees Can Affect Your Plants

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.