Symptoms and Signs
The symptoms that characterize deep bark canker are the deep cracks running down scaffolds and trunks. A reddish brown to dark brown substance oozes from these cankers from late spring through early fall, giving them a "bleeding" appearance. Internally, dark brown to black streaks of varying width extend through the inner bark and may run for many feet up and down the scaffolds and trunks of affected trees. These streaks occur deep in the bark, in the region of the phloem. As a result, the disease was named phloem canker or deep bark canker to distinguish it from shallow bark canker, which is restricted to the outer layers of bark.
Another typical internal symptom of deep bark canker is the numerous small, round, dark spots that extend into the wood beneath the cankered areas.
Deep bark canker infections develop first on the trunk or lower scaffold; only one or two scaffold limbs are affected. As the disease progresses upward, the branch weakens slowly over time. After many years, most branches are affected, and the tree becomes less productive. The symptoms do not extend into the rootstock.
Comments on the Disease
Deep bark canker occurs in all walnut-growing areas of the Central Valley but is rarely a problem in coastal growing areas. Deep bark canker is most common and most severe on the Hartley cultivar. The canker does not kill trees, but it may further debilitate trees already weakened by other factors, including inadequate irrigation, poor water infiltration, restrictive soils, as well as insect and disease pests.
The deep bark canker pathogen is most commonly transmitted in symptomless graft wood used to develop new trees. The disease may also be spread when the bacterial pathogen is introduced into a deep wound that exposes the phloem, such as shaker damage and possibly woodpeckers. Shallow wounds and pruning cuts are not infected.
The pathogen survives the winter in cankers or dried exudate on the tree surface. The bacteria become active in late spring and begin to ooze from the cankers, together with plant sap. At this time bacteria may be spread by windblown rain to wounds on uninfected trees. Trees are susceptible to infection from April to October and almost completely resistant in winter. Cankers may lengthen by about 1 foot in spring and about 2 feet in summer. The bacteria spread within the tree through nonconducting parts of the phloem tissue. Movement of nutrients in the phloem is impaired, slowly weakening the affected branches. As the trees lose vigor, they become more susceptible to sunburn injury and infection by the branch wilt fungus.
Because high temperatures favor the development of the disease, deep bark canker is more prevalent in the Central Valley than in coastal areas. The disease sometimes develops on trees that have not been injured. It is thought that these are latent infections that were introduced when the tree was grafted.
To avoid predisposing trees to infection by deep bark canker, keep them healthy by practicing good water management, fertilization, pruning, and pest control. These management practices also reduce canker development in infected trees and keep them in production in most cases. Only trees growing on poor soils or under particularly adverse conditions may never fully recover. Infected trees may remain free of symptoms unless stressed.
The following guidelines can help identify stress factors that promote disease development:
- Dig a hole 6 to 10 feet deep in several areas of the orchard. Select sites that represent different soil types or differences in tree growth and vigor. Study the soil profile for hardpans, claypans, compaction, or layers of differing texture. Check the rooting depth and health of the root system.
- Maintain adequate moisture in the root zone and avoid over- or under-irrigating by using stem water potential measurements, water marks, water budgeting based on evapotranspiration (ET) estimates, tensiometers, or other established methods to monitor orchard water status and schedule irrigations.
- Check soils a few days after an irrigation to evaluate water infiltration and penetration. Problems can have physical or chemical causes. Contact your UC Cooperative Extension advisor for information on corrective procedures.
- Avoid saturated soil conditions for longer than 18 to 24 hours to minimize Phytophthora crown and root rot development.
- Avoid injuries that expose the inner phloem, like those that may be caused when shaking trees.
- Avoid introducing the pathogen into the orchard; thoroughly scrub and drench shaker pads in a 1:10 dilution of bleach before bringing the shaker in from another orchard.
Deep bark canker cannot be cured by any known chemical means. Cutting away the cankered areas, both with and without applying copper or bleach, has not proven successful and is harmful to the trees. Attempts to halt the disease by injecting antibiotics have also failed.