Walnut fungus Graphium

Graphium juglans is a fungal pathogen known to cause severe damage to walnut trees. Belonging to the Ascomycota division, this organism acts as a vascular parasite, colonizing the bark and cambium layers. Its presence in the woody tissue of the host plant severely impairs the circulation of essential nutrients and water, eventually causing the death of limbs or the entire tree if left untreated.

The primary host for this fungus is the English walnut (Juglans regia). The pathogen targets all ages of the tree, from young seedlings to mature specimens. Infection typically occurs via natural openings or artificial wounds, including pruning cuts, sunscald areas, and insect-inflicted damage. These entry points provide the fungus with the necessary access to the vascular system, where it establishes its mycelium.

The biology of the pathogen is highly dependent on environmental factors, particularly high humidity and moderate temperatures. The life cycle involves the production of conidia, which are efficiently dispersed by splashing rain, wind, and even human activity or insects moving between trees. Once established, the fungus spreads through the internal tissues of the bark, accelerating the breakdown of cellular structures.

Damage is characterized by dark, sunken lesions on the branches and trunk, often accompanied by exudation of sap. Foliage on affected limbs may turn yellow and drop prematurely, leading to reduced vigor and stunted growth. As the disease progresses, the girdling of stems or the main trunk cuts off the supply of nutrients, leading to progressive dieback and eventual mortality of the walnut tree.

Integrated management is essential for successful control and requires the following measures:

• Strict sanitation by pruning and destroying all infected branches, ensuring cuts are made well into healthy wood.
• Immediate treatment of all pruning cuts and bark wounds with copper-based sealants or protective paints.
• Applying preventive fungicide sprays during the dormant season and early spring.
• Managing secondary pests that damage bark and act as carriers of fungal spores.
• Improving overall tree health through balanced fertilization and appropriate irrigation to boost the tree's natural defense mechanisms.