Plant Health Problems
Diseases caused by Fungi:
Butternut canker, Sirococcus clavigignenti-juglandacearum.
This serious disease is a problem for butternuts and does not affect black or English walnuts. Symptoms include dead branches, top dieback, discolored bark which has an inky black fluid coming from cracks in the spring and sooty patches, usually with a whitish margin, in the summer. Cankers are elongated and sunken, and frequently lethal.

Cutting and removing infected wood will slow the spread of infection, but there are no known controls.

Nectria canker, Nectria galligena.
This pathogen attacks trees in many hardwood genera, but is particularly virulent on black walnut. Cankers may be open and look like targets with concentric circles, or be closed with only a horizontal slit showing at the surface.

Cutting and removing infected wood will slow the spread of infection, but there are no known controls.

Anthracnose, Gnomonia leplostyla.
Symptoms appear as irregular brown or blackish leaf spots on all types of walnuts. The fungus infects leaves early in the summer, and if severe, causes them to drop. The fungus can also infect young shoots and cause dieback.

Control is achieved by raking and removing affected leaves and by pruning infected shoots and twigs back to healthy wood. Since this disease is usually not a serious problem for the health of the trees, chemical controls are usually not necessary.

Diseases caused by Bacteria:

Bacterial blight or walnut blight, Xanthomonas juglandis.
All walnuts are susceptible, but Persian walnuts are most susceptible. The bacterium infects new shoots, young leaves, and the husks of developing nuts from spring to fall during wet weather. Older wood is not subject to the disease, and this blight rarely kills a tree. However, destruction of the nut crop can be a serious problem. Infection is first seen as a small water-soaked spot, which enlarges, turns reddish brown, and then black.

Pruning all affected tissue may retard spread of the disease in an orchard, but there are no effective chemical controls.

Diseases caused by Phytoplasmas:

Bunch disease, phytoplasma.
All walnuts are susceptible to this disease but the Japanese walnut is the most often killed. Lateral buds grow, producing a cluster of twigs which looks like a witches'-broom. These brooms are often killed by winter conditions and few nuts are produced.

There are no known controls, and infected trees should be removed from the vicinity of walnut orchards or nurseries.

Insect Problems
For insect problems see Walnut.