Extension Communications Extension 4-H Youth Building Ames, Iowa 50011-3630 (515) 294-9915

12/29/03

Contacts:
Krystal Tentinger, Plant Health and Protection and Horticulture, keeko@iastate.edu
Jean McGuire, Continuing Education and Communication Services, (515) 294-7033, jmcguire@iastate.edu

Yard and Garden Column for the Week Beginning Jan. 2, 2004

The Virtues and Vices of Black Walnut Trees

Krystal Tentinger
Plant Health and Protection and Horticulture Student
Iowa State University Extension

Walnuts are beautiful trees that have a variety of uses in the landscape. Two common species are American black walnut, Juglans nigra and butternut, Juglans cinerea. The name Juglans means Jupiter’s nut since it was thought to feed the “gods” that lived on earth. Walnuts were also used as medicine in earlier times to help with brain problems because the nut resembles a brain. Walnut trees can be used as ornamentals, but regular clean up is necessary due to the litter from the leaves and fruit. Black walnut is prized for its high quality timber and is often used in furniture and veneers. It is also valued for its fruit.

One of the most interesting characteristics of walnut trees is their ability to release a growth inhibiting biochemical known as juglone. This chemical can cause disruptions of normal processes in near-by plants. This process is referred to as allelopathy. Juglone can disrupt respiration, cell division along with water and nutrient uptake. Symptoms in the affected plant include wilting, yellowing and even death.

Juglone is released from all parts of the walnut tree. From the leaves to the roots, this chemical can be found everywhere. Leaching can cause movement of juglone to surrounding soil. It can be very difficult to grow certain plants around walnut trees. Tomato is one plant that is very sensitive to juglone and needs to be planted at least 20 feet from the drip line.

Since juglone is present in the leaves, they should be raked up and parts of the tree should not be used as mulch unless properly composted. Many plants are resistant to the toxicity of juglone, so refer to a list of these plants before planting around a walnut tree. A few of the trees and shrubs suitable for growing around walnuts are the American arborvitae, white ash, hibiscus, flowering dogwood, forsythia, catalpa, crabapple and hawthorn. Herbaceous plants that are tolerant of juglone include asters, astilbe, clematis, hosta, phlox, trillium, tulips and violets.

There are a few fungal diseases that can harm walnut trees. Cankers can be problematic for both walnut and butternut trees and can affect the trunks or branches. They are characterized by rough, sunken areas on the bark and during the fall and winter months, small red fruiting structures of the disease fungus may be found in cankered areas. The primary means of control is to prune the affected branches. Butternut canker can be a major problem and can kill trees within a few years. The bark changes color from green to red and finally to gray. This first appears on the smaller branches and then travels to main branches. When canker diseases are severe, removal of the tree maybe necessary.

Anthracnose is a common leaf disease that attacks walnut and many other shade trees in Iowa. Anthracnose is most severe after a cool, wet spring and summer. Infected leaves have small to large black spots that can merge together in mid to late summer. Infected fruit husks may have black spots, and the nut itself may shrivel. Consecutive years of serious infection can cause yellowing of the leaves and extensive loss of leaves. With walnut tree anthracnose, the defoliation usually occurs at the end of the growing season. This usually gives the tree time to prepare for the long winter, and does not harm the health or growth rate of walnuts; therefore permanent damage from anthracnose is not usually severe.

Management of anthracnose involves a variety of practices. The fungus over winters in fallen leaves, so clean up of the fall leaves is helpful. Pruning can help with light penetration and air circulation, and keeping the tree as healthy as possible keeps the tree ready to ward off fungal infection. Fungicides are not usually warranted, as anthracnose is rarely detrimental to the tree’s life. Minimal control of the disease is usually obtained with fungicides. When planting for ornamental purposes, make sure to select plants that are less susceptible or resistant to anthracnose.

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Editors: A color photo, suitable for publication, is available at right. Click on the thumbnail photo to go to the fullsized photo. The fullsize photo is 236K. Caption: Walnut in decline

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