How to Diagnose Black Walnut Damage

Barbara C. Weber,
Forestry Sciences Laboratory,
Carbondale, Illinois

Robert L. Anderson,

William H. Hoffard,
Southeastern Area State and Private

Asheville, North Carolina

Illustrations by J. Lockyer

North Central Forest Experiment Station
Forest Service, U.S. Department of Agriculture


Black walnut trees, like all other plants, are susceptible to a variety of injuries that reduce or destroy their usefulness. The first step in preventing or controlling these injuries is to identify their cause. Most damage is caused by disease, insects, birds, mammals, or weather.

Presented here is a method for identifying the most common causes of injury to black walnut trees. It was developed primarily for use by walnut growers or foresters who are not specifically trained in insect or disease identification. The method features a "key" that leads the user step-by-step from the visual symptoms to the culprit responsible. Supplementing the key are brief descriptions of the damages and their causes, illustrated in most cases with line drawings.

Once the grower knows what is causing the problem, he can decide what can or should be done about it.

To identify all the insects and diseases that damage black walnut would be impractical and cumbersome. For example, more than 300 species of insects have been found on walnut in southern Illinois alone. So we have confined our efforts to those insects, diseases, and other sources of damage that are commonly found throughout most of the walnut-growing range and hence most likely to be encountered by the grower.

Some control measures are suggested. Generally, however, we refer the grower to the local service forester or county extension agent to find out about chemical control recommendations. This is because some chemicals currently used to control insects and diseases on black walnut may be banned or restricted by the Environmental Protection Agency in the future, thus making specific recommendations obsolete or illegal. Hence, we recommend that the grower always consult with the appropriate agencies before applying chemicals.

We have tried to keep technical terms and jargon to a minimum. Nevertheless, a few terms may still be present that are unfamiliar to some, so a brief glossary is included.


  1. Examine the damage carefully.
  2. Decide whether the damage is to
    1. a. foliage or nuts
      b. shoots, buds, or twigs
      c. trunk or stem
  3. Turn to the appropriate heading in the key and work down through the alternatives.

  • EXAMPLE: Yellow walnut leaf with brown spots

    • Select the proper category - Damage to Foliage and Nuts

      Begin at A. Leaf has not been eaten, so go to B.

      Read through each subheading under B.

      Leaf fits description 4 ("Leaves with spots or blotches"); read through the subheadings under 4.

      Damage fits the description under 4a, so it was caused by walnut anthracnose.

      Compare the description and illustration of walnut anthracnose with the leaf sample.

      If they do not agree, repeat the process or consult an expert.

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