This fungus is a minor pathogen in California. It infects the lower limbs and trunk of walnut.
Identification
The most characteristic symptoms of shallow bark canker are black to brownish lesions on bark that are irregularly shaped to rounded. There are generally several of these discolored bark spots in a group on the lower limbs, trunk, or both. Newly infected areas have a margin of water-soaked bark and a central spot of black ooze that later dries, leaving a tarlike black spot. Just under the surface of the lesions are dark brown areas of dying tissue in outer bark. The cankers generally extend only one-fourth to one-third the depth of the bark and do not occur in wood.
Life cycle
The biology of this weak pathogen has not been well studied. It is not known how the pathogen infects walnut trees or how or why it develops and spreads.
Damage
Shallow bark cankers are superficial and generally occur only in outer bark. Although the cankers can be extensive, they appear to cause little damage to the tree. Unlike deep bark canker that is a more serious walnut disease, shallow bark cankers rarely extends into the inner bark.
Solutions
Because shallow bark canker can often be more severe in stressed trees, provide walnuts with a good growing environment and proper cultural care to improve tree vigor and help the tree produce callus tissue that stops the growth of the cankers. Appropriate irrigation can be especially important.
Cankers can be cut away; but this practice is not recommended because in California the disease is not serious enough to risk the wound damage and other plant pathogens might infect the tree through the canker-removal wounds. No fungicides are known to be effective for managing this problem, and fungicide application is not recommended for shallow bark canker.
Adapted from Integrated Pest Management for Walnuts Third Edition, University of California Statewide Integrated Pest Management Program (UC IPM).
