As mentioned above Black Walnut roots and foliage contain a chemical that is toxic to many other ornamental and vegetable plants. This substance, juglone, causes harmful reactions in neighboring plants within a circle 1 1/2 times the distance from the trunk to the outermost branches of the tree.
However, many plants do not seem to be susceptible to it. Among these are hawthorne, American elm, beech, locust, tulip, white oak, red cedar, gray dogwood, some euonymus species, and common daylily.
It is rare to find healthy butternut trees today. The humidity and relatively low air movement along stream terraces provide an ideal habitat for the fungus that causes cankers to form on the roots, trunk, and limbs of the trees. The cankers cause girdling which eventually will cause the death of the tree.
Leaves Yellow and Curl
Zinc Deficiency - Rapidly growing young Black Walnut trees draw deeply on soil nutrients and occasionally deplete certain trace minerals. If the foliage on an otherwise healthy tree begins to turn yellow in mid-season and the leaf stems curl upward, there may be inadequate zinc in the soil. This is easily corrected by adding zinc sulfate to the soil. It is available at garden centers. Read the label for instructions on its application.
Leaves Eaten, Trees Defoliated
Caterpillars - Various kinds of caterpillars feed on walnut tree foliage. Walnut caterpillars are about 2 inches long and are covered with long white hairs. They emerge from eggs on the undersides of leaves in July and begin to hungrily devour tree foliage. Other caterpillars, the larvae of many different moths, including the gypsy moth, also like walnut foliage. Some emerge from tents, or webbed bags, on the tree branches.
Bark Attacked, Leaves Curled and Distorted
Aphids - There seems to be a type of aphid for every plant and walnut trees are no exception. There is a pale yellow type that attacks English Walnut trees on the West Coast, another, the giant bark aphid sometimes infests all kinds of walnuts. Soft-bodied, pear-shaped sucking insects usually about the size of the head of a pin, aphids typically cluster on the undersides of leaves. They suck cell juices from tender new tissues, retarding or distorting tree growth. Leaves may turn yellow or brown, wilt under bright sunlight, or sometimes curl and pucker. Aphids often secrete quantities of sticky honeydew that promotes mold and compounds the problem.
Yellow Galls on Leaf Undersides
Mites - Walnut blister mites and two other types of mites sometimes infest walnut trees. While blister mites cause yellow or brown felt-like galls (swellings) on the undersides of leaves, other mites cause pale, stippled leaves. Mites are about 1/50 inch long, practically invisible to the unaided eye. In fact, it is often the wispy webbing that some of them make, that signals their presence on plants. They have four pairs of legs and piercing-sucking mouthparts, and very compact bodies. They may be yellow, green, red or brown. Typically they zero in on a tree that is under stress from drought, heat or other pest problems.
Sunken Sores on Trunks, Branches
Canker - Fungus causes cankers. Carried by wind, rain or insects, the spores enter tree bark through injuries. They germinate and gradually penetrate tree wood, killing about 1/2 inch of tissue a year. The infection is signaled on the bark surface by the development of flat sores with sunken centers surrounded by raised ridges. On large, heavily infected trees these cankers may be 4 feet long and over 2 feet wide. On smaller trees they are barely evident. Eventually the sores on the trunk girdle the tree, killing it. The only treatment for this problem is surgery by a professional arborist. Seriously diseased trees should be cut down and their infectious sores dug out and discarded to prevent contagion. To prevent these problems, keep trees vigorous with proper feeding and watering. Site them in the full sun that discourages fungus growth. Most importantly, avoid injury to tree bark from yard care equipment. Prune out rubbing tree branches, and prune ragged breaks of branches from weather injury cleanly.
Irregular Dark Spots on Leaves
Brown Leaf Spot - A fungus sometimes attacks walnut tree foliage early in the summer when it is young. Tender new leaflets develop dark brown or black spots, roughly circular, sometimes with white rings around the edges. This disease is capable of defoliating trees completely. It is extremely important to gather all fallen leaves and yard debris under and around affected trees. Throw it in the trash to prevent the spread of the fungus.