Deep bark canker of walnut—Erwinia rubrifaciens
Deep bark canker caused by the E. rubrifaciens bacterium can occur anywhere walnuts are grown in the Central Valley of California or other interior locations with hot weather. The problem is rare in coastal areas.
Identification
Deep cracks running lengthwise in large limbs or the trunk of walnut are characteristic of deep bark canker. Dark brown to reddish brown liquid oozes from these cankers during late spring through early fall, giving them a "bleeding" appearance. Internally, black to dark brown streaks of varying width extend through the inner bark and may run for many feet up and down the large limbs and trunk of affected walnuts. Numerous dark, round, small spots occur in wood beneath the cankered and cracked areas.
Life cycle
The deep bark canker pathogen is commonly transmitted in symptomless graft wood used to develop new trees. The disease may also be spread when the bacterium is introduced into a deep wound that exposes the phloem. Shallow wounds and pruning cuts are not infected by E. rubrifaciens.
The pathogen survives the winter in cankers or dried exudate on the tree surface. The bacteria become active in late spring when they begin to cause ooze from the cankers. The bacteria may spread from oozing cankers by windblown rain and infect wounds on nearby healthy walnuts. Trees are susceptible to new infections from April through October. Trees are almost completely resistant to new infections during winter.
Cankers may grow about 1 foot during the spring and about 2 feet more during summer. The bacteria spread within the tree through nonconducting parts of the phloem tissue. Movement of nutrients in the phloem is impaired by infections, slowly weakening the affected branches. As the tree loses vigor, it becomes more susceptible to sunburn injury and infection by the branch wilt fungus, Neoscytalidium dimidiatum (=Scytalidium dimidiatum =Hendersonula toruloidea), that causes branch dieback.
The disease sometimes develops on trees that have not been injured. It is thought that these are latent infections that were introduced in the nursery when the tree was grafted.
Damage
Deep bark canker is most common and severe on the Hartley cultivar. Deep bark canker does not kill trees, but it may further debilitate trees already weakened by other factors including other diseases, inadequate irrigation, insect damage, poor water infiltration, and soils that restrict root growth.
Solutions
Provide trees with good growing conditions and appropriate cultural care to keep them growing vigorously. Appropriate irrigation is especially important. Protect limbs and trunks from wounds because deep wounds can become infected by the bacterium.
If walnut is growing in soils that are highly compacted or shallow, the tree may be seriously affected by deep bark canker despite providing good cultural care. Deep bark canker cannot be cured by any known chemical use.
Adapted from Integrated Pest Management for Walnuts Third Edition and Pest Management Guidelines: Walnuts, University of California Statewide Integrated Pest Management Program (UC IPM). |
