Symptoms and Signs
The most characteristic symptoms of shallow bark canker are brownish to black round spots or areas, usually several in a group, on the trunk or lower scaffolds. 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, dark brown areas of dying tissue are formed in the outer bark. These superficial cankers can be extensive, but they seem to cause little damage to the tree. Shallow bark canker rarely extends into the inner bark, as does deep bark canker.
Comments on the Disease
Shallow bark canker affects many commercial walnut cultivars, but the full range of susceptible hosts has not been determined. Because the damage is superficial and does not result in economic loss, shallow bark canker is not considered a major disease of walnut. It is not known how the pathogen infects walnut trees or how it develops and spreads.
As with deep bark canker, shallow bark canker is often severe in stressed trees. Improving tree vigor may help contain the disease. There is no need to cut cankers away because the disease is not serious enough to warrant possible wound damage and other infections.