Walnut canker

Walnut canker is a serious fungal disease that causes necrotic lesions on the bark and cambium of various walnut species. The causal agent, Juglanconis oblonga (formerly Melanconis juglandis), is an ascomycete fungus that infects the vascular system of the tree, significantly impairing its ability to transport nutrients and water.

The primary host for this pathogen is the butternut or grey walnut (Juglans cinerea), which is highly susceptible and often suffers significant mortality. While it can occur on other species like the Persian walnut, the impact is most devastating on susceptible varieties, leading to localized dieback or the eventual death of the entire tree if the trunk becomes girdled.

The symptoms begin with the appearance of small, dark, depressed spots on the bark. As the disease advances, these spots expand into elongated cankers. A distinct characteristic of the disease is the secretion of a dark, oily liquid from the infected bark. As the lesions coalesce and grow, the bark eventually cracks, exposing the underlying dead wood and severely weakening the structure of the tree.

The fungus develops rapidly under conditions of high humidity and moderate temperatures. Spores are primarily spread by rain splashes and wind, entering the tree through wounds, pruning cuts, or insect-caused injuries. Trees already weakened by environmental stresses, such as drought, winter injury, or poor site conditions, are far more likely to develop severe cases of the disease.

Managing walnut canker requires a strict integrated pest management approach:

• Strict sanitation: remove and burn all infected branches immediately.
• Always disinfect pruning tools between trees to prevent cross-contamination.
• Maintain tree health through balanced fertilization and proper irrigation.
• Protect fresh pruning wounds with recommended wound sealants or copper-based fungicides.